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 Using
ecotoxicity data
 How Are Ecotoxicity
Tests Conducted?
How Are LC50 Numbers
Used?
How
Are Ecotoxicity Tests Conducted?
Acute toxicity of a pesticide
is determined by exposing an organism to a measured dose of pesticide
for a specified length of time (the study time), and noting the
dose that causes a measurable effect (usually death or some other
endpoint that can easily be measured) in a predetermined fraction
of the organisms (usually 25% or 50%) within the specified study
time. This dose is expressed in one of several ways.
The LD50 (LD25) is the dose of the pesticide
in milligram (mg), microgram (ug), or nanogram (ng) of pesticide
per kilogram (kg) of body weight that is lethal to 50% (25%) of
the test organisms within the stated study time. Units used are:
ppm (mg/kg), ppb (ug/kg), and ppt (ng/kg). For bees, the dose is
given in microgram per bee (see definition of conversion factors between one unit and
another).
The
LC50 (LC25) is defined as the amount
of pesticide present per liter of aqueous solution that is lethal
to 50% (25%) of the test organisms within the stated study time.
Units used are mg or ug of pesticide per liter of solution. Units
used are: ppm (mg/L) and ppb (ug/L) (see definition of conversion factors between one unit and
another).
The EC50 is the effective concentration
of the pesticide in mg/L or ug/L that produces a specific measurable
effect in 50% of the test organisms within the stated study time.
The measurable effect is lethality for zooplankton and a reduction
in photosynthetic activity by 50% for phytoplankton. Similarly,
the EC25 is the effective concentration
that produces a specific measurable effect in 25% of the test organisms
within the stated study time. (See definition of conversion factors between one unit and
another).
The NOEL (NOEC) is the "no observed effect
level (concentration)," or the level below which, no adverse effects
are observed. Note that this level depends strongly on the sensitivity
of the techniques used to measure the effects.
The LOEL (LOEC) is the "lowest observed effect
level," or the lowest level (concentration) at which adverse effects
are observed. Note that this level (concentration) depends strongly
on the sensitivity of the techniques used to measure the effects.
The MATC is the "maximum acceptable
toxicant concentration" and is a hypothetical threshold concentration
that is the geometric mean between the NOEC and LOEC concentration.
For aquatic organisms (fish,
zooplankton, and phytoplankton), tests are carried out using either
static (S) or flow-through (F) methods. In the static method,
the pesticide and test organisms are added to the test solution
and kept there for the remainder of the study time. In the flow-through
method, a freshly prepared, pesticide-spiked test solution flows
through the test chamber continuously for the duration of the test.
The flow-through method provides a higher continuous dose of the
pesticide; however, the static method does not remove waste products
and may accumulate toxic pesticide breakdown products and metabolites.
Neither method exactly mimics a natural system. For birds, the pesticide
is administered either orally or in a certain concentration in food.
Because there is variability
in susceptibility to pesticides among individuals due to age and
genetic makeup, as well as variability between laboratory test methods
used to measure toxicity endpoints, the toxicities in the database
should not be viewed as a precise measurement; rather, as an approximate
concentration (or dose) that causes the observable adverse effect.
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How
Are LC50 Numbers Used?
The LC50 values given in
the database are generally used as comparison values to actual concentrations
found in the environment. If the environmental value exceeds the
laboratory value known to cause harm, it is generally agreed that
adverse effects are likely to occur to organisms in the environment.
The limitations of this assumption are:
- The species used in laboratory
tests are frequently not the same as those found in the environment,
and
- Conditions in the environment,
particularly the aquatic environment, may either accentuate or
attenuate the effects of the pesticide. Factors that may contribute
to such differences are: pH, temperature, alkalinity, salinity,
dissolved oxygen, and the presence of suspended sediment, dissolved
minerals, and/or other pesticides besides the one in question.
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Last updated
March 7, 2002
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