Risk Assessment — Roof Tar

Risk Assessment - Roof Tar

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 = 2pr 3 /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 M 3 ).

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 10 6 /760 mm Hg) is 6,645 ppm at saturation.  One part per million (ppm) of Stoddard Solvent is equivalent to 5.8 mg/M 3 .  Thus, at saturation, 6,645 ppm would be 38,541 mg/M 3 air.  This same mass of Stoddard Solvent, when spread out into a hemisphere of volume 59.3 M 3. produces a concentration of 650 mg/M 3 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 M 3 ).  Pulling 0.54 M 3 of air having 650 mg/M 3 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/M 3 air, expressed as a time-weighted average (TWA-8) over an 8 hour (480 minutes) period.  An exposure of 10 minutes to 352 mg/M 3 results in a time-weighted average of (352 x 10/480) or 7.3 mg/M 3 (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/M 3 to 35 mg/M 3 which is still well above the calculated 7.3 mg/M 3 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.

1298 Noguchi Mews


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