|
 Summary
ecotoxicity ratings
 Data Structure and Summary
Ratings
Methodology
for Determining Acute Summary Ratings
Data
Structure and Summary Ratings
There are three levels of
detail in the Ecotoxicity section of the PAN Pesticide Database.
Organism
Group Ratings:
Organism group summaries are presented on the Chemical Information
page in the Ecotoxicity summary section. This section provides a
quick overview of the toxicity of a particular chemical to different
types of animals and plants, summarizing the results of toxicity
tests by groups of similar organisms (amphibians, fishes, zooplankton,
etc.). The technique used to determine the acute ratings (using
mortality as the endpoint of the toxicity test) is outlined in the
Methodology section. For
the chronic and other studies (any endpoint except mortality),
the summary ratings only note the different types of effects observed.
Species Summary: This section gives an average
acute toxicity rating for the selected chemical for an individual
species within each organism group.
All Studies: This section provides information
for all studies conducted on the selected chemical, taken directly
from the U.S. EPA AQUIRE database.
Top of
page
Methodology
for Determining Acute Summary Ratings
Step 1: Calculation
of "Average Species LC50" for a particular chemical
The entire set of studies
for a particular chemical from the U.S. EPA AQUIRE database were
used as the starting dataset. Studies were included in the average
calculation if they met all of the following criteria:
- The study endpoint
was mortality, with the endpoint reported as an LC50.
NOTE: Since most phytoplankton studies use the EC50
as an endpoint, they are not included in the summary ratings.
- Results were given
in units that could be converted to ug/L
- The study reported
an average toxicity
endpoint. Those studies reporting only a range were excluded
at this time.
There are approximately
172,000 total studies reported in the U.S. EPA AQUIRE database.
After the above
criteria were applied, approximately 63,000 studies remained, from
which the acute summary toxicity ratings were obtained.
For each chemical/species
combination, the average LC50
value of all remaining studies was then calculated, along with the
population standard deviation. The group of studies averaged together
were for a single chemical/species combination and included:
- all life
stages
- all study
durations
- all study
types
- studies
of both the pure active ingredient (AI) and its formulations with
inerts (the AQUIRE database excludes studies conducted on mixtures
of AIs)
- studies
with any water quality parameters (pH, temperature, alkalinity,
salinity, etc.)
The validity
of this strategy of including studies with such a wide variety of
conditions into a single average value was carefully considered.
For most species, the variability of the study results under different
study conditions was less than an order of magnitude. Since the
narrative toxicity rating steps are based on orders of magnitude,
this approach is sufficiently accurate to assign a toxicity rating.
Elimination
of outliers further reduced the variability of the data.
Any LC50
value for a particular chemical/species combination that was more
than two standard deviations from the mean value was flagged as
an outlier, and a new mean was calculated excluding outliers to
give the Average Species LC50.
For those who wish to independently evaluate this assessment, the
raw data used to calculate the Species Summary (including outlier
status) is available on the "View All Studies" page.
Step 2: Calculation
of an "Average Organism Group LC50" for a particular
chemical
Using
the average species LC50
from Step 1, we calculated an average LC50
for the set of species comprising a particular group of organisms,
as well as the standard deviation. For example, in the Fishes group,
average LC50
values for trout, carp, bluegill sunfish, salmon, etc. were averaged
together to obtain the Average Organism Group toxicity.
Any LC50 value
for a particular chemical/species combination that was more than
two standard deviations from the mean value for that organism group
was flagged as an outlier, and a new mean was calculated excluding
outliers to give the average LC50 for that organism group.
This procedure eliminated particularly sensitive or particularly
insensitive species from the data used to generate the Summary Toxicity
Ratings; however, the Toxicity
Range includes the outliers. For those who are interested, all
species summaries used to calculate Organism Group Toxicity can
be viewed on the "Average Species Ratings" page, and include
outlier status.
Step 3: Assignment
of Average Acute Toxicity Ratings
Narrative descriptions of
toxicity were assigned based on LC50, according to the guidelines
in M. A. Kamrin, Pesticide Profiles: Toxicity, Environmental
Impact, and Fate, Lewis Publishers (Boca Raton, FL, 1997), p.
8 (see table below).
|
Toxicity Category
|
LC50 (ug/L)
|
|
Very highly
toxic
|
<100
|
|
Highly toxic
|
100-1,000
|
|
Moderately
toxic
|
1,000-10,000
|
|
Slightly
toxic
|
10,000-100,000
|
|
Practically
nontoxic
|
>100,000
|
Since any single summary rating
will inherently not be representative of the extremes of the sensitivity
of organisms to chemical exposure, we also provided the Toxicity
Range for species within the Organism Group, as well as the
raw data for the individual studies, so the analyst can evaluate
the studies and make his or her own decision about toxicity to a
particular species.
The Toxicity
Range assigned to each organism group was based on the most
sensitive and least sensitive members of the group, including outlier
species.
Top of
page
Last updated
March 7, 2002
.
|
