By: Vince McLeod, CIH, Glenn Ketcham, CIH
Issue: Winter 2004
The toxicological effects of chemicals can manifest themselves in a number of
ways. The results of exposure from some materials can be felt immediately such
as watering of the eyes with lachrymators.
With other chemicals, there may be both immediate effects such as irritation
and delayed effects such as pulmonary edema. With carcinogens there may be
no symptoms yet effects are seen decades later. Effects of teratogens and
mutagens
are not seen until the next generation. With the thousands of chemicals that
can be encountered and the different effects how does one go about determining
what is a safe exposure?
OSHA Compliance – A Minimum Standard
Exposure concentrations and limits in air are typically given in parts per
million (ppm), milligrams per cubic meter (mg/m3), or fibers/cm3 (fibers per
cubic centimeter). Where additive exposure potential exists via absorption
through the skin, a "[skin]" designation appears with the exposure
limit.
The OSHA permissible exposure limits (PELs) (29 CFR 1910.1000) are typically
the least restrictive exposure values and serve as a minimum performance standard
in the United States. It should be noted that when PELs are established, it
is a political process mixed with scientific evidence. Economic factors related
to compliance are presented by industry groups and influence the final selection
of exposure limits as a compromise value. OSHA limits may also lag behind new
scientific literature as the entire political rulemaking process must be followed
to make changes to a PEL. PELs are typically time-weighted average concentrations
that must not be exceeded during any 8-hour workshift. Short term exposure
limits (STEL) have been established for some materials and are usually measured
over a 15-minute period. OSHA ceiling concentrations must not be exceeded during
any part of the workday.
The American Conference of Governmental Industrial Hygienists (ACGIH) Threshold
Limit Values (TLVs) are also expressed as time-weighted averages and short
term limits. These are determined by ACGIH committees of experts in public
health and related sciences through review of existing scientific literature.
TLVs are based only on health factors and not subjected to a political process.
The TLVs can more rapidly adapt to new scientific information than OSHA PELs.
Most health and safety professionals we have known rely on the more conservative
ACGIH TLVs as minimum protective standards for their clients.
However, TLVs and PELs are not available for most chemicals. The absence of
an exposure limit does not mean a chemical is “safe”; it means
that the chemical has not gone through the rigorous review and standards setting
process. In the absence of exposure limits, the LD50 and LC50 along with physical
parameters such as vapor pressure can help establish the risk of working with
a chemical.
We have found the Poison Information Centers and Teratogen Information Service
excellent sources of information when standard literature searches yield poor
or equivocal results. The professional staff at these centers willing to help,
however, please let them know this is not an emergency and that you are willing
to wait a few days for an answer. We would caution not to request assistance
except when your efforts fail to find results by conventional means or of course
when you have an actual exposure event.
There is a term “ALARA” taken from the nuclear industry. It stands
for “as low as reasonably achievable.” This is a good mantra. Compliance
with OSHA is a legal obligation; meeting ACGIH guidelines can help limit liability
further. But reducing levels as low as one can using reasonable means makes
good sense, further limits potential liability, and helps create a more pleasant
and possibly more productive workplace.
So, at this point we have identified the chemicals we will be using; have
consulted sources of chemical safety information; have an idea of the possible
routes of exposure of concern for the particular chemicals; the types of biological
effects (e.g. simple toxin, sensitizer, carcinogen, etc.); and the relative
hazard of the materials. Next we must select the appropriate procedures to
minimize exposures and prepare for contingencies.
Hierarchy of Protective Methods
There are a variety of means to protect oneself from hazardous chemicals.
These include in order of preference: substitution, engineering controls, administrative
controls, and personal protective equipment. In fact, in terms of airborne
exposures, OSHA requires engineering controls for controlling exposures except
when it can be demonstrated to be infeasible.
Substitution is the most effective method of hazard control. A less hazardous
or relatively harmless material is substituted for a hazardous chemical. An
example would be the use of one of the new glass cleaners as a substitute for
Chromerge (an oxidizer containing hexavalent chromium, a probable human carcinogen).
By using a different material the hazard is largely eliminated.
Engineering controls are the next preference when use of the hazardous chemical
cannot be eliminated. One example most commonly used in the lab is the use
of differential air pressure to control exposures to airborne contaminants.
Where there is the potential for airborne contamination, a chemical fume hood
is a very effective first line of defense. The contaminants generated in the
hood should be retained in the hood, drawn away from the worker, and exhausted
up the stack.
Fume hoods are not foolproof however, and we have seen many exposures to materials
used in hoods. These exposures usually stem from misuse or hoods that have
been compromised in some fashion. The hood should be set up with the work at
least six inches back from the sash. The back baffles (slots) through which
air flows should not be blocked. We have often seen the entire back bottom
slot blocked by reagent and sample bottles. The airfoil on the bottom leading
edge of the hood is also frequently blocked by users placing “benchcoat” (absorbent
pads) on the bottom and taping it to the edge. The proper functioning of the
hood requires airflow passing around the items in the hood and passing directly
out the back slots. Overloading the hood can cause eddies and back drafts allowing
chemicals to escape.
We have also seen many hood users with the sash fully raised. This not only
removes a line of defense against explosion but also reduces the hood face
velocity to a point where simple room currents blowing by the hood may aspirate
contaminates into the open room. The face velocity V= (Q)/(A) where Q is
the volume flow (a constant) and A is the face area of the hood. You can
see, as
the area increases (raising the sash) the velocity decreases. Newer hood
systems are often variable volume systems that maintain a relatively constant
face
velocity regardless of sash position.
The use of administrative controls to prevent chemical exposure is fairly
uncommon in laboratory settings though these may include posting signage
warning of
hazards such as areas where fume hoods exhaust at roof level or limiting
staff participating in a procedure.
Personal protective equipment is widely used in the laboratory and a thorough
discussion would require an article devoted to the topic. Some key points
to make, however, include matching the equipment to the chemicals and
the hazard
they present.
Glove compatibility charts should be consulted and selection made based
in part on chemical permeation resistance. We were involved in a case
where a worker was using an organic solvent with neurotoxic properties.
The worker
was convinced his fume hood was not operating properly and he was exhibiting
symptoms of overexposure. Upon investigation and observing the process
we
determined
that in fact the hood was working well but that he was wearing latex
gloves and his fingers were contacting the solvent. The solvent was
absorbed by
the glove material and actually increased his exposure by holding the
solvent impregnated
glove material against his skin.
Chemicals can be worked with in a safe manner, but to do so the worker
needs to understand the properties of the chemicals with which they
work and the
means to protect themselves. The development and use of the chemical
hygiene plan with standard operating procedures that consider the
hazards associated
with the use of chemicals and protective measures can go a long way
in preventing exposures in the laboratory. Diligence of all workers
in the
lab is necessary
however to protect not only themselves but those around them.
Useful reference information and links:
Prudent Practices in the Laboratory: Handling and Disposal of Chemicals.
Copyright 1995 by the National Academy of Sciences.
Threshold Limit Values for Chemical Substances and Physical Agents in
the Work Environment, American Conference of Governmental Industrial
Hygienists (ACGIH)
(latest edition).
Poison Information Center National Toll-Free Number: 1-800-222-1222
Teratology Information Service in your area, OTIS Information at: (866)
626-OTIS or (866) 626-6847
Questions and comments are always welcome. Email us at info@thesafetyguys.com.
Until the next issue remember – SAFETY FIRST!
Vince McLeod is a Certified Industrial Hygienist by the American Board
of Industrial Hygiene and the senior IH with the University of Florida’s
Environmental Health and Safety Division. He has 15 years of experience in
all facets of
occupational health and safety and specializes in hazard evaluation and exposure
assessments.
Glenn Ketcham is a Certified Industrial Hygienist with 20 years experience
in the health and safety field. He is currently the Risk Manager for the
University of Florida. He has worked as a USDOL/OSHA compliance officer
and has program
management experience in general OSHA compliance, laboratory and chemical
safety, workplace ergonomics, disaster preparedness, and classical industrial
hygiene
with 10 years direct experience in animal care areas.
