Page 86 - Environmental Control in Petroleum Engineering

The Impact of Drilling and Production Operations 73

Table 3-1
Exposure Types

Exposure Type Duration of Exposure
Acute Less than 24 hours
Subacute Less than 1 month
Subchronic 1-3 months
Chronic More than 3 months

sublethal or long-term effects of the tested substances. For sublethal
effects, a related toxicity parameter, EC 50 can also be used. This
parameter is the concentration that would result in adverse effects in
test species after an exposure of a specified duration. Few data are
available on EC values, however, because they are difficult and
expensive to quantify. A related sublethal toxicity parameter is the
NOEC (no observable effect concentration), the concentration at or
below which no effects are observed.
Another limitation to bioassays is that they are conducted in a
laboratory and do not necessarily represent the field conditions that
would actually be encountered if exposure occurred. Field conditions
normally involve different concentrations and different mixtures of
potentially toxic materials. Because regulations are normally based on
laboratory data, these differences can lead to regulations not reflecting
the actual risks in the field.
A third difficulty with laboratory bioassays is that they do not
provide adequate information about chronic effects, including the
mutagenic or carcinogenic activity of a substance. Bioassays normally
consist of exposing the test animal to a single, high-level dose of the
substance in question. Such acute exposure may not induce tumors in
the test animal, even if a chronic exposure of the same substance and
same total dose could. Such time-dependent responses have been
observed with poly aromatic hydrocarbons.
The problem with measuring mutagenic or carcinogenic activity
with bioassays is that such activity takes time to appear and be
identified. Many substances, called mutagens, can alter the structure
of DNA molecules in individual cells. Most mutations result in the
death of the individual cells affected, with no reproduction of the
mutation. If a mutated cell survives and results in future birth defects,