Unformatted text preview: suitable laboratory model for exposure is
a continuous exposure of duration similar to total duration
of all the pulses observed in the field. Exposure Concentration If exposure is variable with time, then the exposure should
be integrated with time to mea sure the area under the curve
(Figure 3-5 D). The time unit used for this integration can
be chosen from appropriate durations of laboratory tests, i.e.
96-h acute LC50 or 21 -d chronic test. Effect C Concentration When refining the characterization of toxicity, it is
appropriate to consider the actual use pattern and physical
and chemical parameters of the substance i n relation to data
Time weighted mean concentration
generation. Exposure is a function of how much of the
compound is present in the environment and how much is
available to the organism. In the case of pesticides, these
factors are related to the use rate and application method,
physical and chemical properties of the substance, and
runoff and drift rates. An example of customizing a
standard acute toxicity test would be to add additional
exposure concentrations, time periods, or measures of effect.
Examples of relationships between
exposure duration and concentration
If the substance in question is used once per year and has a
half-life in water of 1 h, assessing mortality on a shorter time
scale (e.g., 4 h, 8 h, etc) in a 48 - or 96-h study would allow data to be created which compares
short-term exposure to short -term response.
126.96.36.199 RECIPROCITY Direct reciprocity, the
LC50 on day 2 is half
that on day 1 etc. 4 Toxicity (LC50) The degree of response to a substance is a function of
duration and intensity (concentration) of exposure,
especially in situations where organisms are expose d
through their surrounding medium.
concentrations of toxicant, shorter times of exposure
are required to generate a specific response, and vice
versa, and this is usually a reciprocal relationship. The
reciprocity between concentra tion of toxicant and time
to an effect can be used to describe toxicity curves.
Reciprocity is the result of the non -instantaneous
kinetics of uptake and distribution of the sub stance in 5 3 2 1 0 1 2 3 4 Time in days Figure 33
Illustration of direct reciprocity 5 organisms ( see Chapter 2). This ideal relationship of linear reciprocity is shown in Figure 3-6.
Some substances, such as organo phosphorus insecticides, show linear reciprocity. This is, in
part, due to the slow reversibility of the inhibition of the target enzyme, acetylcholinesterase ( see
Chapter 1, Section 1.7.5 ), when it is inhibited by organophosphorus inhibitors. In most other
cases, it is known that there is not linear reciprocity between the duration of exposure for pulsed
exposures (Giesy and Graney 1989). For instance, pulsed exposures to greater concentrations
may be less toxic than longer -term exposures to lesser concentrations (). This may be the result
of several processes. Uptake of the substance may be slow, thus reduc...
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