WHO Regional Publications European Series No. 10
NONIONIZING RADIATION PROTECTION
Edited by
Michael J. Suess
THE UNITED NATIONS ENVIRONMENT PROGRAMME
• ~ ~..,.~ WORLD HEALTH ORGANIZATION REGIONAL OFFICE FOR EUROPE
COPENHAGEN 1982
ISBN 92 890 1101 7
© World Health Organization 1982
Publications of the WHO Regional Office for Europe enjoy copyright protection in accordance with the provisions of Protocol 2 of the Universal Copyright Convention. For rights of reproduction or translation, in part or in toto, of this publication application should be made to the WHO Regional Office for Europe, Scherfigsvej 8, DK-2100 Copen hagen </) , Denmark.The Regional Office welcomes such applications.
The designations employed and the presentation of the material in this publication do not imply the expression of any opinion whatsoever on the part of the Secretariat of the World Health Organization concerning the legal status of any country, territory, city, or area or of its authorities, or concerning the delimitation of its frontiers or boundaries.
The mention of specific companies or of certain manufacturers' products does not imply that they are endorsed or recommended by the World Health Organization in pre ference to others of a similar nature that are not mentioned . Errors and omissions excepted, the names of proprietary products are distinguished by initial capital letters .
The authors alone are responsible for the views expressed in this publication .
TYPESET IN THE UN ITED KINGDOM PRINTED IN SWITZERLAND
79/ 4457 - Acccnl I Alar- 5500
ISSN 0378-2255
Contents
Page Preface ................................................. vii
Note on terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viii
Introduction - M.J. Suess . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ....
CHAPTER l. Ultraviolet radiation - M. Faber. . . . . . . . . . . . . . . . . . . . . . 9
CHAPTER 2. Optical radiation , with particular reference to lasers - L. Goldman, S.M. Michaelson, R.J. Rockwell, D.H. Sliney, B.M. Tengroth & M.L. Wolbarsht . . . . . . . . . . . . . . . . . . . . . 39
CHAPTER 3. Infrared radiation - C.E. Moss, R.J. Ellis, W.E. Murray & W.H. Parr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
CHAPTER 4. Microwave and radio frequency radiation - S.M. Michaelson . . . . 97
CHAPTER 5. Electric and magnetic fields at power frequencies , with particular reference to 50 and 60 Hz - R. Hauf . ... .... .... 175
CHAPTER 6. Ultrasound - C.R. Hill & G. ter Haar . .................. 199
CHAPTER 7. Regulation and enforcement procedures - F Kosse/ . ........ 229
Glossary ............................................... 245
Annex 1.
Annex 2.
Special acknowledgements .. .... ... .. .. . . . ........ . 249
List of Working Groups ... ... ...... ...... . . ....... 259
Index .. .... ... .. . . ........... ... .... . .. ... ............ 261
PREFACE
As human populations multiply and industrialization increases and diversifies new hazards arise, some of which have become more and more critical. To limit and, as far as possible, reverse this trend, the WHO Regional Office for Europe mounted an intensive intercountry programme during the period 1969- 1979, which has culminated in the successful completion of many projects in different sectors.
One of the newer environmental hazards is nonionizing radiation (NIR}, which may lead to adverse effects on human health. Exposure to NIR extends from occupational health right into the field of public health. When considering exposure limits and setting up control programmes, the heterogeneity of the population to be protected has to be kept in mind. Possible genetic and car cinogenic effects, as well as effects on development, have all to be carefully considered and are of prime importance in protecting the public. Therefore, one of the major efforts within the radiation sector of the programme during these years was directed towards the development of this book, which I now introduce with pleasure to a broad professional audience. This publication represents the results of the collaborative effort of over 200 experts from 20 countries, to whom we are indebted not only for their professional competence but for their deep dedication. With equal satisfaction, I wish to commend the work of the WHO staff directly responsible for this activity. The financial assistance of the United Nations Environment Programme in the project on NIR protection, and in the publication of this book, is greatly appreciated.
Protection against exposure to NJR is a subject of increasing concern to European countries, but is also of growing importance to those in other parts of the world. I hope that this work will be of practical value to the many scientists, engineers, physicians and community leaders concerned with and responsible for protection against NIR and human health.
Leo A. Kaprio WHO Regional Director for Europe
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NOTE ON TERMINOLOGY
The policy of the World Health Organization in respect of terminology is to follow the official recommendations of authoritative international bodies such as the International Union of Pure and Applied Physics (IUPAP), the Inter national Commission on Illumination (CIE) and the International Organization for Standardization (ISO). Every effort has been made in this publication to comply with such recommendations.
Nearly all international scientific bodies have now recommended the use of the SI units (Systeme international d'Unites) developed by the Conference generale des Poids et Mesures (CGPM)° and the use of these units was endorsed by the Thirtieth World Health Assembly in May 1977. Only SI units are used in this publication.
In the establishment of scientifically acceptable protection standards, both for workers and for the public, the first requirement is that internationally acceptable units for use in measurements must be available. The introduction of SI units means that this requirement has now largely been satisfied.
a An authoritative account of the SI system entitled The SI for the health professions has been prepared by the World Health Organization. Copies may be obtained through the sales agents listed on the back cover of this book, or direct from the Distribution and Sales Service, World Health Organization, 1211 Geneva 27, Switzerland.
viii
Introduction M.J. Suess 0
In recent years there has been an increase in the development and use of equip ment that produces nonionizing radiant energy, and the question has been raised as to whether adequate measures are being taken to guard the user and the general public from its possible adverse effects. In contrast to ionizing radiation, radiation of longer wavelengths is intrinsically less energetic and usually interacts with human tissue primarily by generating heat. For want of a better collective term, "non ionizing radiation" (NIR) is used to encompass this group of electromagnetic radi ations and also ultrasound. Nonionizing radiation pervades the entire environment but, except for the narrow spectrum of visible radiation, it is unperceived by any of the human senses unless its intensity becomes so great that it is felt as heat. Differences in wavelength, even within a single type of radiation, are particularly important when evaluating hazards from exposure to NIR. The ability of the radi ation to penetrate into the human body and the sites of absorption will depend on this characteristic and will differ from one type of radiation to another.
In developed countries there has been a remarkable growth in the number of processes and devices that utilize or emit NIR. They find an ever increasing use in industry, engineering, telecommunications, medicine, research, education and the home. This gives rise to a number of questions. How serious are the problems linked with NIR, what are their dimensions, and what acute and chronic effects on the human body are involved? Is there sufficient knowledge of occupational risks and public health hazards? How can exposure be reduced? Are national protection standards adequate and, if not, how can better regulations be drafted and enforced to reduce exposure? Because of the rapidly developing technology and the associated health implications, there is a need to develop international cooperation in the use of NIR and measures to prevent overexposure. Moreover , governments will be expected to intensify the establishment of rules and regulations and means of enforcing them.
The NIR part of the electromagnetic spectrum is divided into five regions (Table 1). No exact ranges for these regions can be defined, and those given in Table 1 are only approximations. In some cases, and for various reasons, different international bodies have developed and agreed on slightly different ranges, depend ing on the purpose of the definition. This is similarly true for some of the working
0 Regional Officer for Recognition and Control of Environmental Hazards, WHO Regional Office for Europe, Copenhagen, Denmark .
N Table 1. Ranges of frequency, wavelength and energy for some types of electromagnetic radiation
Type of radiation Frequency range 0 , b Wavelength range 0 Energy range per photon°• b
Ioniz ing > 3000 THz < 100 nm > 12.40 eV
Ultraviolet (UV) (nonionizing part) 3000 - 750 THz 100 - 400 nm 12.40 - 3.10 eV extreme (vacuum) 3 X 1 o' to 30 ooo - 1580 1 to 10 - 190 1240 to 124 6.53 far 1580 - 1000 190 - 300 6 . 53 - 4.13 near 1000 - 750 300 400 4 .13 - 3 .10 UV - Cc 3000 - 1070 100 - 280 12.40 - 4 .43 UV - B c 1070 - 952 280 - 315 4 .43 - 3.94 UV - A c ("black light" ) 952 - 750 315 - 400 3 .94 - 3 .10
V isible ll ghtd 750 - 385 THz 400 - 780 nm 3 .10 - 1.59 eV
Infrared JI R ) 385 - 0 .3 THz 0 ,78 - 1000 µm 1590 - 1.24meV IA -A 385 - 214 0. 78 - 1.4 1590 - 886 IR-Bc 214 - 100 1.4 - 3 886 - 413 IA-Cc 100 - 0,3 3 - 1000 413 - 1.24 near 385 100 0 .78 - 3 1590 - 413 middle 100 - 10 3 - 30 413 - 41 .33 far 10 - 0 .3 30 - 1000 41 - 1.24
Lasers 1500 - 15 0.2 - 20 6200 - 62 Class 1 - non -risk laser devices Class 2 -low-risk, low-power laser devices Class 3a - low-risk, medium -power laser devices Class 3b - moderate-risk , medium -power laser devices Class 4 -h igh -risk , high -power laser devices
Microwave (MW) 300 - 0 .3 GHz 1 - 1000 mm 1240 - 1.24µeV EHF (extremely high frequenc ies) 300 - 30 1 - 10 1240 - 124 SHF (super-high frequencies) 30 - 3 10 - 100 124 - 12.40 UHF (ultra-high frequencies) 3 - 0 .3 100 1000 12 - 1.24
Radar 56 - 0.23 5.4 - 1300 230 - 0.95
Aadiofrequency ( AF) 300 - 0 .1 MHz 1 - 3000 m 1240 - 0.41 neV VHF (very high frequencies) 300 - 30 1 - 10 1240 - 124 HF (high frequencies) 30 - 3 10 - 100 124 - 12.4 MF (medium frequencies) 3 - 0 ,3 100 - 1000 12 1.24
a The ranges given are only approximations, since no precise limits can be defined . b The figures given here have generally been rounded up or down to the third sign ificant d igit. ~ Radiation bands of biological sign ificance designated by the International Commission on Illumination (CIE) (Chapter 3 , Ref. 11) ,
The vlslbilitY lim its of the human eye vary among individuals between about 380 - 400 nm and 750- 780 nm.
groups involved in the preparation of this book. Ultraviolet (UV) radiation has been used extensively for sterilizing equipment
and air, and in different types of medical apparatus. In recent years industrial uses of UV sources has greatly increased, and a certain risk still exists for workers from open UV sources. Damage is confined to the eye and the skin, but there is a certain long-term risk of skin cancer. The largest exposed group comprises those who spend a great deal of time in the sun, and attention should be drawn to protective measures. As far as human cancer is concerned, little quantitative knowledge of dose-effect relationships and latency periods is available. Ultraviolet lamps for private use are widely distributed among the public and should be supplied with appropriate warn ings.
Exposure to infrared (IR) radiation can occur in almost any industry from direct IR sources as well as from other heat sources, and the risks under certain working conditions are well known. Still unanswered is the question of whether IR radiation has produced lenticular cataract. In any case, the presence of well developed temperature sensors in the skin around the eye provides a good bio logical warning system.
Risks from the use of lasers should receive more attention because of the rapid development of these instruments and their increased use for both military and non military applications. The emitted light can damage the eye and the skin, and under certain conditions perhaps also internal tissues. The difficulty in evaluating the risks from lasers is due partly to the difficulty of extrapolating the results of animal experiments to man.
Microwave (MW) and radiofrequency (RF) radiation are recognized as the types of NIR that present the greatest perceived risk. The expansion in the use of MW in the communications field and of MW ovens, if appropriate safety devices are not present in such ovens, presents a possible public health hazard.
Some questions have been raised with respect to possible adverse effects of electric and magnetic fields, particularly those at low frequencies, in connexion with high-voltage lines. However, no effects due to occupational exposure have been reported, nor are there any indications of injuries to human organs. At present, there is still a dearth of information, and there is a need for better experimentation and more objective and relevant observations.
The versatility of ultrasound has led to its widespread employment in industry, medicine and science for measurement, scanning and control applications, and for thermally modifying material. Ultrasound is relatively safe because of its inability to pass an air-water interface. Whether potential adverse effects exist from im mersion of hands in ultrasonic cleaning baths is not known. It has been claimed that chromosome aberrations can be produced by ultrasound radiation, but the evidence tends to be negative. So far, no major adverse effects have been recognized following diagnostic exposure of children in utero, and there is general agreement that the risk is much less than from X-ray examinations.
The World Health Organization has always recognized that the protection of the environment is an integral part of protecting the health of the people who live in it. Conscious of the seriousness of the issue, the WHO Regional Committee for Europe decided in 1969 to adopt a comprehensive long-term programme on environ mental health and pollution control, including NIR protection. The main aim of the programme was to develop management guides and decision aids for use by
3
government administrations, executive agencies, scientific institutions and individual specialists concerned with the quality of the environment and the protection of public health.
Development of the NIR protection sector of the programme began with a working group which was held in The Hague in November 1971 to consider the health effects of ionizing and nonionizing radiation. This meeting could perhaps be considered the beginning of international activities on NIR protection. The working group reviewed the existing situation with regard to the use of NIR and concluded that although the existing codes and guidelines were sufficient to prevent injury, it was doubtful whether they would be adequate in the future in view of the expected growth in the use of all types of NIR. The group went on to make a number of specific recommendations with respect to surveys in the field of health protection, dissemination of information, and the preparation of codes of practice. The major project that followed this meeting consisted of a series of activities which have led to the publication of this book.
This book is the culmination of a decade of concerted effort by over 200 scientists from 20 countries in Europe, North America and Asia (Annex 1). Five scientific groups (Annex 2), each addressing the subject matter of one or two chapters , met at various times to review draft chapters and to recommend ways and means of finalizing them. Drafts went through several reviews before and after the group meetings , followed by revisions and final editing. The purpose of this book is to provide practical information on health aspects of exposure to NIR energies. Descriptions are given of the physical characteristics, biophysical principles and bio logical effects of exposure to these energies. Public and occupational health im plications and protective measures against the hazards of exposure are also described. Existing standards for general public and occupational exposures are described to inform the reader about the maxinmm permissible exposure levels, or threshold limit values, at present accepted in various countries. The book is intended to provide information and recommendations that will assist in setting up NIR control programmes and establishing a unified system for the recording and evaluation of results. It is also designed to serve as a technical guide for scientists, engineers and medical personnel active in the field of NIR protection and control. Evidence is presented on possible adverse health effects of NIR, so that where appropriate standards and legislation may be enacted to protect health. Although the book was originally planned by the Regional Office for use in European countries, it should be of value to countries in other parts of the world and to the other international organizations concerned . Moreover, it should provide NIR specialists everywhere with helpful information based on up-to-date international practice.
Work began in 1973, when Professor Michaelson was invited to prepare a document on the potential hazards and safety considerations of human exposure to NIR energies. The first draft was ready in May 1973 and was sent to specialists for review. Moreover , advantage was taken of the International Symposium on Biologic Effects and Health Hazards of Microwave Radiation, held in Warsaw in 1973, to convene a small ad hoc group with the participation of Professors Czerski, Faber, Gordon, Michaelson and Schwan. It was decided to divide the document into two parts : that on MW and RF would be subjected to further revision and review, and a second on lasers would eventually be submitted to a working group on that subject. The final manuscript on MW and RF, following significant and
4
elaborate changes, was ready for final editing in February 1976 and was issued by this Office in 1977. Some of the major contributions to the completeness and accuracy of this version were made by Professors Gordon and Schwan, and Dr Silverman.
However , this document had neither the advantage of being discussed by a working group, nor was it up to date by the time other chapters were completed. An ad hoc meeting in Freiburg in May 1978 , attended by Czerski , Harlen , Kosse) , Michaelson , Repacholi , Shore and Suess, decided to submit the MW/RF document for further review and revision . Consequently , Professor Michaelson convened a group of experts from Europe and North America for a review meeting, which was held in 1978 at the Bureau of Radiological Health in Washington, DC. This meeting led to a new, rather lengthy and detailed text, but one that was very comprehensive and acceptable to all the points of view represented. The new chapter was revised and corrected, and submitted for final editing in December 1979. Mr Harlen in particular was very helpful in checking the nonbiological sections of the manuscript for adequacy and accuracy.
The Working Group on Health Effects from Lasers , held in Dublin in October 1974, considered the revised working document provided by Professor Michaelson , and the authorship was then enlarged and the material redrafted. After further review and revision, the chapter was submitted for final editing in February 1976 and was issued by this Office in a provisional form in 1977. Mr Sliney and Dr Wolbarsht were particularly instrumental in giving this chapter its present form , while Professor Michaelson was the coordinator of the team and technical editor of the material.
The chapters on UV and IR radiation were both commissioned in 1976. The former was reviewed by correspondence and examined thereafter by a working group in Sofia, in February 1978. After further revision , the chapter was submitted for final editing in January 1979. The draft chapter on IR radiation went through a number of reviews and revisions before submission to the same working group. It was revised , reviewed and corrected thereafter, and submitted for final editing in October 1978.
The chapter on ultrasound was commissioned in 197 5 and was passed through a similar process. It was discussed by a working group in London in October 1976, and after further review and corrections, was submitted for final editing in May 1977. A provisional edition of this chapter was issued by this Office that same year.
The two last chapters to be mentioned , which were discussed by the working group in Freiburg in May 1978, are concerned with electric and magnetic fields at low frequencies, and with regulations and enforcement procedures. They were com missioned in 1976, went through a similar review, revision , and correction process, and were submitted for final editing in December and October 1978, respectively.
Some chapters contain conclusions and/or recommendations of various types and purpose . These were agreed on, as were the contents of the chapters themselves, by the participants in the various working groups , and presented by the respective authors in their chapters . It will be obvious, considering the time spent in compiling this work, that some literature references are not as recent as one would wish. However, the reader will appreciate that the objective of this book is not to provide ( even if it were possible) an up to date literature review. What is important is that the fundamental information contained in this publication is sound and will remain
5
valid for some years. The updating of the material and the references will be dealt with in future revisions.
A great effort has been made to present a uniform text as regards style and terminology . Moreover, the general rule of WHO in its publications is to follow authoritative, internationally approved scientific terms and units, some of which are rather new. A short glossary provides definitions of some essential terms. However, terms in common use in related fields, such as physics, optics, radio sciences, ionizing radiation and medicine, and their definitions in other glossaries, have been excluded.
This Office has also embarked on a survey of national legislation and regu lations, and of institutions and specialists concerned with one or more sectors of the NIR field. The survey is based on replies to two questionnaires received from various governments, and on information from many experts involved in this project. The survey has revealed that only in a relatively small number of countries are there institutions dealing with NIR. In addition to the two countries that have long been involved in this area, namely the United States and the USSR, only about two dozen other countries in the world are known to have institutions that are concerned with the study of NIR, and of these about three quarters are European countries. Since the listing at present is incomplete, and the first attempt will inevitably contain errors and omissions , the results will be issued separately so that it can easily be revised and updated.
The World Health Organization is grateful to the Governments of Bulgaria, the Federal Republic of Germany, Ireland and the United Kingdom for support through their kind agreement to host working group meetings in their countries, and to the Government of The Netherlands for financial support . Also recognized with appreciation is the United Nations Environment Programme which, through its significant financial assistance from 1978 onwards, made the completion of this project and the publication of this book possible. The consistent support given to this project by Mr J .C. Villforth, Director of the Bureau of Radiological Health (BRH) of the US Public Health Service, and members of his staff, is greatly appreci ated. In its capacity as the WHO collaborating centre for standardization of non ionizing radiation , the BRH kindly hosted a review meeting on health effects of exposure to microwave radiation. My thanks go also to Dr W.H. Parr, Chief of the Physical Agents Effects Branch, National Institute for Occupational Safety and Health (NlOSH), and his staff for their work on and contribution to the chapter on IR radiation .
Some of the figures and tables in this book have been reproduced from pub lished works. In all cases this has been acknowledged by means of a suitable refer ence. The authors and WHO wish to thank the publishers for granting permission for such use to be made of copyright material.
I should like to record my personal indebtedness to all the colleagues, reviewers, and participants in the meetings who have contributed in many ways and at various stages. All of these are listed in Annex 1, and I offer my sincere apologies to any body whose name may have been unintentionally omitted. The very close collab oration with the authors of the chapters and other particularly active contributors among the working group participants, some of whom have become personal friends in the course of the work, and their endless efforts to upgrade and update the material, are warmly acknowledged.
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The friendly assistance and goodwill of various members of the Regional Office staff on different occasions is greatly appreciated. They include secretaries, draftsmen , publications and reproduction staff, registry and mailing personnel , and the administration. Though too numerous to mention individually, all have con tributed toward the successful implementation of this project. I am also grateful to Mr J . Kumpf and Mr J.I. Waddington,former Chief and present Director, respect ively, of the environmental health team in the Regional Office , for their support during the implementation and completion of this work. The understanding of Dr Leo A. Kaprio, WHO Regional Director for Europe, and the late Dr F. Bauhofer, former Director of Health Services, of the potential value of this work in promoting NIR protection in many countries was a great encouragement.
Finally , acknowledgements and thanks should go to Dr Shore for his farsighted ness and recognition of the significance and benefits of this work to international understanding and cooperation, and for his repeated encouragement and support; to Professor Michaelson for sparing no time and effort in providing special advice and essential assistance; to Dr R.B. Dean for his editorial assistance; and to Christiane S¢rensen who , as my secretary from 1974 to 1978, worked tirelessly on behalf of this project and helped with the organization of all four working groups.
A revised and updated edition of this book may be prepared in the future and readers are invited to submit comments, corrections and observations, as well as suggestions for additional material, to the World Health Organization , Regional Office for Europe, 8 Scherfigsvej, 2100 Copenhagen</), Denmark.
7
CHAPTER 1
Ultraviolet radiation M. Faber 0
CONTENTS
Page Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Physical description .. . . . . ... .... . ... .. . . . .. .. . .. .. ...... . ... I 0 Production . .. .. . ..... . .. . ...... . . . .. . ... .. . .. .... . ... . .. . I 0 Natural ultraviolet radiation .... .. ....... .. ... . .. . . . .... ... . .. . 14 Transmission and absorption in biological tissue . .. .... .. . . ....... . . .. 17 Absorption and photochemical effects . . ... . .. .. ... . . . .. . ..... . . . .. 17 Pathological effects in man ......... . .. . ..... . ..... . ........ . .. 19
Non-stochastic effects ..... ... . . ...... . .. .. . .. . . ... .... .... 19 Chemical photosensitization . ........ .. .......... .. . . ....... . 23 Late effects . . . .. ... . .. . .. . ........ . . . . . .... . ... .. ... .. . 24
Areas of risk from overexposure ....... . .... . ........ . ........ .. . 27 Dosimetry .. .. ............ . .. . ...... ... .... . ... . . . . . ... .. 29 Safety standards .. . ..... . . . .... . ........ . ... . . .. ...... . . . . . 29 Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . 3 I
Solar ultraviolet radiation ..... . . . ....... .. ... . . . . .. . .. . .. . . . 31 Industrial sources .... . ..... . .... .. ..... . ..... . .... . . . .. . . 31
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2
INTRODUCTION
Of the various types of nonionizing radiation , ultraviolet (UV) is of special interest because of its relatively high photon energy as compared to the other types included in this group. This could lead to greater variation in biological response. On the other hand , the low penetration will restrict most of the direct biological responses to the superficial tissues.
Although man-made UV radiation can arise from a large number of sources, it is the sun which is the main source and both the general public and people working out of doors will be exposed to it . This natural background radiation and the variations in its magnitude must be taken into account when exposure limits are discussed.
0 Professor and Director, The Finsen Laboratory, The Finsen Institute, Copenhagen , Denmark .
9
It is well known that UV can initiate photochemical reactions and that some of these take place in the skin. The best known is the production of vitamin D3,
which is necessary for the prevention of rickets in man. The full extent to which UV affects human well-being is difficult to quantify. Artificially produced UV has, however, been used in mines and cellars as a supplement to combat functional impairment among the workers (26, 27) . Many of the observed effects, such as a decrease in the incidence of infectious diseases and in absenteeism, may be due to the bactericidal nature of the radiation (9 I) . On the other hand, large doses of UV have an acute destructive effect on the skin and eye . Doses so low that they give rise only to normally acceptable acute symptoms can, if repeated, induce changes resulting in late effects such as elastosis of the skin , keratosis and the various skin cancers. These effects will be of greatest significance in people with lightly pig mented skin .
Our goal in protecting the population against the harmful effects of UV is to establish the most appropriate exposure limits based on a biological risk- benefit analysis of all these factors in as quantitative a fashion as possible ( 77).
PHYSICAL DESCRIPTION
Ultraviolet radiation is that part of the electromagnetic spectrum lying between the softest ionizing radiation on the one side and visible radiation on the other. For biological purposes, it is convenient to regard the range of wavelengths from 100 to 380- 400 nm as constituting UV. The lower limit of 100 nm is equivalent to photon energies of 12.4 eV, which corresponds approximately to the limit for the produc tion of ionization in biologically important materials. At the other end, the limit is the shortest visible wavelength; this varies slightly from individual to individual, but lies between 380 and 400 nm.
Because of differences in physical properties and in biological effects, the UV region has been subdivided. Wavelengths shorter than approximately 190 nm con stitute the region of vacuum UV. This radiation is absorbed by air to such an extent that no biological effects would be expected , unless very powerful sources are used . The remainder can then be divided into the far-UV region between 190 and 300 nm and the near-UV region between 300 and 400 nm.
A somewhat different way of dividing up the UV region takes some of the biological effects into account. In this arrangement the range 400- 315 nm, the so-called "black light" region, is called UV-A. In this wavelength region , fluorescence can be induced in many substances. UV-B covers the range 315- 280 nm (the skin erythemal region). Most of the biologically active and potentially harmful UV from the sun reaching the surface of the earth comes within this spectral region. UV-C includes the radiation of wavelength less than 280 nm (the germicidal region). These divisions are, however, arbitrary and usage varies from one worker to another.
PRODUCTION
Matter at a temperature of 2 500 K or higher may emit a significant number of photons with energies inside the UV range . Such incandescent sources emit a
smooth spectrum, a continuum, possibly with superimposed lines. Most man-made sources of UV radiation can be grouped together in the cat
egories shown below:
Gas discharges Mercury lamps (low-, medium- and high-pressure). Mercury lamps with metal halides Noble gas lamps Flash tubes Hydrogen and deuterium lamps Welding arcs
Incandescent sources Tungsten halogen lamps
Fluorescent lamps Fluorescent tubes Fluorescent sun UV emitters Black-light UV emitters
Mixed sources Carbon arc
The spectrum of the UV emitted varies from one source to another. In the case of low-pressure mercury lamps, a line spectrum will be emitted. These lines are broadened into bands in high-pressure lamps ( often called medium-pressure lamps by photobiologists) and there may also be emission of a continuum over a wide range of wavelengths. This continuum is most strongly marked in the highest pressure lamps. Addition of metal halides will increase both the continuum and the number of superimposed lines in the spectrum. In high-pressure xenon or hydrogen lamps there is also a combination of disinct bands with a continuum (93) (see Fig. I and 2).
The arc produced during welding will depend not so much on the atmosphere in which the welding takes place but rather on the composition of the electrodes. An example of an emission spectrum produced by a welding process is shown in Fig. 3.
The spectrum of fluorescent lamps depends on the properties of the fluorescent phosphors employed in the envelope. The small UV part of the spectrum of day light fluorescent lamps stems, however, from the mercury discharge inside the lamp. The amount of UV depends to some extent on the glass used in the fluor escent tube.
In the case of lasers, the line spectrum will depend on the active medium and on the operating conditions.
To permit the transmission of U¥ when the discharge does not take place in free air, gas discharge arcs and other UV sources must be contained within an en velope of quartz or UV-transmitting glass. On the other hand, sources that are designed primarily to emit visible radiation but which also produce significant but unwanted amounts of UV should be provided with an external screen of glass that does not permit the passage of UV-Band UV-C radiation.
11
12
1,0 --- --------------------
0 ,8
t ·.; 0,6 C:
e C:
~ 0 ,4 ·;, ., .; a:
0 ,2
Hg low pressure lamp (germ icidal lamp) pHg "' 1 Pa
• I]__ D __n_.lj_ 300 400 500 600 700
Wavelength (nm)
1,0-----------------------
0 ,8
t ·.; 0 ,6 C:
e C: ., > 0 ,4 ·;, .. .; a:
0 ,2
300
Hg high pressure lamp (spectral lamp)
PHg "" 10 6
Pa
400 500 600 700 Wavelength (nm)
1 ,0 -----------------------
0,8
t -~ 0,6 i C: ., -~ 0,4 .. .; a:
0 ,2
300
I/ L.r
400
Hg highest pressure lamp (short arc) pHg > 106 Pa
,._ 500 600 700
Wavelength (nm)
Fig. 1. Emission spectra of mercury lamps (courtesy K. Bischoff)
20
N
E u 15 i :t ., u C
=ij 10
~
5
~ .; C ., .... C ., >
·;::; CD
~ a:
1 .o Hg hi gh pressure lamp with metal
0,8 halides
0 ,6
0 .4
0 ,2
300 400 500 600 700
Wavelengt h (n m)
Fig. 2. Emission spectrum from mercury lamp with halides (courtesy K. Bischoff)
Process : GMAW Arc c u r ren t : 150 A E lectrode : LINDE 8 5 Shielding gas : CO , at 0 .0 2 m 3 / s
200 300 400 500 600 700 800
W ave length (nm)
Fig. 3. Emission spectrum from gas tungsten arc welding (courtesy D. Sliney)
13
.,
NATURAL ULTRAVIOLET RADIATION
The sun is the most important UV source. The broad spectrum and the intensity of the UV from the sun are due to the high temperature at its surface and its size. The intensity is such that the UV radiation reaching the earth's atmosphere would probably be lethal to most living organisms on the surface . Fortunately, they are shielded by the atmosphere. The ozone layer in the upper atmosphere is particularly important in this connexion. The spectrum, both before passage through the atmosphere and at sea level, is shown in Fig. 4. The path length traversed in the
.~ C:
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24
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0 UV Visible
100 300 600 900
~ 3 C:
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Terrestrial /",
/a:', I_, , ~ \
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300 700 3000 10 000 30,000
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1200 1800
Wavelength (nm)
2400 3000
Fig. 4. The sun's spectrum at the outer surface of the atmosphere and at sea level ( 122)
12 000~-------------~-----------------,
11 000
10000
9 000
C 8 000 :, g 7000
> 6 000 ::> >,. 5000
d 4000
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2000
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14
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Fig. 5. Daily total UV count for 1974, Minneapolis, USA (94)
atmosphere by the UV determines the intensity of the radiation at the surface of the earth, but it is also affected by geographical latitude , altitude above sea level and time of year. Scattering and absorption by dust , smoke and rain are also important. It should be noted that practically no UV from the sun with a wavelength below 290 nm reaches the surface of the earth. The changes over the year in one location, as measured by the Robertson-Berger apparatus, are shown in Fig. 5 (94); it will be seen that large day-to-day changes occur. When the yearly changes for discrete wavelengths (Fig. 6) are given as monthly averages, it can be seen that such
1200
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I II 11 1 I V V VI V II V III IX X X I X II
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Fig. 6. Monthly average UV intensity in Sofia, Bulgaria and in adjacent open country
changes are not the same for all wavelengths. A comparison between the curves ob tained in open country and in an adjacent city (Sofia) show the effect of polluted air in reducing the UV in the city (Fig. 6) . Again, there is a difference between the wavelengths, the shortest showing the greatest reduction. Dependence on latitude is shown in Fig. 7 (94).
If the ozone layer were to decrease in thickness, a decrease in the absorption in the critical UV bands and a considerable increase in the photo-effects from UV would be expected (52).
The ozone is produced photochemically by UV from the sun at wavelengths largely below 242 nm. It is present in a concentration which is the outcome of a dynamic equilibrium between production and spontaneous breakdown; there are both daily and seasonal variations in this concentration. The rate of production is 4000t/s.
Volatile substances that are inert in the atmosphere can reach the ozone layer from the earth or as a result of other human activities, and are decomposed there by photochemical reactions. lnterest has been concentrated on halogen containing fluorocarbons, of which 1 million tons were manufactured in 1975, since photochemically produced chlorine or free radicals can attack the ozone and result in a shift of equilibrium to lower values (92). Nitrogen oxides produced by internal combustion engines and by bacteria have also been mentioned in this connexion.
15
3 _5,---------------------------,
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• Maune Loa
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o ~--~1 __ ..__1 __ ..__1 __ ..__1 __ ..__l __ ..__1 _ __, 15 20 25 30 35 40 45 50
Degrees north latitude
Fig. 7. Annual erythemal UV count by latitude (94)
The final result, when equilibrium has again been reached after a number of years, will, it is suggested, be a decrease of the order of 10- 15%. This will affect the UV passing through the ozone layer and increase the UV-B level at the surface of the earth; it will also shift the spectrum towards the lower wavelengths, with possible adverse consequences for all forms of life.
The consequences for man are, of course, difficult to evaluate. It has, however, been suggested on the basis of data for the USA on skin cancer in relation to UV exposure, that a 1 % reduction in the average thickness of the ozone layer could result in a 2% (range 0.7- 5%) increase in non-melanoma skin cancer in the USA (52, 96).
A recent study by the National Academy of Sciences of the USA (121) suggests that the potential for damage to the stratospheric ozone layer by chlorofluoro carbon emissions may be much greater than previously suspected. The study revealed that if emissions continue at the present rate, ozone levels could be reduced by more than 16% by the middle of the next century; this in turn could increase the worldwide incidence of skin cancer by as much as 65%. If emissions rise by 7% between now and the year 2000, more than half the ozone in the stratosphere could eventually be eliminated.
16
TRANSMISSION AND ABSORPTION IN BIOLOGICAL TISSUE
Penetration of the human body by shorter-wavelength UV is restricted to the epidermis. Penetration is somewhat deeper at longer wavelengths and in non pigmented subjects, where there is some penetration into the dermis, especially at wavelengths greater than 300nm (115) (see Fig. 8).
The same is true for the eye. Most of the UV will be absorbed by the cornea. The lens and the tissues in the anterior part of the eye may, however, be exposed to UV at wavelengths above 295 nm. The final absorption takes place in the lens, and the retina can be exposed only under special circumstances. The penetration of different wavelengths into the eye is given in Table 1. Some doubt exists, however as to the high transmission given for the vitreous humour at the longer wavelengths (7).
Stratum corneum
Wavelength (nm)
_ 3~1:'!' _ Rete --t---'la--9---,.,!A..AI---II'"""""" Malpighii
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Fig. 8. Penetration of UV radiation through skin. Drawn from 108 (courtesy A. Wiskemann)
ABSORPTION AND PHOTOCHEMICAL EFFECTS
.. I a, "O ci w
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The absorption of UV depends on the wavelength. The absorption spectrum describes this relationship (71) .
Proteins and nucleic acids are the most important biological absorbers. Nucleic acids have their main absorption peak close to 265 nm , due to the pyrimidine structure. The aromatic amino-acids are the absorbing sites in protein , with tyrosine at 275 nm and tryptophane at 280 nm.
The action spectrum gives the relative responses of a system to irradiation at different wavelengths. In simple physical systems , the action spectrum will be the
17
Table 1. Percentage of energy incident on the corneal epithelium that impinges on the anterior surface of the various ocular media (modified from 57)
Wavelength Corneal Aqueous Lens Vitreous Retina (nm) stroma humour humour
230 3 235 11 240 19 245 26 250 26 260 26 265 27 270 29 275 31 280 33 285 42 290 52 2 0.4 295 63 9 3 300 70 27 14 305 75 50 37 310 78 64 51 320 81 78 74 0.3 0 .3 330 84 80 77 0.5 0 .5 350 87 86 83 2 2 360 89 88 85 4 4 370 90 90 87 12 11 380 93 91 88 28 26 390 94 93 91 49 45 400 95 94 93 69 64 450 96 96 84 81 500 96 96 87 84
same as the absorption spectrum; this is only partly true in biological systems, however, since energy transfer, shielding and other factors ( 65) not fully understood may complicate the result and may even lead to effects at non-absorbing wave lengths. For a discussion of these effects in photokeratitis, see Kurzel et al. (61). Proteins combined with DNA, e.g., histones, may be photochemically bound to DNA to form DNA- protein cross-links. The DNA appears to be the chromophore most often responsible for UV absorption (98, 100).
After absorption, a number of photochemical reactions occur, of which those taking place in the nucleic acids are of the greatest biological importance. The num ber of possible photoproducts is large. DNA interstrand cross-links and DNA strand breaks are produced in vitro. Hydrates of cytosine and uracil may be produced, but have not been shown to produce biological effects after direct absorption of UV energy. Of greatest interest, at least in the UV-8 region, is the photoproduction of covalent dimers of thymidine and other pyrimidines (7). The biological effects of these lesions were studied by Wacker et al. (J 12) in irradiated bacteria.
Defects in DNA are repaired in living cells principally by excision repair (dark repair) , a process in which it has been possible to isolate at least five participating enzymes. They act by excision of thymidine dimers and dimers of other pyrimidines and restitution of the intact DNA strand. The primary biochemical mechanism has
18
been studied both in bacteria and in mammalian and human cells and is now reasonably well understood (31). Defects in the repair system are present in a num ber of rare diseases, of which xeroderma pigmentosum is the most important in relation to UV damage. Evidence has been presented to show that the genetic con stitution, as measured by the ability of lymphocytes to repair UV lesions, may be of importance in relation to the development of UV-induced diseases (62). When repair is in progress, it is easy to recognize in autoradiograms as unscheduled DNA synthesis after uptake of tritiated thymidine. The unscheduled synthesis disappears gradually during the 24 hours following the irradiation. The repair system is limited in its efficiency. In He La cells, the initial rate of repair increases linearly after doses of up to 30 J/m2
. At higher doses, no further increase in the rate of thymidine uptake is seen (31) . The incident dose needed for the induction of a given number of dimers is wavelength-dependent ; at wavelengths of 254, 290 and 310 nm, the doses are in the ratio 1 :30:6000. In relation to the number of induced dimers, both the lethality and the mutagenicity are greatest at longer wavelengths. Other types of DNA lesion apparently become of increasing and ultimately of dominant importance here (69) . •
In addition, post-replication repair is active in mammalian cells and may be a more error-prone step. This repair system can also be deficient in xeroderma pig mentosum.
Protein- DNA cross-links can also be repaired . The mechanism is not yet under stood (60) .
Photoreactivation is another independent repair system where exposure at longer wavelengths, whether UV or visible, can induce enzymatic repair. The enzyme active in this system has so far not been identified in mammalian cells (J 06). Full agreement has, however, not yet been reached as to the significance of these results. It has also been claimed that the enzyme is absent in xeroderma pigmentosum cells (J 05).
Photochemical reactions induced by UV that have biological consequences are, however, not restricted to cellular constituents alone. Photo-oxidation of atmos pheric constituents at 316 nm may, for example, indirectly influence human health (23).
PA THO LOGICAL EFFECTS IN MAN
For protection purposes these effects can be divided into two main groups , namely non-stochastic and stochastic (51). The non-stochastic effects are related directly to the radiant exposure and are generally acute , while the stochastic effects take the form of an increase in the risk of contracting certain diseases of which the late appearance of cancer is the most important.
Non-stochastic effects
In this group of effects , the severity of the effects in the exposed individual varies with dose and there may be a threshold below which no effects occur (51). These effects may be either acute or late and can occur in any cell or tissue that can be reached by radiation .
19
Couum
Subc u11 s
Hydrohp,d mantle
S1ra1um corneum
Stratum luc1dum
S1ra1um granulosum
S1ra1um spmosum
S1ra1um basale
Papilla and capillary space
Sebaceous gland
Hau
Collagenous fibre
Arrec1or p1h muscle
Swea1 gland
Hai r fo lli cle
Fatly 11ssue
- ~-. -~- :j,, ~ ' • -~< : .,- .. : .. :~ •
• -~
,• ·)~ I . . . . . ..
I . ' . .. . . . . . • ,/
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Fig. 9. Cross section of human skin ( 7 23)
Because of its limited penetration , the effects of UV in man will essentially be restricted to the skin and eyes; only under special circumstances may such effects also extend to the oral cavity.
Skin Four types of immediate changes occur in the skin: (a) darkening of pre
existing melanin pigment; (b) the production of erythema (sunburn); (c) the upward migration and production of melanin granules (suntanning); and (d) changes in epidermal cell growth. The structure of the skin is shown in Fig. 9.
20
The darkening of existing pigment takes place immediately after irradiation . This reaction has a wavelength maximum near 360 nm , but can be induced at wave lengths above 400 nm. It is probably the result of an oxidation of existing premelanin and may be the visible result of the free radical scavenger effect of melanin (37, 79). It is most strongly marked in pigmented skin.
The vascular reaction of the skin to UV known as erythema comprises vaso dilatation, augmented blood flow , and increased vascular permeability leading to cellular exudation, e.g., neutrophil leukocytes.
The erythema is made up of two components which occur in succession (22, 67, 78). It is initiated by an immediate erythema of short duration (I - 2 hours), which is produced mainly by UV at wavelengths above 300 nm. After a latency period of 2- IO hours the more important late erythema, which lasts for one or more days, appears. The duration of the latency period varies inversely with the dose. The intensity of the erythema increases with increasing radiation dose , and at higher doses is complicated by increased vascular permeability, as is shown by oedema and blistering. The mechanism underlying the increased vascular permeability is unknown. The involvement of histamine has been suggested (67), but the immediate reaction alone has been found to respond to antihistamines in certain animals. In the rat however, 5-hydroxytryptarnine appears to be the mediator. The mediator involved in the late response is unknown ( 68).
The minimal dose that will provoke an erythema (the minimal erythema dose, MED) is the best defined threshold dose for acute effects following UV exposure in man , but only if a consistent measurement technique is used. The MED depends on absorption and scattering in the skin as affected by age , pigmentation, previous UV exposure, thickness of skin and location of area irradiated. In lightly pigmented Europeans , the MED for the untanned skin of the trunk is approximately 100 J /m 2
for wavelengths between 240 and 290 nm (J 11). At the lowest solar wavelengths of 300nm, the MED is still approximately 1S0J/m2 but rises sharply at higher wave lengths (72) .
The third reaction of the skin to UV exposure is an increase in pigmentation or "suntan", probably with an action spectrum close to that of erythema. It is initiated by a spreading of existing pigment granules into neighbouring cells throughout the exposed skin. Production of new pigment granules occurs later in the process.
UV radiation interferes with cell growth in skin . Immediately after irradiation there is a cessation of cell growth (16, 34) for 24 hours or longer, depending on dose, followed by an increase in mitoses (J 1). This increase reaches a maximum at 72 hours as compared to a maximum at 42- 48 hours when normal regulation is interrupted by stripping of the superficial layers of the skin (24). The increase in cell production after a single dose of only short duration gives rise to hyperplasia of the epidermis with a maximum at 5- 6 days (13), often observed as a shedding of superfluous cellular material (scaling). During more continuous exposure, the epidermis may thicken. This thickening will reduce the sensitivity to subsequent exposures to UV.
The sunburn cells constitute a special histological feature; these are cells characterized by dense nuclei lying in isolation within the middle layers of the epidermis. These severely damaged cells may be dead and will be removed, but little direct information on them is available. The wavelengths between 260 and 290 nm
21
appear to be of equal importance and photosensitizers, such as 8-methoxypsoralen, will increase the yield (116).
In certain countries in recent years there has been a great increase in the use of UV-A fluorescent sources to produce rapid skin pigmentation. The high intensity of some of these lamps, the presence of a small amount of UV-B, and an imperfect knowledge of the effect of high-dose UV-A on the skin and eye has resulted in widespread concern as to whether this type of cosmetic UV exposure is advisable. At the very least, the apparatus should be subject to stringent regulations; the risk to photosensitive individuals must also be ta.ken into account.
After single very large doses, the outcome of UV irradiation in animals such as mice is ulceration and scar formation, but this occurs in man only if there is a secondary bacterial infection.
Mouth As a consequence of recent dental practices, where plastic materials are
hardened ( or cured) by UV treatment, the mucous membrane of the mouth may now be exposed to unwanted UV irradiation when defective apparatus is used (34). To date severe erythema of the skin around the mouth has alone been observed under such conditions.
Eye The structure of the eye is shown in Fig. 10. The main clinical effects of UV
on the eye are photokeratitis and conjunctivitis, which appear 2- 24 hours after irradiation. The symptoms are acute hyperaemia, photophobia, and blepharospasm, which last from 1 to 5 days. In general, there is no residual lesion. Photokeratitis is caused preferentially by UV-B but also by UV-A, though with decreasing sensi tivity. The peak sensitivity of the cornea is given as 270 nm (86) or 288 nm (20). The effect is dose-related, which means that tissue damage depends on the total
22
AQUEOUS HU
LENS
MACU
-=--=-==-:.-=--=.------ --- CORNEA FOV
VITREOUS BODY
Fig. 10. Cross section of the human eye
OPTIC NERVE
energy absorbed, not on the rate of absorption. Accordingly, the effect also depends on the duration of exposure. Fairly accurate measurements have been made of the energy necessary for minimal photokeratitis. The threshold has a minimum at 270nm of 50J/m2 (87) rising to 550J/m2 at 310nm, and followed by a steep in crease to 22 500J/m2 at 315 nm.
Irreversible damage to the rabbit cornea requires twice the radiant exposure necessary for the minimal effect. Anterior uveitis was not found with exposures at 315- 325 nm until about four times this dose was achieved. Studies on primate eyes tend to indicate a slightly lower threshold (87).
The problem of acute or lasting damage to deeper-lying ocular structures has become of increasing importance with the availability of strong UV sources, includ ing lasers. The effects will depend on the transmittance of UV through the ocular media, figures for which are given in Table 1, which shows that an effect on the lens is possible at wavelengths longer than 290 nm. The action spectrum for damage to the lens lies between 295 and 320nm (61). Animal experiments show that both a single, high radiant exposure and exposures of long duration but of low irradiance induce opacities in the lens. Some epidemiological studies on man suggest that sun light, especially UV-A, could also be active in producing cataract (85, 119). One particular type of cataract may be due to photochemical processes in the lens in volving tryptophane, resulting in the formation of a brown pigment (brunescent cataract). A discussion of the possible biochemical mechanism has been presented by Kurzel et al. ( 61) and Zigman (J 19). The most efficient wavelength for the production of transient lenticular opacities was 300 nm, but only when the ex posure exceeded a threshold value of 1500J/m2 (87). The damage became per manent at radiant exposure levels of approximately twice this figure.
The problem of retinal damage is still unsolved, but should be amenable to solution for the near-UV range and short-wave visible light, in the latter case es pecially after exposures of short duration. It is known that UV can produce transient red vision (erythropsia) (55). This effect, however, is common only in the aphakic eye, where there is no protection from the lens. Recent studies have shown that exposure of the eyes of experimental animals at 350- 365 nm has resulted in damage to the basal parts of the blue sensory cells of the retina (118, 120).
Chemical photosensitization
Photosensitization is both an area of great interest and one of importance in industrial hygiene. It manifests itself in a number of ways, involving somewhat different mechanisms.
The photodynarnic effects of organic substances all appear to be based on the same principle and are associated with the UV-A and its extension into the visible light region. The energy is first absorbed by the photosensitizer, a relatively low molecular-weight substance present in the cells, e.g., a fluorescent dye (acridine orange, riboflavines). lt can then be tra.Q.sferred to a target molecule, oxygen serving as an intermediate. The effect of this transfer is most serious when the target is DNA in the nucleus or in a virus, when chain breaks and cross-linking may result (99).
Other substances, such as the naturally occurring psoralens, will be bound to DNA after irradiation and will react with thymine and cytosine. After irradiation at a wavelength of 365 nm, they form covalent photo-adducts, resulting in inter-
23
strand linking. This results in a high degree of DNA damage. From a clinical point of view, photosensitivity is a general term describing the
combined action of UV and a chemical substance ; this can lead to either photo toxic or photo-allergic reactions. Phototoxicity is the common response in all those whose skin is irradiated with sufficient energy and at the appropriate wavelength in the presence of a phototoxic substance. Although the result may only be the common sunburn reaction, it will be more marked with the photosensitizer. Photo allergy is less common and is thought to depend on an acquired altered reactivity due to an antigen- antibody or cell-mediated hypersensitivity to the photosensitizer. Clinically immediate urticaria! or delayed papular or eczematous reactions may appear. Photo-allergy may result in an increased sensitivity to light even without the sensitizer. The incident exposure may be relatively small. The UV of wavelengths 300- 320 nm present in white fluorescent lamps (58) may suffice.
Late effects
Late non-stochastic effects These occur in the skin and in the eye. Skin. After prolonged exposure to sunlight over a period of years, the dermis
will begin to degenerate , with a decrease in elasticity due to degeneration of the collagen fibres combined with other histological changes (25, 80). The visible symp toms will be deep furrows in the skin giving an appearance of premature ageing. No dose- effect relationship is known for this reaction in man.
The epidermis may also be involved, with the development of actinic keratosis. The importance of this lesion is difficult to evaluate but the occurrence of an in creased cellular proliferation rate (114) and a certain amount of cellular atypia (50) suggests that it may represent a precancerous stage in the development of squamous cell carcinoma. Many of these carcinomas are surrounded by areas of actinic keratosis (50). It has been suggested that actinic keratosis may be a universal dis order of epithelial growth ( 84).
Eye. As mentioned earlier, some of the types of cataract seen in elderly people may be due to repeated exposure to UV, and especially UV-A, over many years. The quantatitive dose- effect relationship is unknown.
Late stochastic effects Late stochastic effects are those for which the probability of an effect occurring,
rather than its severity, is regarded as a function of dose. A threshold cannot be expected in lesions of this type (51) . The typical lesion following UV exposure is a malignant tumour. There is no reason to expect genetic damage since UV cannot reach the gonads but is absorbed in the overlying tissues.
Skin. Cancer of the skin is a well-recognized effect of UV irradiation, both in experimental animals and in man.
Three types of cancer are concerned, namely, basal-cell carcinoma, squamous cell carcinoma and malignant melanoma. The evidence for the production of cancer by UV is good for all types of malignancies (J 2, 104). It has been difficult to induce melanomas by UV alone, but this has now been described in hairless mice (49).
That the wavelengths suitable for tumour induction are below 320 nm was first found for animals by Roffo (89, 90) and by Funding et al . (44). The peak in the
24
-------------
action spectrum appears to be at 280-320 nm ( 42) and the important shorter wave lengths from the sun, i.e., 295 - 320 nm, are thus active if the dose is high enough (I 10). Irradiation is, in general, effective only when the dose is spread over an ex tended period (9, JO, 114), and the carcinogenic effect will depend on the number of doses and the duration of the irradiation. The radiation dose for continuous irradiation until tumours appear will contain a certain amount of wasted irradiation. If the radiation is at an appropriate time, the tumours will appear after that time, even without further irradiation (45). At higher doses, hyperplasia of the epidermis will develop during the irradiation due to an increase in the thickness of the stratum corneum. At low doses, the tumour may appear on nearly normal skin (14). There is thus no simple correlation between hyperplasia and tumour formation in the experimental animal (103).
Photosensitization can affect carcinogenesis, but binding of the sensitizer to DNA may then be necessary. Thus irradiation at wavelengths between 300 and 400 nm after either local or systemic application of 8-methoxypsoralen can give rise to skin tumours in experimental animals (47). The development of skin cancer will also, apart from photosensitizers, be affected by other carcinogens (28), immunosuppressants, etc. (26), as with other experimental cancers.
In man, the evidence for UV induction of skin cancer is both clinical and epidemiological. Basal-cell and squamous-cell carcinomas appear preferentially on the uncovered skin of lightly pigmented individuals. The incidence of such carci nomas, furthermore, depends on the cumulative exposure. It should be possible to document this as an inverse relationship between incidence and geographical latitude . The strongest evidence should be obtained for melanomas {Fig. 11) (32, 33, 63, 64, 66, 73, 74, 75), where the information is more reliable than is the case with other skin tumours, as the disease is more severe, requires immediate treatment and will probably be better reported to cancer registries . Furthermore , the shorter latency period will make epidemiological studies more realistic. The latitude correlations are, however, significant only in reasonably homogeneous populations. A different population will show a completely different picture, a fact which demonstrates the importance of other factors in the dose- effect relationship.
It has not been possible to establish quantitative dose- effect relationships in man since the number of biological parameters affecting the outcome is large. Any increase in pigmentation of the skin will result in a decrease in incidence. It has been observed in the indigenous population of Uganda that unpigmented scars are the preferential areas for UV-induced skin tumours (29). Even if the pigmentation is light, factors such as an increase in skin temperature, which could be produced by infrared irradiation (41, 83), will enhance the carcinogenic action of UV. Apart from pigmentation, the part of the body irradiated will also be of importance, probably as a result of differences in skin thickness.
The latency period in relation to the radiation dose is also of signficance. Epidemiological evidence from Denmark supports a latency period of 10- 15 years for malignant melanoma (J 8, 19, 35). It is difficult, however, to evaluate the latency period for other skin tumours that usually occur later in life, may be preceded by long-lasting epidermal precancerous abnormalities, e.g., actinic keratosis, and be produced by repeated episodes of radiant exposure. Indeed, the appropriate groups for such a study have not even been defined as yet.
Diseases such as certain types of inherited albinism and xeroderma pigmen-
25
26
10
:. 9 Cl)
> ~ 8 a. C: • Ft . Worth ~ 7 ., ., ::, a.
6 0 a. 0 0 0 5 0 0
~ Cl)
a. e 4
E "O
~ ::,
u 3 .. ., Cl <(
2.5 30 35 40 45
Degrees north latitude
10 ~
9 .. Cl)
> ~ 8 • Ft.Worth a. C: 0 7 -;:; !! ::, a. 0 6 a. Oakland 0 . •Colorado 0 0 0 5 0
~ Cl)
a. e 4 E "O
~ ::,
u Pittsburgh• .. 3 ., Cl <(
2.5 30 35 40 45
D egrees north latitude
Fig. 11. Annual age-adjusted skin melanoma incidence by latitude for white females (top figure) and white males in the United States in 1970
tosum (56) result in an increase in the incidence of actinic keratosis and skin tumours, probably as a result of defects in different types of repair mechanism.
While an increase in cellular activity may often represent a precancerous state, this is apparently not the case in psoriasis, where there is a generalized hyperactive epidermis. The incidence of actinic keratosis in this disease is low (59), and this is the case whether untreated areas of the skin (F . Urbach, personal communication, 1978) or the whole of the skin, including treated areas , is taken into consideration (53). It is generally assumed that incomplete or erroneous repair of DNA defects is important in the development of malignant skin tumours after UV irradiation. Studies showing a decrease in the incidence of skin cancers in mice when excision repair was inhibited by caffeine (I 17) suggest that complete repair together with a large number of surviving cells is an important factor in carcinogenesis, an observa tion which may explain why repeated smaller exposures are more effective in the induction of cancers in animal experiments than single large doses. The theoretically small sites suggested by Blum (I 1) as the sensitive loci for carcinogenesis are in harmony with the concept that unusual unrepaired lesions may constitute the primary defect.
Whether dermatological phototherapy with 8-methoxypsoralen will result in an increase in malignant tumours in man is not yet known, but this has been shown to be the case in animal experiments (45, 48). The wavelength dependency is interest ing (46) ; there was no increase in the incidence of tumours in methoxylen-treated animals at the normal carcinogenic wavelength of 254 nm but a marked increase after exposure to UV-A ( 45). A similar problem may exist in relation to other photo toxic drugs or other chemical substances, whether the UV exposure takes place under medical surveillance, or in an industrial environment during the production or handling of such substances. Work with photosensitizers or skin irritants in the open air, e.g., in the case of asphalt workers, appears to result in an increase in risk. Animal experiments have suggested that fluorescent whitening agents may increase the risk of skin cancer (8, 36). This conclusion could not, however, be confirmed in subsequent experiments (38, 39).
Eye. So far no direct evidence has been presented that tumours in the anterior chamber of the eye, and especially melanomas at this site, can be produced by UV irradiation. It should be noted that melanoma of the eye is commonest in blue eyed individuals, and that melanoma of the iris occurred only in blue eyes in a series of ocular melanomas (54). Tumours of the cornea, both fibrosarcomas and haemangioendotheliomas, have been induced in experimental animals (40) and tumours, including melanomas, are known to occur in domestic animals and in man in tropical regions (29).
AREAS OF RISK FROM OVEREXPOSURE
The risk of damage following either acute or chronic exposure to UV is en countered in a number of situations.
Natural UV is the most important source of such exposures. All those who work out of doors are potentially at risk from overexposure , the consequences of which may be both acute and long-term effects. The fashion of exposing a large part of the body to sunlight has during recent years increased the exposure of the
27
skin, resulting in quite high UV doses . This is true not only for out-door work but is now also normal during leisure periods, as exemplified by the holiday exodus of a large part of the population of the northern European countries to the coast of the Mediterranean.
UV-emitting arcs are an integral part of the working conditions at a number of workplaces. In welding, such arcs constitute a serious risk. Not only UV, but also visible and infrared radiation as line spectra with a continuous component are emitted during welding. The shape of such spectra will be different for the different welding procedures (107).
The use of UV in some graphic reproduction techniques also represents an ex posure risk for the workers concerned.
UV, and especially UV-C , is used for the sterilization of food and air, and pathological effects due to accidental exposures, often as small doses but of long duration , may result.
A number of sources available to the general population are known to emit either as a normal part of the emission or after accidental breakdown. Thus the normal white-light fluorescent tubes may emit a small amount of UV. This may be enough to induce a phototoxic reaction if a photosensitizer is present. In addition, UV sources have for many years been available for home use , partly for the semi cosmetic purpose of suntanning. This may result in overexposure if the instructions on the equipment are not followed . The spectrum of fluorescent sunlight tubes is only rarely restricted to the sun spectrum, and UV-B and UV-C components may be present.
The use of black-light lamps to control the effectiveness of teeth brushing will, in general, not constitute a hazard as the sources used are weak and the exposure short. The same is true where fluorescent black-light tubes are used, for instance, for crack detection , chromatography, philately, mineral identification, document inspection, etc.
Accidental short-term exposure with resultant symptoms has, however, occurred after breakage of the protective shield around high-intensity UV lamps (109).
Medical irradiation in the form of phototherapy is at present expanding. In the phototherapy of infants with neonatal jaundice (hyperbilirubinaemia), incorrectly selected fluorescent tubes have given rise to erythema. The use of certain photo dynamic dyes and of light for the treatment of herpes, or of psoralens and UV-A for other skin diseases, such as psoriasis (PUV A), will expose the patient to a number of risks, but there is so far only experimental evidence as to their nature . Up to the present, the action spectrum of erythema is the only effect that has been well studied quantitatively (80). Neither the action spectrum of the effect on psoriasis nor of the potential carcinogenic spectrum is known (17). If adequate safety precautions are not taken, the treatment personnel may also be at risk.
UV lasers are considered in detail in Chapter 2. In the evaluation of overexposure, it must not be forgotten that all those factors
tending to reduce the penetration of the atmosphere by UV from the sun can reduce the irradiance to man, resulting in an exposure close to or below that necess ary for the maintenance of life. Some of the above-mentioned man-made sources may then have to be used under controlled conditions as a supplement. In any given situation , therefore, it is necessary to balance the advantages obtained against the risks incurred.
28
DOSIMETRY
From the point of view of establishing standards, it is desirable not only to be able to make measurements of emitted dose but also to be able to record , by means of some form of personal dosimetry , the doses received by the persons to be pro tected. It must, however, be appreciated that the dose absorbed by the sensitive cells may be difficult to estimate. Most of the equipment available for dose measure ments in relation to protection is cumbersome and delicate and not too well adapted for field use. In general , a phototube (photomultiplier) or photodiode detector is used. When it is necessary to determine the spectral distribution , the different wavelengths can be separated, e.g., by a diffraction grating monochromator or by filters, and the transmitting optics must be made of quartz so as to allow all the UV to pass . The quality of broad-band measurements can be good but consider able errors may be introduced when a full spectral description is required. One such error is the lack of precision in narrow wavelength bands when filters are used.
Detailed information on the natural erythema-producing irradiance from the sun is of interest in connexion with dosimetry. To obtain such information, an analogue integrating dosimeter (Robertson-Berger) has been constructed (6) and yearly curves of the effectiveness of the sun in producing erythema at various places on the earth are now being published (94) .
A broad-band filter apparatus , having 270 nm as the most sensitive wavelength and essentially no response above 325 nm , has been developed as a hazard moni tor (88) . It corresponds to the standard guides mentioned below, and should be well suited for monitoring workplaces .
Although portable, none of these instruments can be used for permanent personal dosimetry , whether for experimental or , in special cases, protective pur poses. It is therefore of interest that work is now in progress on the development of thermoluminescent dosimeters sensitive in the UV range (2). This work has only just been started. Another type of portable broad-band dosimeter, using the darkening of a plastic film, has been used with some success (21, 30), but is still in the experi mental stage. Before it can be used in the protection of workers, the correct method of use must be defined, e.g., the way in which the shielding effect of the body resulting even from small movements is to be taken into account.
SAFETY ST AND ARDS
Any safety standards developed must take into consideration not only the harmful effects of UV radiation but also the need for a certain minimal irradiation, so as to ensure that sufficient vitamin D3 is produced; this is of greatest importance during infancy and childhood.
Maximal vitamin D production in man during irradiation at wavelengths below 366 nm has been found (3, 4, 5) to be of the order of 2- 18 IU/cm2 after a UV dose of 4- 6 J, but the values vary widely for different parts of the skin and there is a lower yield in pigmented skin.
The daily requirement is not easy to define but an additional supply of 100- 250 IU, at least in lightly pigmented children, will suffice to prevent rickets (95).
If it is assumed that all the UV radiation recorded by the Robertson-Berger
29
apparatus is equally active in producing vitamin D3 , it can be calculated that the daily requirement is only of the order of a small percentage of the MED delivered to the face alone. This calculation applies only to children, but the requirements of adults and especially of the elderly should also be taken into account.
The serious effects consequent upon a combination of pigmentation, bad social habits and environmental pollution, resulting in an insufficient UV irradiance, have been discussed by Loomis (70). The effect of the polluted air over a large city on the UV spectrum can be seen from Fig. 6.
Up to the present, the standard established by the American Conference of Governmental Industrial Hygienists (ACGIH), which specifies a threshold limit value, has been used in preparing guidelines for other countries (J). This is based on the action spectrum for the MED and the minimal photokeratitis dose in normal white-skinned individuals. This means that acute effects have alone been taken into consideration . For the UV-B region and at lower wavelengths, it states that the radiant exposure in an 8-hour period must not exceed the value given in Table 2. For the wavelength range 320-400 nm , the total irradiance on the unprotected skin or eye must not exceed IO W/m2 for periods exceeding 103 s {about 17 min). For radiant exposures of shorter durations , it should not exceed 10 kJ /m2
.
30
Table 2. Threshold limit values (TL V) and relative spectral effectiveness by wavelength for any 8-hour period of exposure
Wavelength, ;>.. TLV Relative spectral (nm) J/m2 effectiveness, S ">-.
200 1000 0.03 210 400 0.075 220 250 0.12 230 160 0.19 240 100 0.30 250 70 0.43 254 60 0.50 260 46 0.65 270 30 1.00 280 34 0.88 290 47 0.64 300 100 0.30 305 500 0.06 310 2000 0.015 315 10000 0.003
A procedure has been suggested for characterizing the relative levels of UV from illumination sources and derived guideline numbers given for the maximum illumination level of the source which will not exceed the ACGIH standards (15).
The values given in Table 2 apply only to sources emitting essentially mono chromatic UV. The maximal permissible exposure for a broad-band source can be calculated by summing the relative contributions from all its spectral components, each contribution being weighted by means of the relative spectral effectiveness, as given in Table 2. In addition , the guidelines should not be used for determining exposure limits for photosensitive individuals.
Up to the present, none of the guidelines take into account the risk of carcino genesis, as neither the action spectrum nor the dose- response curve for man is known. It is hoped that this will become possible from a comparison of cancer incidence and UV irradiance, as measured by the Robertson-Berger method (94) .
PROTECTION
Solar ultraviolet radiation
The weak penetration of UV makes a simple form of protection possible , since it is excluded by most types of clothing. This , of course, may not provide protection for the face and hands during work out of doors. Furthermore , it should be re membered that not all clothing will adequately exclude UV, e.g., there is good reason to believe that transparent stockings permit sufficient UV to be transmitted to produce skin melanomas. It is also the experience of dermatologists that syn thetic materials used for dresses and shirts permit sufficient UV to pass for a skin reaction to occur when phototoxic substances are being tested (83). Apart from clothing, protection may also be afforded by the application of sun-blocking sub stances that absorb the UV or sun-screening substances that act by reflecting the radiation. Of the former, p-arninobenzoic acid or some of its esters have proved to be the most successful. They can easily be applied as a lotion, a cream or preferably in an alcoholic solution (82). The results are a decrease in the UV-B-induced ery thema and a slower rate of suntanning.
It has been suggested that (3 carotene could act as a systemic protector (76). Since even its possible effectiveness in erythropoietic protoporphyria, a photo sensitive disease, has been questioned, the evidence in support of the general use fulness of this treatment is not very strong. The recent observation that the related retinoic acid decreases the dose necessary for the successful PUV A treatment of psoriasis (43) is interesting, in view of the inhibiting effect of this substance on skin tumour production (J 01, 113). Since animal experiments suggest that retinoic acid is carcinogenic (F. Urbach, personal communication, 1978) and may increase cell growth (101) , some caution is necessary.
Industrial sources
Protection against UV in the working environment should primarily consist of containment of the radiation by appropriate design of the source or of the apparatus in which it is placed.
31
As mentioned previously , a large number of sources can be shielded by the use of an appropriate glass covering, which may be a filter that selects only those wave lengths corresponding to the purpose for which the lamp is to be used. In the case of high-intensity lamps containing UV sources, attempts are being made to introduce safety devices that will interrupt the emission if the covering glass is broken.
The radiation facilities used in dermatology for the treatment of psoriasis (PUV A) provide an example where containment is necessary for the protection of the hospital employee. This appears to be difficult but, as the UV emitted is mainly UV-A, may be of minor importance.
When containment is not possible and the irradiance is high, appropriate eye protection is mandatory together with protection of the skin. Standards for eye protection exist in most countries. Welding is an example of a type of work where sufficient protection can be obtained by suitably designed and fitted welding masks or hoods. When welding is started, however, the shield may have to be removed, and this can give rise to photolesions of the eye.
It is fortunate that a certain degree of control of the hazard can be achieved by the welder himself, as the eye will reject filters of insufficient absorbing capacity in the visible light at wavelengths close to UV-A (97). It is not sufficient to protect the welder himself; the surrounding area must also be monitored and screened so as to ensure that nobody is accidentally exposed. This requires fixed shielding between and around welders; such shielding work must be treated with non-reflecting paint in order to protect the neck of the welder from exposure by reflection.
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