Michael Woods and Jeff Corbett, SLAC National Accelerator Laboratory
The words “laser safety” bring to mind eye protection, but protecting the skin is also important, especially with UV lasers, where moderate-to-long exposures to diffuse reflections must be considered. Skin-hazard descriptions and control requirements related to UV laser radiation can be found in the ANSI Z136 safety standard, the OSHA Technical Manual and the ACGIH handbook of threshold limit values. These references provide practical guidance on UV skin hazards. Controls – based on these references and on a few example calculations – can help keep UV laser users safe.
MPE values, calculated NHZs
Figure 1 shows the maximum permissible exposure (MPE) values for the eye and skin for an exposure duration of
1000 s, which could represent a prolonged exposure to diffuse laser reflections or to an incoherent source. These MPEs are the same for eye and skin outside of the retinal hazard region, from 400 to 1400 nm.
Figure 1. Maximum permissible exposure (MPE) for the eye and skin for a 1000-s period exposure, using Tables 5a and 7 in ANSI Z136.1-2007. The MPEs are the same outside of the retinal hazard region (400-1400 nm).
The nominal hazard zone (NHZ) is a region in which exposures can exceed the MPE. Table 1 gives examples of NHZ distances for diffuse reflections from a small-diameter UV laser beam incident on a diffuse target for an exposure duration of 1000 s. The examples assume a reflectivity of 50 percent for the target and a viewing angle of 45°. The calculations show that for the NHZ to be 20 cm or greater at 380 nm, the incident beam power on the target must be at least 3.5 W. But at 255 nm, the same 20-cm NHZ occurs for only 10 mW of incident power.
Consideration of the examples in Table 1 and the information available in the ANSI, OSHA and ACGIH references led to the following summary comments:
• MPEs in the UV depend on cumulative exposure. For example, the MPE is just 3 mJ/cm2 between 180 and 300 nm for exposures from 10-9 to 1000 s.
• Eye and skin MPEs are the same in the UV (and in the far-IR, beyond the retinal hazard region) for exposures up to 1000 s. In the retinal hazard region, 400 to 1400 nm, eye MPEs are approximately 100 to 1000 times less than skin MPEs for a 1000-s exposure.
• MPEs in the UV are the same for coherent (laser) and incoherent sources.
• MPEs drop by a factor of 300 at wavelengths below 300 nm because photochemical effects become important – chemical bonds can break, resulting in cancer risk. UV-B radiation between 280 and 315 nm seems to have the most hazardous consequences.
• Skin injuries are less serious than eye injuries because vision impairment has much greater consequences. Many skin injuries tend to be self-repairing.
• Skin injuries also are much more likely to occur than eye injuries. This is because of the much larger surface area; also, skin personal protective equipment (PPE) can be neglected more than eye protection, and hands are generally closer to the laser beam than eyes are.
• Chronic skin damage from UV laser operations is not well-documented in the literature, but MPEs and injury thresholds are fairly well understood from studies with incoherent light sources, sunlight and more.
Skin injury and UV exposure incident reports
Sources of hazardous UV radiation extend beyond conventional laser systems and include diffuse and incoherent sources. Examples include plasma sources for spectroscopy research, collateral and plasma radiation from cutting and welding processes, and UV transilluminators. A consistent theme among reported UV skin and eye injuries with these sources is that personnel were not fully aware of the inherent UV hazards.
Overexposure can lead to skin and eye irritation or tissue damage. The most common reaction is thermal “sunburn” (erythema) that can exhibit delayed symptoms of several hours or more. Long-term effects from chronic exposures are possible, and laboratory workers need to be concerned about skin cancer risks, as UV radiation can photochemically break bonds.
An increasingly common source of UV overexposure is the UV transilluminator found in life sciences laboratories. This device typically is used to view gels containing DNA samples. They are equipped with clear plastic safety covers to attenuate the UV light, and come with explicit instructions for skin and eye PPE measures.
Nevertheless, UV transilluminator overexposure incidents are occurring. In 2006, a Department of Energy employee received first-degree sunburn symptoms to the face after working on a transilluminator with the lid open for only three minutes. In this incident, the worker wore UV-protective eyewear and latex gloves but no protective face shield. Subsequent measurements indicated UV exposure levels of 200 μW/cm2 at a distance of
18 inches from the source without the protective lid in place, and a level of <0.02 μW/cm2 at the lid position with the lid in place.
A similar incident at a DoE lab occurred in 2010 when an intern donned protective gloves, a full face shield and laboratory coat while again working with a UV transilluminator. After an estimated 30-minute exposure time, “sunburn” conditions developed on the lower neck area left exposed by the face shield. Subsequent measurements indicated that the UV intensity was ~20 μW/cm2, for which the maximum permissible exposure time is only 2.5 minutes.
In 2011, several university undergraduate students received corneal eye injuries while working with UV transilluminators in an introductory science laboratory. Some of the students performed work without the protective lid in place, while others performed work without protective UV goggles. Fortunately, after hospital treatment, the resulting eye injuries healed without chronic symptoms. The root cause of the incident was identified as lack of instructions in the lab manuals on the use of the protective lid or skin/eye safety equipment.
Skin injuries are also a concern with high-power industrial lasers. In 2011, at a DoE lab, a technician received second-degree burns to the hand while inserting a target in a Class 4 CO2 laser beam during a maintenance procedure; the technician thought the laser beam had been disabled. The technician was performing the task from memory rather than following a written procedure and did not perform a zero-energy verification.
Class 3B, Class 4 UV laser radiation
Laser personnel must be aware that high-power lasers have significant potential to be a skin as well as eye hazard.
If laser beam paths are open, PPE for the eye and skin provide primary protection against accessible diffuse reflections. Eye PPE should be mandatory whenever Class 3B or Class 4 laser beams may be accessible. Additional guidance for eye and skin protection include engineering controls (enclosures and barriers), administrative procedures, PPE for skin, medical exams and site-specific training (see sidebar).
Meet the authors
Dr. Michael Woods is the laser safety officer at the SLAC National Accelerator Laboratory; email: [email protected]. Dr. Jeff Corbett is an accelerator physicist, laser system supervisor and deputy laser safety officer at SLAC; email: [email protected]. This work is supported by the U.S. Department of Energy under contract number DE-AC02-76SF0.
Reference
This article is adapted from a paper presented at the 2013 International Laser Safety Conference, SLAC-PUB-15357, http://slac.stanford.edu/pubs/slacpubs/15250/slac-pub-15357.pdf.
Additional guidance for Class 3B or Class 4 UV laser beams
Engineering controls
(enclosures and barriers):
• Enclose laser beam paths to the extent practical. If the radiation cannot be enclosed, implement adequate barriers to minimize potential exposure to the direct beam and to beam losses and other sources of diffuse reflections.
• Beam dumps. Design barriers or enclosures for beam dumps to minimize potential exposure to diffuse reflections.
Administrative procedures:
• Attenuate the laser beam to the minimum power required, when there are open beams.
• Use remote steering controls and diagnostics for alignment as often as practical.
• Plan work to minimize time with potential skin exposure to hazardous beams.
• Keep exposed skin as far from open beams as practical.
PPE for skin:
• Wear long-sleeved shirts.
• Use gloves when working with hands near accessible laser beams (direct beam exposure hazard for primary or stray beams).
• Use gloves when the diffuse reflection NHZ is greater than 20 cm if hands may be within this distance of an open beam path when diffuse reflections may not be well-shielded. (Nitrile gloves work well for UV protection from diffuse reflections.)
• Use face shields when the diffuse reflection NHZ is greater than 1 m if working within this distance of an open beam path when diffuse reflections may not be well-shielded.
Medical exams:
• Periodic skin exams are recommended for laser personnel who may have chronic exposures exceeding MPE values.
Site-specific training:
• On-the-job training and the standard operating procedures need to describe the potential for skin injury and the controls to use. They must emphasize barriers and enclosures for UV beams, and when to use skin PPE.