Chapter 14
LASER RADIATION

David H. Sliney and James A. Hathaway

Lasers are devices that produce an intense, coherent, directional beam of light by stimulating electronic or molecular transitions to lower energy levels.1 The beam of radiation emitted by lasers in common use may have a wavelength anywhere from the ultraviolet (UV) region of the electromagnetic (EM) spectrum to the far-infrared (FIR) region. This includes numerous lasers operating in the visible light portion of the EM spectrum. Lasers vary widely in the intensity of their outputs; they may generate brief bursts or pulses of energy or operate continuously. The potential hazard of laser radiation depends on all of these factors.

OCCUPATIONAL SETTING

The use of lasers in industry, construction, research, medicine, and the military is widespread and increasing. Lasers are used in alignment, welding, trimming, spectrophotometry, range-finding, interferometry, flash photolysis, fiber-optic communication systems, and surgical removal or repair procedures.2,3 Low-power lasers are also widely used in commercial activities and consumer applications, including supermarket checkout counters, detection of motor vehicle speed, as pointers for presentations, in CD-ROM drives for computers, and in CD, DVD, and laser disk players for home entertainment. Specific occupational titles may not be particularly helpful in identifying where lasers may be used. In industries using high-technology processes, various craftsmen, operators, and service workers may be expected to use lasers.4 Lasers are used for range-finding in advanced weapon systems by military personnel. Maintenance personnel may actually be at higher risk of accidental exposure than the operators because they may need to remove protective shielding and interlocks to repair the equipment. Similarly, the nature of laboratory research often precludes the use of engineering safeguards and may increase the risk of accidental exposure.2 Medical uses usually require lasers with sufficient power to damage tissue. Accidental exposures have the potential to injure operating room personnel as well as patients.

MEASUREMENT ISSUES AND CLASSIFICATION OF LASER POWER

In general, measurements of laser radiation are not necessary. The laser classification scheme described in the following paragraph was designed to minimize the need for measurements. It is the responsibility of laser manufacturers to perform measurements and classify their products. The classification system allows the user to determine potential risks and provide for the necessary safeguards, procedures, and personal protective equipment. Measurements are required only when information from a manufacturer is not available or when a laser system has been modified. Detailed information on measurement can be found in Section 9 of the ANSI Z136.1 standard1 on the safe use of lasers. Appendix H4 of the same document provides a listing of catalogs on commercially available laser-measuring instruments.

The primary hazard from laser radiation is from exposure to the eye and, to a lesser extent, the skin. Therefore, the classification is based on the laser’s capability of injuring the eye or the skin. Lasers manufactured in the United States are classified in accordance with the Federal Laser Product Performance Standard.5,6 The actual process is somewhat complex, because numerous types of lasers have been developed that operate at different wavelengths. The threshold for biological injury varies with the wavelength of radiation. It is also dependent on the operating conditions of the laser—that is, on whether the radiation is continuous or pulsed. If it is pulsed, the duration and repetition rate of the pulse must also be considered. Details of the classification scheme are described in ANSI Z136.1. The following outline provides a somewhat simplified view of the classification scheme:

  • Class 1 laser—Will not produce injury even if the direct beam is looked at for the maximum possible duration inherent in the design of the laser. For many lasers, this essentially amounts to an unlimited viewing time.
  • Class 1M laser—Will not produce injury except potentially when viewed by binoculars or telescopes from within the beam.
  • Class 2 laser—Will not produce injury if the direct beam is viewed for 0.25 second, the time period necessary for a protective aversion response. Class 2 lasers are limited to lasers emitting visible light on a continuous basis.
  • Class 2M laser—Is equivalent to class 2 unless viewed from within the beam with a telescope.
  • Class 3 laser—Can produce eye damage if the direct beam is viewed, but would almost never pose a risk to the skin. This classification is subdivided into classes 3R and 3B. Class 3R, which is limited to the lower accessible outputs of this class, is believed to present less risk of actual injury from a practical standpoint. Class 3B represents those class 3 lasers with higher outputs where the risk of real ocular injury from even momentary viewing of the direct beam is high.
  • Class 4 laser—Even the diffuse reflection of some pulsed lasers with this level of power output can produce biological damage to the eye. The direct laser beam can injure the skin or pose a fire hazard.

Control measures apply primarily to lasers in class 3B or 4. Limited precautions such as product labeling apply to classes 2 and 3R lasers.

EXPOSURE GUIDELINES

Exposure guidelines have been developed by the American Conference of Governmental Industrial Hygienists (ACGIH) and by the American National Standards Institute (ANSI) Z136 Committee on the Safe Use of Lasers.1,7 Both of these organizations have issued guidelines for safe laser use. The Occupational Safety and Health Administration (OSHA) does not specifically regulate laser radiation, although the ANSI standard would be consulted in cases where the OSHA “general duty” clause is applied.

The ACGIH standards are called threshold limit values (TLVs®). They vary, depending on the wavelength of the laser radiation and depending on whether the radiation is pulsed or continuous. Certain assumptions are also made regarding aversion time (0.25 second) and the size of the pupil under various exposure conditions. ACGIH Tables 2 and 3 list the TLVs® for either eye or skin exposure by wavelength and exposure time. The output of pulsed lasers is described in terms of energy (joules), while the output from continuous wave lasers is described in terms of power (watts). The TLVs® are expressed as radiant exposure in joules per square centimeter (J/cm2) or as irradiance in watts per square centimeter (W/cm2). The ANSI Z136 committee has labeled their exposure limits as maximum permissible exposures (MPEs). The MPEs for various conditions and types of lasers are listed according to wavelength in Table 5-7 of the ANSI Z136.1 standard.1 The complexity of these tables and those of the ACGIH preclude them from being summarized here.

PATHOPHYSIOLOGY OF INJURY

Research on the biological effects of laser radiation has been directed toward determination of the thresholds for tissue damage. The threshold for identifying damage has typically been grossly apparent findings or findings observable using instruments such as microscopes, slit lamps, and ophthalmoscopes. The ANSI Z136 committee used these data to determine exposure levels that produce damage 50% of the time. This would be analogous to an ED50, or a dose that produces an adverse effect 50% of the time in experimental animals exposed to a chemical. A factor of 10 below the 50% damage level was then typically used to arrive at the MPE level, where the probability of damage was negligible. Actual regression lines were used to determine the slope of the dose–response curve where possible; when this slope was very steep, a factor <10 was used.8 The principal biological hazards associated with laser radiation occur with acute short-term or intermittent exposures. Chronic effects are theoretically possible based on results of exposures to experimental animals or based on analogy to the chronic effects produced by ambient or artificial sources of UV, visible, or IR radiation. Chronic exposure to laser radiation of sufficient power to be of concern is rare in occupational settings, because laser beams have very limited spatial extent. Therefore, we will focus on the acute biological effects of laser radiation.

Corneal damage from the infrared region (1400 nm to 1.0 mm)

Depending on the power level of the laser, tissue damage to the cornea from acute IR laser radiation can range from a minimal lesion involving only the epithelium, which appears as a small white area, to massive destruction of the cornea with severe burns to adjacent structures of the eye such as the conjunctiva and lids. Damage results from absorption of energy by tears and tissue water in the cornea. The heat is diffusely absorbed; a simple heat flow model is believed to explain the observed effects adequately.1 Minor damage may heal completely within 48 hours; more severe damage will have permanent sequelae.

Corneal damage from the ultraviolet region (100–400 nm)

Biological damage from UV laser radiation is similar to that caused by other artificial or ambient sources of UV. Corneal effects following acute or subacute exposures include epithelial stippling, granules, haze, debris, exfoliation, and stromal haze and opacities.8 Clinical symptoms and findings may include photophobia, tearing, conjunctival discharge, and redness. The damage caused by UV radiation is not due to heating effects but rather to photochemical denaturation of proteins and other macromolecules, such as DNA and RNA. Thermal damage can occur from pulsed lasers at some wavelengths necessitating dual exposure limits.

Retinal damage from the visible and near-infrared region (400–1400 nm)

The cornea, lens, and ocular media are mostly transparent to visible light in the 400–700-nm wavelength range. Nearly all of the visible energy reaches the retina. Near-IR radiation in the 700–1400-nm range also reaches the retina in significant amounts and produces damage similar to that caused by visible radiation. Clinically, the minimal lesion is a small white patch apparently caused by the coagulation of protein. It may be asymptomatic. It is visible within 24 hours of exposure. More significant exposures may produce immediate symptoms, such as loss of vision in the visual fields, spots (scotomata) in the field of vision, or persistent afterimages.2 More severe exposure can cause substantial damage, including significant hemorrhage from the retina into the vitreous humor.

Laser radiation in the visible and near-IR can cause damage by a variety of mechanisms, depending on the type of laser. Damage has been attributed to thermal, thermoacoustic, and photochemical phenomena.3 Lasers with short-pulse durations of <10−9 seconds may cause “blast” damage through nonlinear mechanisms such as ultrasonic resonance and acoustic shock waves.9 Most of the radiation is absorbed in the melanin granules of the retina in the retinal pigment epithelium and choroid. This structure underlies the cones and rods. When damage is caused by a thermal mechanism, it is due to protein denaturation. Damage caused by heat or photochemical mechanisms is similar to what would be expected from equally intense doses of noncoherent light.

Examination of an injured individual typically reveals a blind spot (scotoma) or spots in one or both eyes. Visual acuity may or may not be decreased, depending on the proximity of the injury to the macula. Obviously, injuries to or near the macula produce greater functional loss than injuries in the periphery of the retina. Fundoscopic examination may show retinal or subretinal hemorrhages and hemorrhage into the vitreous. More minor injuries may not be immediately obvious on fundoscopy, or they may present as minor retinal burns with edema. Healing takes place over a course of weeks. Some improvement in visual acuity may occur as the edema subsides. Generally, a blind spot remains in the visual field. The extent of functional loss depends on the size and location of the injury.

There are two reports on series of patients injured from exposure primarily to Nd:YAG lasers operating at 1064 nm.10,11 In one report, 8 of 12 patients had macular lesions. Visual loss ranged from minimal to severe. There was no improvement in vision over time, in spite of vasoprotective and corticosteroid treatments. In two cases, the extent of the injury was progressive. In one case, there was hemorrhage in the vitreous humor. In the other report, 25 of 31 eye injuries resulted in macular damage. Macular damage was progressive over a 1-week time period in seven cases. Ten eye injuries were followed for 4–10 years. The extent of injury remained stable over this time for nine of the eye injuries. Another report indicates that the presence of a hemorrhage, which may initially severely impair vision, does not preclude the possibility of a return to normal vision. When a short-pulse laser produces a microscopic retinal hole, recovery can be remarkable.12

Other ocular damage

Radiation in the near-UV zone and radiation in the zone between near-IR and IR have absorption characteristics such that significant levels of energy may be absorbed in structures of the eye between the cornea and the retina, including the lens and the iris. Acute damage to these structures would be expected from very high-energy lasers (e.g., at 1315 nm, iodine laser wavelength). Concurrent damage to the lens or the retina (depending on the wavelength) would also be expected. Chronic exposure to noncoherent sources of UV or IR radiation in these wavelength regions causes lenticular damage leading to cataracts. Theoretically, the same damage could be incurred from coherent laser radiation at similar wavelengths. In actual occupational settings, chronic exposure of unprotected workers is unlikely. Chronic effects on the lens have not been studied.

Skin damage

Laser radiation can cause injury to the skin. Higher levels of energy are required to produce skin damage than for eye injury. The focusing power of the cornea and lens of the eye increases the energy density reaching the retina, thus allowing lower levels of total energy to produce localized injuries. However, UV lasers can cause photochemical damage to the skin similar to acute sunburn. Visible and IR lasers can produce thermal burns from acute exposures. The power output of the laser determines whether accidental exposure to the skin produces a minor injury or a more severe one. Theoretically, chronic exposure to UV lasers would have the same risk of causing premature aging of the skin and increase the risk of skin cancer; however, exposure conditions that would result in chronic exposures are unlikely, given the current uses of lasers.

TREATMENT

Individuals with suspected injuries to the retina should be referred to an ophthalmologist. In many cases, no treatment is required, but continued follow-up is important to evaluate functional loss (both visual acuity and blind spots in visual fields). Complications such as growth of new blood vessels in the vicinity of the injury may require treatment. More severe retinal injuries from very high-power lasers require immediate evaluation by an ophthalmologist.

Minor UV injuries to the cornea can be treated in the same way as photokeratitis from other sources of UV radiation. Patching the eye and using cycloplegics are recommended. Anesthetic drops should not be used. Complete recovery takes about 48 hours. More severe corneal injuries from either UV or IR radiation require specialized treatment from an ophthalmologist.

The following precautions should be taken when dealing with eye injuries that require referral to an ophthalmologist:13

  • Eye ointments should never be used because they make clear visualization of the retina very difficult.
  • Topical anesthetics should not be used to relieve pain from a UV injury.
  • Prolonged use of these anesthetics can cause corneal breakdown and lead to blindness.
  • Topical steroids should never be used unless prescribed by an ophthalmologist.
  • If in doubt about the seriousness of an injury, err on the side of caution and refer the patient to an ophthalmologist.
  • Keep in mind that some suspected laser-induced ocular injuries may not actually originate from laser exposure.14

Megadose intravenous methylprednisolone has been used in studies with cynomolgus monkeys to determine if it might improve healing of retinal laser burns caused by visible or near-IR laser radiation. An overall beneficial effect was noted. The authors indicated that the effect might be ascribed to the anti-inflammatory action, protection of microcirculation, and antilipid peroxidation effects.15

Skin injuries from visible or IR radiation can be treated in the same way as localized thermal burns. Intramuscular vitamin E and/or use of vitamin E and an occlusive dressing have been reported to improve wound healing in miniature swine following exposure to IR lasers.16 UV radiation can produce a localized injury equivalent to sunburn; it should be treated accordingly.

MEDICAL SURVEILLANCE

At one time, medical surveillance requirements were included in the ANSI Z136.1 standard.1 They were required only for individuals working with class 3B or 4 lasers. Medical surveillance was never required for use of class 1, 2, or 3R lasers. Examinations were required before work with lasers and after suspected injuries. No periodic examinations were required. At this time the ANSI Z136.1 standard requires no medical surveillance.

Although some wavelengths of laser radiation have the theoretical potential to produce chronic effects, the nature of their use makes this unlikely. A few studies have looked for possible chronic effects in laser workers, but no evidence of adverse chronic effects has been found.17–20

At this time, the preplacement evaluation of laser workers may be used to establish a baseline against which the effects of accidental injury could be compared. Some employers or institutions may require this for potential legal protection against claims for preexisting damage to parts of the eye.

Medical examinations should be required following accidental exposures to the eye or skin. It is recommended that an ophthalmologist conduct the exam if there are injuries to the retina.

PREVENTION

Protecting the skin, and particularly the eye, from high-power laser radiation is critical because permanent damage—including blindness—can result. There are a number of excellent references on laser safety that can be consulted. Other references provide considerably more details on preventive measures than are appropriate here.1,21–26

In their review of reported accidental exposures to laser radiation, the ANSI Z136 committee specifically noted the following important causes of the incidents:1

  • Unanticipated eye exposure during alignment
  • Available eye protection not used
  • Equipment malfunction
  • Intentional exposure of unprotected persons
  • Operators unfamiliar with laser equipment
  • Improper restoration of equipment following service

The ANSI committee also noted that several serious accidents were traceable to ancillary hazards such as electric shock, toxic gas exposure, and vaporized tissue exposure from medical procedures. These topics are covered in several reports.1,22,25,27

Obviously, the preferred method of prevention is to incorporate engineering control measures that limit access to laser radiation. Indeed, many laser systems are designed to embed more powerful class 3B and 4 lasers within shields or enclosures. This safeguard eliminates the risk of accidental operator exposure, but it does not eliminate the risk to persons servicing the equipment. For many applications of lasers, however, it is not feasible to rely on enclosure; other methods of engineering control must be used, along with training, administrative procedures, personal protective equipment, and warning systems.

The ANSI Z136.1 standard requires the appointment of a laser safety officer (LSO) to monitor and enforce the control of laser hazards. This task may involve training requirements, administrative procedures, standard operating procedures, and selection of engineering control measures.

Depending on how the laser is used, numerous engineering control measures may be necessary. Examples include protective housing, interlocks on protective housings, interlocked service access panels, master switches that are disabled when the laser is out of use, interlocks, filters or attenuators for viewing portals and display screens and collecting optics, enclosed beam paths, remote interlock connectors, beam stops or attenuators, emission delay systems, and remote firing and monitoring.

Under certain circumstances, some of these engineering controls may not be feasible and alternative methods will be necessary. One important control measure is the establishment of what is called a laser controlled area. Access to the area is limited to personnel who have been specially trained. These workers must have appropriate protective equipment, and they must follow all applicable administrative and procedural controls. Controlled areas need to be posted with warning signs. They must have limited access, be operated by qualified and authorized personnel, and be under the supervision of someone specially trained in laser safety. They should use beam stops of appropriate material, diffuse reflecting materials where feasible, and appropriate eye or skin protection. Furthermore, these areas limit the beam path to above or below eye level except as required for medical use; they eliminate the possibility of transmission of laser radiation through doors, windows, and they include a system that can disable the laser to prevent unauthorized use. Class 4 laser controlled areas require safety controls to allow rapid egress, emergency alarms, nondefeatable area/entry controls where feasible, and other controls for particular operations. Inherent in the controlled area concept is the need for rigorous compliance with training, administrative, and procedural requirements. Protective equipment is mandatory whenever it is needed.

Industrial employers have generally complied well with the ANSI Z136.1 consensus standard. This has not always been the case with research laboratories, particularly those in university settings. One article described a number of injuries in university research laboratories where persons using lasers had not received proper training and appropriate protective eyewear was not used.2 The authors proposed a registration system for research lasers that would ensure that laser personnel receive proper training and that appropriate protective equipment is available. In recent years, a number of other application-specific safety standards were developed in the ANSI Z136.1 series. These include ANSI Z136.5 (educational institutions), ANSI Z136.6 (outdoor use), ANSI Z136.8 (research, development, and testing), and ANSI Z136.9 (manufacturing environments).

References

  1. 1. Laser Institute of America. American national standard for the safe use of lasers. ANSI Z 136.1. Orlando, FL: Laser Institute of America, 2014.
  2. 2. Barbanel CS, Ducatman AM, Garston MJ, et al. Laser hazards in research laboratories. J Occup Med 1993; 35:369–74.
  3. 3. Wilkening GM. Nonionizing radiation. In: Clayton GD, Clayton FE, eds. Patty’s industrial hygiene and toxicology, Vol. 1, Part B, 4th edn. New York: John Wiley & Sons, Inc., 1991:657–742.
  4. 4. US Department of Labor. Guidelines for laser safety and hazard assessment. OSHA instruction PUB 8-1.7. Washington, DC: US Government Printing Office, 1991.
  5. 5. Code of Federal Regulations, Title 21, Subchapter J, Part 1040. Laser product performance standard. Washington, DC: US Government Printing Office, 1999.
  6. 6. Code of Federal Regulations, Title 21, Parts 1000 and 1040. Laser products: amendments to performance standard. Washington, DC: US Government Printing Office, 2015.
  7. 7. American Conference of Governmental Industrial Hygienists. Threshold limit values, 2015. Cincinnati, OH: American Conference of Governmental Industrial Hygienists, 2015.
  8. 8. Sliney DH, Mellerio J, Gabel VP, et al. What is the meaning of threshold in laser injury experiments? Implications for human exposure limits. Health Phys 2002; 82(3):335–47.
  9. 9. Ham WT, Williams RC, Mueller HA, et al. Effects of laser radiation on the mammalian eye. Trans N Y Acad Sci 1966; 28:517–26.
  10. 10. Pariselle J, Sastourne JC, Bidaux F, et al. Eye injuries caused by lasers in military and industrial environment. J Fr Ophtalmol 1998; 21:661–9.
  11. 11. Lui HF, Gao GH, Wu DC, et al. Ocular injuries from accidental laser injuries. Health Phys 1989; 56:711–6.
  12. 12. Hirsch DR, Booth DG, Schockett S, et al. Recovery from pulsed dye laser retinal injury. Arch Ophthalmol 1992; 110:6188.
  13. 13. Vinger PF, Sliney DH. Eye disorders. In: Levy BS, Wegman DH, eds. Occupational health, 2nd edn. Boston, MA: Little, Brown, 1988:387–97.
  14. 14. Mainster MA, Sliney DH, Marshall J, et al. But is it really light damage? Ophthalmology 1997; 104:179–80.
  15. 15. Takahashi K, Lam TT, Tso MO. The effect of high dose methylprednisolone on laser-induced retinal injury in primates: an electron microscopic study. Graefes Arch Clin Exp Ophthalmol 1997; 253:723–32.
  16. 16. Simon GA, Scmid P, Reifenrath WG, et al. Wound healing after laser injury to skin—the effect of occlusion and vitamin E. J Pharm Sci 1994; 83:1101–6.
  17. 17. Wolbarsht WL, Sliney DH. Historical development of the ANSI laser safety standard. J Laser Appl 1991; 3:5–11.
  18. 18. Hathaway JA, Stein N, Soles EM, et al. Ocular medical surveillance on microwave and laser workers. J Occup Med 1977; 19:683–8.
  19. 19. Friedman Al. The ophthalmic screening of laser workers. Ann Occup Hyg 1978; 21:277–9.
  20. 20. Hathaway JA. The needs for medical surveillance of laser and microwave workers. In: Tengroth B, ed. Current concepts in ergophthalmology. Stockholm: Societas Ergophthalmologica Internationalis, 1978:139–60.
  21. 21. Laser Institute of America. Laser safety guide. Cincinnati, OH: Laser Institute of America, 2015.
  22. 22. Sliney DH, Wolbarsht ML. Safety with lasers and other optical sources: a comprehensive handbook. New York: Plenum Press, 1980.
  23. 23. Sliney DH, LeBodo H. Laser eye protectors. J Laser Appl 1990; 2:9–13.
  24. 24. Laser Institute of America. American national standard for the safe use of optical fiber communication systems utilizing laser diode and LEP sources. ANSI Z 136.2. Orlando, FL: Laser Institute of America, 2014.
  25. 25. Laser Institute of America. American national standard for the safe use of lasers in health care facilities. ANSI Z 136.3. Cincinnati, OH: Laser Institute of America, 2005.
  26. 26. Thach AB. Laser injuries of the eye. Int Ophthalmol Clin 1999; 39:13–27.
  27. 27. Sliney DH, Clapham T. Safety of medical excimer laser with an emphasis on compressed gases. Ophthalmol Technol 1991; 1423:157–62.
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