Page 85 - Environmental Control in Petroleum Engineering

72 Environmental Control in Petroleum Engineering

(Pimephales promelas) and daphnid shrimp (Ceriodaphnia dubia) are
used for fresh waters.
Two types of toxicity measurements are commonly used: dose and
concentration. The dose is the concentration of a substance that has
been absorbed into the tissue of the test species, while the concentration
is a measure of the concentration of a substance in the environment
that the species lives in. Toxicity measurements using concentration
also include a time interval of exposure.
The dose is the mass fraction of the substance in the animal tissue
(milligram of substance per gram of tissue, mg/g) when a particular
effect has been observed. A dose that is lethal to 50% of the animals
is called LD 50, while the lowest dose that is lethal, i.e., the dose
resulting in the first death, is called LDLO. The dose levels required
for any particular effect also depend on how the animal is exposed—
by injection, ingestion, or inhalation.
The concentration is the fraction of the substance in air or water
that causes a particular effect when the target animal is placed in that
environment. It is normally given either as a mass fraction in parts
per million (ppm) or as mass per unit volume in milligrams per liter
(mg/1). A lethal concentration that kills 50% of the animals within a
given period of time is called LC 5(). Similarly, the lowest lethal
concentration for the same period of time is called LCLO. Concen–
tration is the toxicity measure most commonly used for materials
associated with the petroleum industry.
If a material is highly toxic, then only a small concentration will
be lethal and the numerical values of the lethal doses and concentrations—
LD 50, LDLO, LC 50, and LCLO—would be low. Conversely, a high
value of these parameters indicates low toxicity. LC 50 values on the
order of 10 are normally considered highly toxic, while values on the
order of 100,000 are considered nontoxic. The length of exposure to
a substance can be divided into descriptive types, as indicated in
Table 3 -1. Exposure that causes an immediate effect is called acute,
while repeated, long-term exposure is called chronic.
There are a number of significant limitations to bioassays for
toxicity testing. These limitations must be considered when
new regulations are being considered or laboratory test protocols are
being developed.
One limitation to most bioassay testing for toxicity is that the tests
yield only acute lethal concentrations. They provide no data on the