Risk
Assessment - Roof Tar
Exposure Reconstruction Case Study
Roof Tarring (Stoddard Solvent)
By: Henry P. Shotwell,
Ph.D., CIH, Senior Vice-President, Atlantic Environmental,
Inc.
Facts: a large retail chain
store had its roof re-tarred on a weekend near
Birmingham, AL. The contractor used Stoddard
Solvent to thin the tar for easier application.
The skies were clear, with a 5-10 mph wind. Temperature
was 83°F.
A photo development center was located under
a section of roof that was being re-tarred. It
had a photo developer machine which chronically
leaked glacial acetic acid. On the day in question,
it was leaking.
Two young girls in their early teens passed by
the photo department, noted a strange odor, and
went into asthmatic distress. The parents sued,
claiming the Stoddard Solvent used by the roofers
caused the girls’ attacks.
At the time of the incident, the roofers were
working in the vicinity of an air intake that
supplied air to the photo department. Equal numbers
of witnesses testified that the fan was on and
that the fan was off.
Approach: Review medical literature
for associations between asthma and exposure to
asthma. Estimate the maximum concentration of
Stoddard Solvent at the air intake, and the time
it took to move out of the immediate vicinity
of the air intake. Calculate the dilution effect
of bringing Stoddard Solvent from the roof to
the photo department.
Assumptions: The distance from
the air intake to the point where the presence
of Stoddard Solvent in the photo department could
be dictated was 10 feet before and 10 feet past
the air intake. The total residence time was
5 minutes and we assume the maximum concentration
was achieved immediately and remained at that
level for the 5 minutes in which it would be detected.
Calculations: The volume of
a hemisphere (V = 2pr3/3) having
height (radius) of 10 feet is assumed to cover
the area within which roofing operations could
produce a detectable odor in the store below.
The calculated volume is 2,094.4 cu. Ft. (59.3
M3).
The vapor pressure exerted by Stoddard Solvent
at 83°F is approximately 5 mmHg. Thus, at 83°F,
the maximum concentration that can be produced
(5 mm Hg x 106/760 mm Hg) is 6,645
ppm at saturation. One part per million (ppm)
of Stoddard Solvent is equivalent to 5.8 mg/M3.
Thus, at saturation, 6,645 ppm would be 38,541
mg/M3 air. This same mass of Stoddard
Solvent, when spread out into a hemisphere of
volume 59.3 M3, produces a concentration
of 650 mg/M3 air.
The air intake system leading from the roof to
the air supply grating in the ceiling of the photo
department consists of 35 feet of 10-inch diameter
duct. The volume of this run of duct is (duct
area x length) 35 ft x 0.5454 square feet, or
19.1 cubic feet (0.54 M3). Pulling
0.54 M3 of air having 650 mg/M3
through the system yields a concentration of 352
mg Stoddard solvent per cubic meter of air. This
concentration would have been present over a 10
minute window of time.
The current OSHA Permissible Exposure Limit for
Stoddard Solvent is 350 mg/M3 air,
expressed as a time-weighted average (TWA-8) over
an 8 hour (480 minutes) period. An exposure of
10 minutes to 352 mg/M3 results in
a time-weighted average of (352 x 10/480) or 7.3
mg/M3 (1.26 ppm).
Discussion: All assumptions
and calculations assumed worst-possible case scenarios.
The approach and retrocession over time, of a
source of contamination with respect to a fixed
point would describe a graph similar to a bell-shaped
curve or a parabola. We assumed a graph which
is described by a rectangle. That is, instead
of considering the contaminant concentration to
rise from zero to a maximum and then decreasing
to zero, we assumed an instantaneous rise to the
maximum during the entire exposure duration.
This maximizes the estimates.
Even maximizing the concentrations failed to
produce a TWA that approached the OSHA PEL. Granted
that OSHA PELs pertain to healthy adults, they
are nonetheless based on human physiologic responses.
Factoring in a safety margin of 10 would lower
the PEL from 350 mg/M3 to 35 mg/M3
which is still well above the calculated 7.3 mg/M3
by a factor of almost 5.
The medical literature did not report asthmatic
attacks as associated with human exposure to Stoddard
Solvent. We concluded that Stoddard Solvent was
probably not the causative agent.
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