EAS Inc. Technical Reports (no charge)
Hewett, P. (2005): Technical Report 05–02 - Equations for Calculating
Exposure Management Objectives. Exposure Assessment Solutions, Inc. (www.oesh.com).
download pdf
ABSTRACT
The majority of exposure limits for gases, vapors, and particulates have as
their implicit or explicit goal the control of exposures for each exposed employee.
One measure of compliance with this goal - for a typical single shift, TWA exposure
limit (LTWA) - is the probability ( P) that a randomly
selected worker has a 95th percentile exposure greater than LTWA.
In principle, the goal of an exposure assessment program should be to determine
if P is small, say 0.05 or less. One method for determining if this goal has
been achieved is to directly estimate P through the application of expensive,
complex sampling strategies that require repeat sampling of randomly selected
workers, followed by a components-of-variance analysis. The purpose of this
paper is to present equations for calculating site specific exposure management
objectives that accomplish the same goal, but in principle require simpler strategies
and fewer resources to evaluate. These objectives can be calculated for both
single-shift, TWA exposure limits and the less common long-term average exposure
limits, and include (a) a target group exceedance fraction, (b) a target group
95th percentile, (c) a target group geometric mean, and (d) a target group mean.
The author suggests that each of these exposure management objectives can be
evaluated using off-the-shelf sampling strategies and robust data analysis procedures.
If the site-specific control objective is met, the overall goal of exposure
control for at least 95% of the employees is likewise achieved. Examples are
provided for single shift exposures limits, long-term average exposure limits,
and for dual limits, where both a single shift and long-term average limit apply.
One rule-of-thumb that results from this analysis is that the traditional single
shift, TWA exposure limits should be interpreted statistically as the 99th percentile
exposure, rather than the 95th percentile exposure as is recommended by various
organizations and authorities.
|
