When someone becomes ill and it’s not something that
can be easily traced to a source, the question always
arises—where did this come from? For example, a cold
or flu can generally be traced to an event or source—kids,
work, airline travel, etc. But, what if it’s cancer
or emphysema? These illnesses are known not to be caused
by a recent event but due to some event or series of
events over the distant past. They possibly can be caused
by genetics with no known environmental cause.
Attempting to sort out possible causes is a complex
task. First, a physician needs to define the condition
and suggest possible causes—Genetics? Life style?, Living
environment, Work place? Next, the individual must consider
the past to see if anything that could be a cause actually
occurred. If there is a possibility that the environment
or the work place could have been a source, then an
industrial hygienist, a toxicologist or an epidemiologist
must try to determine if there is a link between the
disease or condition—and possible sources of exposure.
The next step would be dose reconstruction. That is—was
the past event or exposure a possible cause?
If the condition or illness could have been caused
by the agent in question, then it must be asked if the
amount of agent present at the time of exposure was
sufficient to have caused the symptoms. Almost invariably,
no measurements of the agent’s concentration in the
breathing zone air had been made, and no data are available
for evaluation. Sometimes, the exact conditions of the
exposure can be reproduced and air samples collected
to be measured. Often, the literature reports levels
at which certain symptoms can be expected to appear
in an exposed group. If the lab data show much lower
levels of the agent in the breathing zone air than the
levels that typically are associated with the complaint
symptom, then it’s a fairly safe bet that the symptoms
were not caused by the agent in question.
What if the exposure conditions can’t be reproduced,
sampled and evaluated? Then, we may need to rely on
some mathematics to help us estimate what the exposure
may have been. Let’s take a hypothetical example. In
a room of known dimensions, some solvent vapors are
released into the air. An employee was in the room for
a known period of time, breathing the solvent containing
air. We also have a good estimate of the amount of solvent
that had been spilled and some of its physical and chemical
properties. We also know if the room was ventilated
mechanically, and the rate and volume of dilution air
that was introduced. From this information, we can begin
to estimate the concentration of the solvent in the
room air, and from that, the probable exposure dose.
Before launching into lengthy and involved calculations,
it is almost always profitable to make some simple calculations
to define the “worst case” scenario. Let’s assume (by
way of illustration) that we spilled a 1.5 fluid ounce
bottle of nail polish remover (acetone) onto the floor
of a room whose dimensions are 14 feet by 18 feet by
8 feet. The volume is therefore 2,016 cubic feet (57.1
cubic Meters, M3). One-and-one-half fluid
ounces is equal to 44 milliliters (mL) of acetone. The
worst case condition is that all 44 mL of acetone are
instantaneously evaporated into the 57.1 M3
room, producing a uniform concentration of acetone throughout
the room. Acetone weighs 790 milligrams Lmg) per mL
of liquid. So, 44 mL of liquid acetone weighs 34,760
mg, which is instantly converted to a vapor, producing
a concentration of 34,760/57.1M3 (609 mg
acetone per M3 of air). This can also be
expressed as 256 parts per million (ppm). The current
Occupational Safety and Health Administration’s (OSHA)
Permissible Exposure Limit (PEL) averaged over any 8-hour
period of exposure, is 1,000 ppm (2,400 mg/M3).
Since the maximum concentration of acetone that could
have been produced was 256 ppm (609 mg/M3),
which is about ¼ of the OSHA PEL, it’s probably unlikely
that our employee, even if in the room for a full 8
hours, had enough of an exposure to cause a headache.
If we take into consideration the diluting effect of
the ventilation system, then these levels would become
even lower.
Addressing such complex issues is no easy task but
a qualified professional—generally an industrial hygienist
or epidemiologist has the technical knowledge and resources
to do such calculations.
If further refinements to these initial calculations
are needed or desired, the American Industrial Hygiene
Association’s publication, “Mathematical Models for
Estimating Occupational Exposure to Chemicals” (AIHA
Press, Fairfax, VA, ISBN 0-932-627-99-4, Stock No. 379-BP-99)
is highly recommended. You can also contact us at info@atlenv.com.
