EH-16 Letters About Laser Hazards
ENVIRONMENT, SAFETY & HEALTH
BULLETIN
Assistant Secretary for U.S. Department of Energy
Environment, Safety, & Health Washington, D.C. 20858
DOE/EH-0012 Issue No. 16 June 1986
LETTERS ABOUT LASER HAZARDS
Many readers expressed particular interest in Serious Accident bulletin.
Issue No. 11, "Improper Laser Safety Glasses Key Factor in Serious Eye
Injury." Following are four of the letters we received.
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Key Factor in Accident Was Failure
to Follow Standard Safety Procedures
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To the Editor:
Hanford Environmental Health Foundation recently received DOE "Serious
Accidents" Bulletin No. DOE/EH-0007, Issue No. 11 dated January 1986. This
bulletin contained a summary of information concerning a recent laser accident
in a DOE laboratory. The report suggested that a major factor in this
accident was the plastic laser safety goggles which the researcher was wearing
when the accident occurred. The bulletin also suggested that glass laser
safety goggles would have prevented the eye injury and recommended that glass
safety goggles be used when working with high-powered lasers.
The DOE bulletin did not contain enough information to determine if a hazard
evaluation had been conducted on this particular laser experiment prior to
use, or if direct intrabeam viewing of the laser was a predetermined
requirement. The bulletin also did not state if written laser safety
procedures were developed or if other aspects of laser safety contained in
ANSI Z136.1-1980 were complied with.
HEHF provides technical assistance to DOE/Richland contractors for laser
safety. We have never observed a reasonable requirement to directly view a
coherent laser beam, nor would we ever recommend using protective goggles for
direct viewing of a high-powered laser beam. The glass laser goggles do
provide some added measure of protection for reflected beam exposures to
infrared lasers, but would offer negligible additional protection for direct
beam viewing of kilowatt or megawatt lasers. The resulting eye injury from
this specific laser accident might have been reduced by the use of glass
safety goggles. However, this conclusion should not be applied to all
high-powered laser systems.
We conclude from our review of this accident bulletin that the key factor in
this accident was failure to comply with laser safety procedures specified
in ANSI Z136.1-1980. It is recommended that a qualified person be assigned as
a Laser Safety Officer (LSO) at all DOE laser facilities with Class III or IV
lasers and that the LSO conduct a hazard evaluation of all high-powered laser
applications prior to energizing of equipment. A comprehensive laser hazard
evaluation includes:
1. Classification of the laser or laser system or verification of labeled
classification.
2. Evaluation of the environment in which the laser will be used.
3. Determination of required laser beam access and beam viewing requirements.
4. Calculation of the applicable Maximum Permissible Exposure Limits (MPELs).
5. Development of engineering control measures and written safety procedures
for Class III and IV lasers.
6. Calculation of appropriate optical density of protective goggles, if
required.
7. Identification of employees exposed to laser radiation and recommendations
for appropriate medical examination.
S.R. Coleman, Manager
Engineering and Special Projects
Hanford Environmental Health
Foundation
Richland, April 22, 1986
Editor's Note: Since, in this accident, the injury probably would have been
less severe had filter-glass eye protection been worn, we titled the article,
"Safety Glasses Key Factor in Serious Eye Injury." (Of course, even
filter-glass safety glasses would have failed had the researcher continued to
look into the beam long enough.) We agree with Mr. Coleman that the key
factor in the accident itself was the researcher's failure to follow standard
laser safety procedures. Filter-glass was recommended as extra protection
against just such human error.
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Limitations of Plastic Laser Goggles Documented
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To the Editor:
Thank you for sending a copy of the Serious Accidents report on the laser eye
injury. It certainly is a horror story, one that of course should never have
happened since laser protective eyewear should not be considered to be a
primary protective measure in laser safety....
In discussing this incident and the report with the members of our Laser
Safety Committee, I find an indication contrary to the statement in the
report that to the knowledge of the report writer the limitations of plastic
lens goggles in high peak power and high average power beams were not
documented. One of our committee members indicates that Bureau of
Radiological Health has conducted tests that document the failure mechanisms
in such eyewear and have cautioned against the use of plastic absorbing type
filters in high power density, high peak, and high average power
applications. He tells me that a reference to this work is in R. L. Elder,
Applied Optics, vol. 13, p. 725, 1974. He also tells me that there is
reference to the need for specially constructed filter glass laser protective
eyewear in the article "Laser Protective Eyewear," U.S. Army Environmental
Hygiene Agency, Aberdeen Proving Ground, Maryland, 1975, and also cites a
reference "Evaluation of Commercially Available Laser Protective Eyewear
(FDA-BRH)," available as NTIS Publication PB80-103039, 1979.
A.S. Garrett, Jr., M.D.,
Medical Director
Oak Ridge National Laboratory
Oak Ridge, March 13, 1986
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Guide from Laser Institute Available
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To the Editor:
I read your Serious Accidents, Issue 11 with interest. I wish to raise a few
points: Some plastic formulations are indeed suitable protection from
specified laser exposures; and no manufacturer will specify the power-, or
energy-density that filter media will withstand without degradation. There
are too many variables. Also, laser eyewear is intended as protection from
accidental exposures, not deliberate viewing.
For a good discussion of the above, may I suggest "Guide for the Selection of
Laser Eye Protection" available from the Laser Institute of America, 5151
Monroe St., Toledo, Ohio, 43623.
T.E. Ehrenkranz
Los Alamos National Laboratory
Los Alamos, January 27, 1986
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Don't Forget Electrical Shock Hazards
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To the Editor:
In addition to the eye injury hazards described in Issue No. 11 (January
1986), laser equipment also presents serious occupational hazards to workers
from exposure to high voltage currents. Although many safety controls are
incorporated in the design of laser equipment and electrical safety
guidelines have long been established for lasers, accidents still occur. The
most commonly used lasers are high-powered units, classified as Class IV
lasers, which have high voltage (10 to 30 kV) power supplies and storage
capacitors that can produce lethal electrical shocks.
These devices are normally placed in secluded areas, locked in cabinets
isolated from the operators. Installation and maintenance personnel,
however, may be exposed to voltages sufficiently high enough to create fatal
currents. AC currents just above 6 mA produce painful shock. The internal
body resistance is between 200 and 500 ohms, and contact resistance may vary
between 1000 and 100,000 ohms. Assuming the high resistance value and a
current threshold level of 6 mA, it takes a voltage of 600 V to produce
dangerous currents. A laser power supply carrying a voltage of 30 kV will
create dangerous currents (>6 mA) in the human body when the contact
resistance is lower than 5 x 10-to-the-6 ohms. Obviously, even touching the
terminals of the power supply with insulating material can be dangerous if the
insulator does not provide the required resistance.
In capacitor bank discharge (pulsed electrical discharge), electrical energy,
not current, determines the physiological effect of a shock. A discharge of
only 50 J may cause ventricular fibrillation; a capacitor bank usually
contains energy in the range of 3 to 25 kJ.
A review of laser accident reports indicates that in the case of at least
four fatal accidents, electrical safety guidelines were not strictly followed
(see 1-3 below). In one case the power supply safety covers were not in
place. In another case, OSHA noted that the industry where the accident
happened failed to provide a remote mirror adjustment system for the laser's
mirrors, which bear up to 25 kV potential when the laser is in operation.
To help protect workers from electrical shock hazards, the following
administrative and engineering safety options are suggested.
1. Conduct laser safety seminars at the beginning of employment and annually
thereafter.
2. Inspect frequently for proper operation of controls and interlocks.
3. Limit access to laser power supply sources to authorized persons.
4. Do not service operating lasers in humid or contaminated atmospheres
since humidity decreases body resistance and both dust and humidity
increase electrical conductivity.
5. Exercise extreme caution during calibration, adjustments, and other work
performed on an operating laser; use appropriate (e.g., remote control)
devices.
6. Design and position laser cabinet interlocks so that accidential bypass
cannot occur.
7. Incorporate shut-off devices in laser equipment which function in case of
severe mechanical shock.
8. Design power supply circuits to prevent accidental capacitor discharge.
9. Inspect and maintain cables, connectors, cabinets, and switches.
Maintenance and repair services should take place only with the equipment
shut down and the capacitors totally discharged.
10. At shutdowns, operators should not leave the equipment until all power is
removed from the capacitors as indicated by a zero voltage reading on the
meter. Similarly, before they begin working on power supplies,
maintenance workers should confirm that capacitors have been discharged by
reading the voltage meter.
11. Covers should be placed over high voltage circuits to prevent access to
energized components. All components that do not normally carry current
should be grounded.
References
1. Sliney, D. and Wobarsht, M., Safety with Lasers and Other Optical
Sources, Plenum Press, New York, 1982.
2. Laser Focus. "Post Deadline Reports," p. 4, June 1983.
3. Laser Focus, "Post Deadline Reports," p. 4, November 1982.
4. Lasers and Applications, "Late News," p. 4, December 1982.
Paul D. Moskowitz,
Environmental Health Scientist,
and Vasilis M. Fthenakis,
Research Engineer
Biomedical and Environmental
Assessment Division
Brookhaven National Laboratory
Upton, May 8, 1986
Share Your Knowledge and Experience
We appreciate hearing from our readers and encourage them to write. Send
letters to: Nona F. Shepard, Office of Operational Safety, EH-34, Assistant
Secretary for Environment, Safety, and Health, U.S. Department of Energy,
Washington, D.C. 20545.
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Bulletin is published so that DOE program managers and contractors can share
information about potential occupational safety problems relevant to DOE
operations. For more information or additional copies, contact Nona Gilbert
Shepard, Editor, Office of Operational Safety, Assistant Secretary for
Environment, Safety & Health, U.S. Department of Energy, Washington, D.C.
20545, telephone FTS 233-2958; Commercial (301) 353-2958.