Page 143 - Environmental Control in Petroleum Engineering

130 Environmental Control in Petroleum Engineering

The hazards of noncarcinogenic substances are evaluated relative
to an allowable daily exposure level called the reference dose. The
reference dose is the maximum daily dose of a substance to which a
human may be exposed and not be adversely affected. In most cases,
this dose is based on nontoxic exposure levels in animals that are
extrapolated to humans with safety factors. This method assumes that
exposures have a threshold below which no adverse effects will occur.
Carcinogenic substances are evaluated using a model for the prob-
ability of a human developing cancer. Either animal or human data
(when available) are used in developing the probabilities. These
substances are normally assumed to have no threshold levels, and all
effects are extrapolated to zero exposure levels. Regulatory agencies
establish quantitative limits for exposure that are based on the pro-
jected "excess cancer risk" caused by exposure to individual sources.
Cancer risk is typically estimated for a lifetime exposure (70 years)
and is expressed as a probability of developing cancer within a lifetime
as in the term "one chance in one million." The U.S. Environmental
Protection Agency has developed dose-response relationships for many
compounds, but their values should be critically reviewed before being
used because they change as new data become available.

Exposure Assessment determines the extent of potential human exposure
to any emitted substances. Its goal is to accurately estimate both the
dose that reaches the person (the administered dose) and the dose that
reaches the target tissue within the body (the target dose). It quantifies
all potential transport routes for each substance, e.g., groundwater or
airborne transport, and considers three types of exposure—ingestion,
inhalation, and dermal (skin) adsorption. Human exposures are reported
as maximum daily doses for noncarcinogens and lifetime average daily
doses for carcinogens.
Exposure assessment includes characterizing the emissions, model-
ing dispersion of the emissions, and quantifying the resulting exposures
from each pathway. It estimates the probable magnitude, duration,
timing, and route of exposure and the size and nature of the population
exposed, and provides uncertainties for these estimates.
A critical part of exposure assessment includes working with the
relevant regulatory agencies to ensure that the proper type of tests and
measurements are conducted. Unfortunately, there are few standard expo-
sure evaluation methods for most substances. The U.S. Environmental