1
in the Chemical laboratory
XIV. Lasers and Masers-Control
Edited by NORMAN V. STEERE, School of Public Heolh, University of Minnesota, Minneapolis, Minn., 55455
of Health Hazards*
Roswell G. Daniels, MD; DPH-Bernard Less than four years ago, July, 1960 (1) T. H. Maiman announced the development of the first operating laser. This achievement resulted in the scientific introduction of the now familiar rubylaser. Since then many additional laser sources have been discovered. Fabrication of these devices opened the door to an entirely new scientific technology, with obviously immense, and essentially immediate, practical uses in the fields of communication, welding, biological cauterization! photography, spectroscopy, and space navigation. The objective of this presentation is to delineate tthe several variables that influence the possibility of tissue damage resulting from exposure to laser and maser sources, and to present an acceptable interim control program. For t,his assessment it is appropriate to use the epidemiological approach and method-an evslustion of all the definable aspects of the interaction between the agent (laser beam), the host (man), and the environment involved (the Laboratory and the field).
Laser and Maser Emission First, what is distinctive about this specific type of emission? I t i~ monochraatic, thus i t is of a single wavelength, or of a very narrow band of wavelengths of visible light, ultraviolet, or near infrared energy. I t is d i m a t e d with such a small angle of divergence a t t,he source that the rays are so nearly parallel that the beam may
* This article is a minor modification of the presentation, Lasela and MasersHealth Hazords and Their Control, which is one component presentation of the proceedings of the First Annual Conference on Biological Effects of Lasers which is published in toto, as Supplement 14 to the January-February issue of Federation ProceedCngs of the Federation of American Societies for Experimental Biology. Copies of Federation Proceeding.? 24: No. 1, Part 111, January-February, l9G5, can be obtained for $3.00 from the Federation a t 9650 Wisconsin Ave., Washington, D. C. 20014. This article w a also presented in an address to the 5Znd National Safety Congress in Chicago, October 29, 1964, in a session jointly sponsored by the Chemical Section of the National Safety Council and the Campus Safety Association. A1 82
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1. Goldstein, BSCE, MPH
spread only one to two hundred feet aver a distance of twenty miles. T r a a d o z r s energy fluses can be produced. Power densities many times that found a t the surface of the sun may be aequired (B). I t is cohemnl with the individual waves in phase along the axis oi the beam. Spatial coherence, or coherence in two or more planes, may initiate a uniphasic wave front, with single phase across an entire wave front (3). gradient,^ or modes of spatial coherence may also be created (5). Laser radiation complies with the physical laws of optics (3). Thus high energy fluxes can be furthe? concentrated by a eonvex lens which may focus this energy onto submicroscopic targets. There are three categories of laser devices which have been developed and are in use: the solid-state laser, the gaslaser, and the injection laser. Of these, the solid-state laser is generally the one with which most are familiar. Much biological data has been derived from work with this source. These produce a pulsed emission a t a variety of specific frequencies depending upon the type of crystal source. Modifying the device by Q-switching or Qspoiling permits the acquisition of a considerable increase in power flux (intensity per unit time) b y shortening the pulse duration. The gas-laser, exemplified by the helium-neon device, produces a continuous wave rather than a pulsed wave. Energy outputs thus far have been several magnitudes less than those of the solidstate laser, and are generally down in the range of 0.01 watt (joule per see). These devices have the advantage of a range of wavelengths for each device and produce a relatively spatially coherent wave. The newest type laser, the injection or junction laser, also produces a. continuous beam. The emission from the gallium-arsenide device of this type has a wavelena-h in the near infrared area, of 8400-9000 A which varies directly with temperature (4). Currently, the energy output is in the range of 0.1 watt. Since this type device is relatively efficient-up to 20% (4, 6) versus 1% for solid-state lasers and slightly less than 1% for the gas-laserand it is easily tuned for frequency, rapid development of the device for cornmunice tion purposes is anticipated. Thus we have multiple sources of laser-maser radiation; the beams from which are either pulsed or oontinuous, with varying degrees of spatial coherence. A spectrum of frequencies is available, with a vast array of energy levels involving several orders of magnitude.
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Variable Factors in Tissue The most susceptible body organ appears to be the human eye, since the cornea and lens of the eye focun this radiant energy upon the retina coucentrating i t several fold. What exactly are the host factors influencing the occurrence of hody damage upon exposure? We have divided t,hese factors into two components: general factors apparently applicable to all tissue, and organ-specific factors applicable specifically to the eye. For the former the following are pertinent: The pigmentation of the tissue. The more concentrated the melanin in the tissue generally the more absorption (6,7). The vasevlarilg or blood circulation afforded the tissue (8). The available blood flow arts as a homeostatic mech* nism which tends t o maintain tissue temperature and reduce damage from heat stress. Spectral absorption of the energy. Only data on retinal t i ~ s u eare available in this regard, and far this tissue absorption is a t a maximum far wavelengths of about 4000-5500 A and diminishes as Wavelength increases (8, 9). The following are relevant to harmful effects on the eye: The pupil siae. The smaller the pupil dirtmeter the proportionately less energy permitted to enter the eye from a -ride beam flooding the pupil. The convergence power af the cornea and the lens. These affect the size of the retinal image, and determine the point of focus. Distance from the lens to the retina. This distance influences the siae of the retinal image. The attenuation of the energy transmitted through the eye to the retina. This is influenced by the fluids and tissues of the eye and the laser wavelength involved (8, 9, 10).
Variables of Beam Characteristics I t is apparent that certain characteristics of this type of radiation beam influence directly the possibility of biological damage. These are: Pulse duration. Pulse repetifion rate, or continuous wave. Frequency (wavelengths). Magnitude of the energy flvz per square centimeter as affected by source energy or type of equipment, angle of divergence of the primary beam from the source, attenuation in the atmosphere, and optical modifications by lenses. With the ruby laser, variability in the energy output between individual pulses is also 8. factor.
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The location of the beam source, whether close to the eye or in the far field. Energy homogeneity throughout the beam cross section. Imperfections in the crystal structure of the ruby have been noted to cause inhomogeneity of intensity (9) and propagation of the beam through the atmosphere also produces heterogeneity with "hot spots" and "cold spots," apparently as a result of the effect of air turbulence (11). I t should be noted that all of the factors eoneerning the agent (beam) affect the quantity of energy available for interaction. This directs us to an evaluation of our current measurement of beam intensities. A review of this subject with some of the physical scientists a t the Harry Diamond Laboratories reveals that the primmy calibration device, the conical black body radiation calorimeter, is considered to be quite reliable (*I%), and that secondary mensuration devices, such as photoelectric cells supplemented with appropriately calibrated ascillograpbs, are accurate enough t,o meet present requirements.
Environmental Considerations Here, relationships become intangible because of the variety of locations and environments which may be involved. Application of these devices is taking these instruments from the more easily controlled use in laboratories out into the field. Thus new methods and situations for use are occurring and complicating factors will probably soon become apparent. For instance, what is the effect of snow and other meteorological conditions on the various laser beams?
Thermal Effects Only? An understanding of the physical, chemical, or enzymatic mechanisms a t the cellular level by which this emision interacts with the individual cell and destroys i t may provide valuable evidence which will assist one in developing specific controls or prophylatie measures. The pathology and histology of the coagulation necrosis resulting from laser exposure have been described (6, 7, l 2 , l S ) . A conclusive answer has not been provided to the most basic question. Are certain additional athermal effects superimposed upon the thermal changes? Currentlv the vast maioritv of the
Retinal damage, however, causes a functional impairment-% subjective visual loss, which requires patient cooperation 10 qumtitutc b\. me&s of msppi& visual fiehl~. Tnis is nnt tllc tvpr of inlpairnmn s h i d l rarr he diwctlv rxrmun1;~trJ.with confidence, t o man from animal studies. From our knowledge of anatomy and physiology, i t is apparent that a small lesion on the fovea will produce a more serious visual loss than an equally small lesion in other retinal areas providing peripheral vision. The exact size of the smallest lesion required to cause a. measurable visual loss in man can only be
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hypothesized a t the present. Another consideration that is generally less well appreciated is thst there is evidence ill animals that destructive lesions of a small diameter may not becameevident until additional lesions in adjacent tissue become superimposed so as to provide an additive effect. Thus, subclinical effects, evident only upon pathological study, may occur. By no means, does one wish to permit an employee to become exposed to a laser beam of sufficient intensity to cause even such subclinical retinal change. In this matter of binlogical effects resulting from laper radiation esposure one must keep in mind the pammount considemtion that these devices have been used far so brief a period of time that knowledge is lacking concerning the occurrence of suhacute or chronic effects. The point concerning knowledge of the biological effects of lasers brings to mind an introductory comment recently posed by Brigadier General Joe M. Blumberg, MC, U.S. Army, Director of the Armed Forces Institute of Pathology, in his opening remssks on April 30, 1964, a t the First Annual Conference on Biological Effects of Laser Radiation. General Blumberg discreetly stated that in many respects laser technology as it concerns biological effects upon man is analogous today to that situation that occurred in the years immediately after Roentgen's discovery of the X-ray. It took decades to acquire our current knowledge of the harmful effects of X-ray radiation on man. Often this informat,ion resulted from an assessment of the unfortunate consequences thst arose from overexposures tu the individuals involved in the researrh, development, and use of various types of X-ray equipment. Today we stand with laser knowledge in a comparable situation to that which our forefathers faced in 1899 with X-ray. We have discovered .z new modality, we know biological effects can occur, we have knowledge about some of the acute effects, but much remains to he determined as to the existence of subacute and chronic effects, possibly after long latent periods. Much more remains to be ascertained. A multiplicity of factors influencing the resultant interaction of agent and host have now been delineated. Some of these have already been made quantitative relatively accurately, others have been approximated, and others remain to be measured. Additional information is expecaed to be available in the near future concerning environmental variables and the mechanism of action a t the cellular level. Let us not forget hhat interactions between variables may occur and pose additional complexities for any extrapolation of research data on animals to hazards controls for man.
Need for Control I n attempting to initiate controls for any radiation hazard, one is anxious to derive a "threshold level" which is an acceptable level of exposure without resultant physiological loss or pathology. As
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of this date, no satisfactory tolerable level h a been established, and we do not feel that sufficient data. are zvailable now to establish such s. level with reliability. Attention has been directed t o the multiplicity of variables influencing the p n b ability of tissue damage from an exposure; these must also be taken intu considerat,ion in the future upon any extrapolation to man from the experimental anrtest,hesir;edrabbit eye in n state of cyeloplegia. Although no number for such a level is available, the unitage for this level can be established. That is, this t,hreshold level must measure the energy level in terms of two other variablestime and cross sectional ares. Consequently, any satisfactory allowable energy level for exposure should be expressed in terms of joules per nsnoneamd or other time unit, and per unit cross sectional area of the beam. Alternate terminology would he watts/cmz or ealories/cmP per second. One watt is ident,ical t,o one joule per second; one calorie equals 0.24 joule*. There is no question that exposure to the laser or maser radiation may present s serious health hazard to man. Although additional research remains to be accomplished to establish dose-response relationshipn quantitatively, and to assess some of the variables influencing this hazard, definite controls for these hazards are necessary today-not tomorrow. Consequently, recognizing that immediate guides for control were necessary, the Occupational Health Branrh, Preventive Medicine Division, Oflire of The Surgeon General, Department of the Army, formulated general tentative guidelines rovering all types of devires. These generalized control measures were proposed to the Commission on Environmentd Hygiene of the Armed Forces Epidemiulogieal Board in October, 1963, for their consideration. The following tentative guidelines were constructed by that commission far handling lasers in laboratories. We quote these guides with minor modifications: 1. "The laser beam should be discharged into a background that is nonreflective and fire resistant." 2. "An area, should be cleared of personnel fur a reasonable distance on all sides of the anticipated path of thelaser beam." 3. "Looking into the primary beam must be avoided a t all times, and equal care should be exerted to avoid looking a t specular reflections of the beam, including those from lens surfaces." 4. "Avoid aiming the laser with the eye, to prevent looking dong the axis of the beam, which increases the hazard from reflections." 5. "Work with lasers should be done in areas of high general illuminhtion to keep pupils constricted and thus limit the energy which might inadvertently enter the eyes." 6. "Some laser electronic-firing systems may sttore a charge, and caution must be exercised to avoid accidental pulsing of the laser and to avoid electric shock. Systems should be designed to prevent this hazard and to establish a "fail-safe" condition." 7. "Individuals working in h e r test pmredures, and others frequent,ly exposed to h e r discharges, should be hlVolume 42, Number 3, March 1965
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cluded in zn occupational vision program which encompasses thorough general ophthalmologie examinations a t regular intervals. Such examinations will not neglect sliLlamp and funduscopic studies and mapping of the visual fields." 8. "Safety eyewear desimed to filter out specific frequencies characteristic of the system affords protection, but i t may be only partial." In applying these guides the following fadsshould he kept in mind: 1. An aceentable threshold limit for exposure has not been established. Possibly a series of levels for different types of pulses, milliexposures-nanosecond second pulses, continuous w a v e m a y be indicated. In the meantime, it is suggested that the conservative approach delineated ahove be used. 2. The effects of atmospheric diffraction upon the beam still remains to he measured. 3. Atmospheric attenuation needs t n he measured quantitatively. 4. Mirror reflections may cause as much biological damage as the original beam. Glass surfaces may reflect 4% of the beam energy, which itself may he suficient to cause retinal damage. The fact that the beam may he invisible from certain sources (such a3 the neodymium solid-state laser) makes recognition of reflections even more difficult. In conclusion, let us stress that the ultimate standards far controls of these hazards may best he designed to fit the specific type of laser device under consideration. Wide variation between types of beams and intensities make this specific "tailoring" appear desirable. The general cont,rol program, submitted above, can be quite effective long before the resolution of all the problematic details leading into a sophisticated program. Let us also not forget, however, that in the application of any health control program, history has repeatedly demonstrated that apathy, inertia, and ignorance may he the most important impediments to program implementrttion.
Literature Cited (1) "Masers and Lasen," MaserlLaser Associrttes, Cambridge, Mass., 1962. (2) MALT, R. A,, AND TOWNES,C. hf., The Arm England Journal of Medicine, 269, 26 (1417) (1963). (3) REXPEL,R. C., "Optical Properties of Lasers as Compared to Conventional Radiators," Laser Technical Bulletin #1, Spectra-Physics, Inc., Calif., 1963, pp. 1-7. (4) TEBROCK,H. E., YOUNG,W. N., AND MACHLO, W., Journal of Occupational Medicine, 5 , 12, p. 564, Dec., 1963. (5) LAX,B., Science, 141, 1247 (1963). (6) GOLDMAN, L., ET u.,The Journal
of Investigrative D m a t o l o g y , ~ O , 121, March, 1963. (7) FINE,S., ET AL., The Jownal'of Investigative Demalolog.y, 40, 123, March, 1963. (8) Aerospace Medical Division, Radia-
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tion Thresholds for Chorioretical Bums, Rept. #AMRLTDR-GS-71, 6570th Aerospace Medical Research Laboratories, WrighbPatterson Air Force Base, Ohio, 1963, 38 PP. (9) HAM,W. T., ET AL., A d a Ophthab n~ologim, Aupplementum 76, 60 (1963). (10) SOLON,L. R., ARONSON, R., AND GOULD. , G... Science. 134. 1506 (1961). (11) STRAW, H. W., "Effects of Air
Turbulence Upon Propagation of Light," Verbal Presentation, Optien1 Society of America, Oct., 1962. (12) ZARET, M. M., ET AL., Arch of Ophthalmologv, 69, 97, Jan., 1963. (13) ZARET,17. IT., ET AL., Science, 134, 1525 (1961).
Bibliography DULBERGER, L. H., Electronics, Janumy 26, 1962, p. 7. SOLON,L. R., The Journal of the dmeriean Society of Safetv Engineers, December, 1962, p. 30. SOLON,L. R., Axhives of Environmenlnl Health. 6.414(1963). . . Association of casualty and Surety Companies, "Lasers and Masers," Special Hxsards Bulletin No. Z12.5, May, 1963. KAPANY, N. S., ET AL., Nature, 199, 146, 13 July, 1963. STRAUB,H. W., Protection of the Human Eye From Laser Radiation, Harry Diamond Laboratories, AMC (TR 1153), 10 July, 1963. LEY~NE, A. K., Amevimn Scientist 51, 14, March, 1063. OLIVER,B. iM., "Some Potentialities of Optical Masers," Proceedings of the IRE. February, 1962, p. 135.
