ENVIRONMENTAL EFFECTS BRANCH (7403)
HEALTH AND ENVIRONMENT REVIEW DIVISION
FEBRUARY, 1984
ESTIMATING "CONCERN LEVELS" FOR CONCENTRATIONS
OF CHEMICAL SUBSTANCES IN THE ENVIRONMENT
OFFICE OF POLLUTION PREVENTION AND TOXICS
U.S. ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, DC 20460-0001
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The Environmental Effects Branch Position on
Estimating "Concern Levels" for Concentrations of
Chemical Substances in the Aquatic Environment
Background and Purpose
In evaluating PMNs, the Environmental Effects Branch attempts
to estimate levels of a given compound which, if met or exceeded
in the environment, could cause adverse effects. The data set
from which such concentrations are estimated may run the gamut
from fairly complete packages, containing both acute and chronic
test data, to only the structure of the chemical itself.
The purpose of this position document is to describe the
procedure and rationale followed to estimate environmental
concentrations of concern for a particular chemical. The
methodology described should identify (at least 95% of the time)
concentration concern levels for TSCA chemicals which may cause
adverse environmental effects.
This procedure is based upon generally accepted scientific
rationale and currently available data. The attached Support
Document provides a detailed discussion of these data. As
additional data become available and are analyzed, any revisions
necessary to maintain this procedure at the state-of-the-art will
be made.
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Definitions, Rationale, and Assumptions Employed
The number used to adjust effective concentrations
(laboratory LC50, MATCs, etc.) to arrive at environmental
concentrations of concern is defined as an assessment factor.
Before proceeding further, we will attempt' to describe what a
concern level for concentrations of a specific chemical would be
in the natural environment.
An environmental concentration of concern is that
concentration of a chemical at which populations of organisms are
adversely affected as found in a field study conducted under
simulated or actual conditions of production, use, and disposal
(PUD) of a particular TSCA chemical. Thus, a field study would
measure what would be expected to occur normally in the natural
environment. Changes in PUD or site specificity could alter
exposure concentrations but should not significantly change the
actual concentrations at which effects occur.
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The assessment factors described in this paper take into
•
account the uncertainties due to such variables as test species
sensitivity with regard to acute and chronic toxicity, laboratory
test conditions, and age-group susceptibility. For the purposes
of ease and simplicity, each of the assessment factors used in
this.document has been rounded off to the nearest whole number.
The rationale for using assessment factors is similar to that
of any other government agency charged with protecting the
environment or public health. Simply stated, the more the
*
toxicological endpoints of given chemicals are characterized, the
less there is a need for an assessment factor in estimating the
concentration of chemical which would be expected to cause
adverse environmental effects in nature. A chemical with a no-
effect level established by both acute and chronic testing would
require a relatively smaller assessment factor to establish the
concern level. But a chemical with a more scanty data base
(perhaps only a QSAR estimate of acute toxicity) would require a
relatively larger assessment factor to establish the concern
level.
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The diagram below shows how each assessment factor decreases
as the data improves. Note that the assessment factor for each
given data set relates only back to natural environment
conditions. The assessment factors are not intended to relate
among each data set, even though the magnitude of the assessment
factors are partially derived from relationships between the
different types of data.
One Acute Concentration or QSAR
Natural
Environment
Lowest Effect Based On Many
Acute Concentration
Lowest Chronic Effect
Concentration
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The following assumptions are used in selecting the most
appropriate assessment factor to be employed in establishing the
concentration of concern.
1. The actual test data or QSAR employed in applying
assessment factors will be valid. In the absence of suitable
information or data, assessment factors should not be used.
2. The assessment factor is used to predict a
concentration of a given chemical which would be expected to
cause adverse effects on a population, rather than individual
organisms.
3. For purpose of applying these assessment factors, and
only in the absence of data on the chemical under review, data on
an appropriate analog chemical will be treated as if it were data
on the chemical itself. For example, if there are no ecotoxicity
data on a PMN chemical but there are data on an analog (single or
multiple LC50, MATC), the assessment factor appropriate to the
kind of (analog) data available would be applied.
Finally, it should be emphasized that the Assessment Factors,
as described here and in the attached Support Document, are not
the same as "S«f«ty Factors" <«lso c«ll«d "Margin of S«f«ty").
Traditionally, safety factor means a factor (usually 100 or 1000)
applied to a no-observed-effect-level (NOEL) for an adverse
(human health or environmental) effect to give a figure below
which exposures are presumed to be "safe." Additionally, when
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safety factors are used, there is usually a good set of hazard
data available. BEB's assessment factor, however, means a factor
applied to an MATC or to effect levels (e.g., an LC-50) to give a
figure which if met or exceeded by exposures could cause adverse
effects. Also, assessment factors are used when there is only
limited hazard data available. Note also that use of assessment
factors does not identify a "safe" exposure level. EEB cannot
make any statement about the safety or risk or exposures which
fall below concern levels derived from use of assessment
factors. Assessment factors are intended to be used in the PMN
process to identify those chemicals which should be tested under
TSCA Section 5 to more fully characterize their ecotoxicity
hazards.
The Assessment Factors
One of three assessment factors is used in estimating the
chemical concentration of concern based on the following types of
data: 1) Lowest chronic effect concentration; 2) Lowest LC-50
concentration of many acute tests; or 3) One LC5Q from an acute
test, or Quantitative structure-activity relationships.
1. Lowest Chronic Effect Concentration
If available information on a chemical includes chronic
effect and no-effect concentrations, an assessment factor of 10
is proposed to estimate concern levels for concentrations of the
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chemical in the natural environment. For example, if the lowest
available no-observable-effect concentration (NOEC) were for
daphnid reproduction, and it was 0.1 mg/L, a concentration of
0.01 mg/L (i.e., 0.1/10 = 0.01) would be used in preliminary
hazard assessment.
In addition to the data discussed in the Support
Document (attached), the following issues were considered in
arriving at this particular assessment factor.
a) Other species from the same environment may be
more sensitive than the species tested.
b) Test conditions may not fully represent the
natural environment. Typically, laboratory test
conditions are more optimal for survival of
organisms exposed to the chemical than those in
the natural environment.
c) Other adverse effects may occur at levels lower
than the effect measured.
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d) The presence of other toxic chemicals in the
natural environment may amplify the effects of
the tested chemical. For example, fish are more
sensitive to aryl phosphates in the presence of
certain pesticides.
2. Multiple Acute Toxicity Data
When there are acute toxicity data for several species,
an assessment factor of 100 is used to estimate a concern level
for enviromental concentrations. The assessment factor will be
applied to data from the most sensitive species. For example, if
the EC5o values for at least 5 species of fish and invertebrates
range from 20 mg/L to 1 mg/L, the 1 mg/L would be adjusted to
0.01 mg/L (i.e., 1/100 =0.01). This value (0.01 mg/L) would be
used in any preliminary hazard assessment.
For purposes of applying this assessment factor, the
term "multiple acute toxicity data" refers to either of the
following two cases: 1) at least one acute test for each of
three taxonomic groups (fish, invertebrates, and algae); or 2) at
least five acute tests divided among two of the three taxonomic
groups listed above. Note that two or more acute tests in only
one taxonomic group are not ^multiple acutes* un^er this
definition; in such a case the lowest LC5Q and the assessment
factor for a single LC50 would be used (see below). A detailed
discussion of the "species cluster" concept, upon which the
multiple-acute assessment factor is based, is presented in the
attached Support Document.
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In addition to the data discussed in the attached
Support Document and the four uncertainty factors a - d
enumerated under Lowest Chronic Effect Concentration, the
following additional issues were considered in arriving at the
above figure:
e) Acute toxicity tests are short-term tests, while
chronic tests are conducted under long-term
exposures. Total exposure in a chronic test is
greater than in an acute test at higher
concentrations because of the longer duration of
exposure. Long-term exposures generally result
in lower concentrations causing effects.
Therefore, acute LC/EC5Qs are adjusted downward
by assessment factors to account for longer
exposure. Even in the absence of prolonged
exposure due to persistence, repeated or
intermittent exposure may result in delayed
effects or effects at concentrations lower than
that indicated by acute tests.
f) Acute tests generally measure only lethality as
an endpoint, while chronic tests generally
measure sublethal effects which normally occur at
lower concentrations than those which cause
lethality. Also an LC50 is based on median
response. Chronic tests often seek effects on
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less than 50% of the population (e.g., an avian
reproduction test has the goal of detecting
effects on 25% or less of a population; a fish
early life stage may get significant results'at
an even lower percentage response).
g) EEB has compiled data on the acute-to-chronic
ratios (ACR) of selected toxicants on fathead
minnows. Additional information on ACRs of
various fish species for organic chemials is
undergoing analysis. Although there is
substantial variation in the ACRs for various
chemicals, these data support an assessment
factor of 100 when going from the lowest of
several LC50 values to natural environments.
h) Acute toxicity tests generally use juvenile and
adult organisms. Early-life stages (eggs and
embryos) are generally more sensitive to toxic
chemicals than are other life stages (juveniles
and adults).
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3. Acute Toxicity Data
An assessment factor of 1000 is used for estimating a
concern level for environmental concentrations of a chemical when
only one acute test is available. For example, if the acute LC
or EC5Q for a single species of fish or invertebrate is 8 ppm,
then the concern level for environmental concentrations is 0.008
ppm or 8 ppb (8/1000 = 0.008).
In addition to the data discussed in the attached
Support Document and the uncertainty factors a - g enumerated
earlier, the following additional issue was considered in
arriving at this figure of 1000.
h) There are abundant data showing that different
species have different sensitivities to a
chemical, but no one or two species are
consistently more sensitive to a broad array of
chemicals. This natural variation in
sensitivity, along with previously noted
uncertainty, supports an assessment factor of
1000 when going from a single LC5Q value to
natural environments.
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4.
QSAR
An assessment factor of 1,000 is used to predict a
concern level for environmental concentrations when
extrapolations are made from quantitative structural activity
relationships (QSAR).
In addition to the data discussed in the attached
Support Document, the rationale for the above figure is as
follows:
i) QSAR estimates an acute toxicity level based upon
certain structural correlates. Within a given
class of organic compounds, the actual versus
estimated acute toxicity (LC^) may vary 10-
fold. Considered in conjunction with previous
areas of uncertainty, such variation supports an
assessment factor of 1,0OOX when going from an
acute toxicity value derived from QSAR to natural
environments.
SUPPORT DOCUMENT
Introduction
EEB has devised a simple but reliable system to estimate
concern levels for environmental effects of chemicals when
minimal data are available. This approach is intended for use in
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the PMN review process when the available ecotoxicity data
consist of either a single LC5Q, multiple LC5() values, QSAR data
or an acceptable MATC value. The result is that a determination
is made relative to exposure levels as to whether some effects
testing is appropriate.
The basis for the system is the use of "assessment factors"
to apply to the available aquatic toxicity data to arrive at a
level which is likely to cause adverse environmental effects in
the field. An adverse field effect is defined as any effect upon
growth, reproduction, or survival which would either temporarily
or permanently alter the population level of wild plants or
animals. The laws supporting this concept are summarized in
"Environmental Effects of Regulatory Concern under TSCA" (EEB,
1983).
If one has in situ evidence of population effects such as
might be derived by biological monitoring or full scale field
studies, the results are taken at face value and thus an
assessment factor of one is applied to determine the significant
field effect level.
If the best evidence of what the field effect level would be
were limited to a valid MATC*, an assessment factor of 10 is
* To be valid, an MATC must (among other things) be preceded by
acute toxicity tests on several species; the "most sensitive"
species would then be used in the chronic test.
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applied to account for the various laboratory-to-field
variables. Similarly, if the best evidence of what the field
effect level would be were limited to a cluster of acceptable
acute LC50 values, a factor of 100 is applied to the lowest LC5Q
to project the concern level for the field. Finally, a factor of
1000 is appropriate when only a single acute aquatic toxicity
value or QSAR-based LC50 are known.
These assessment factors are derived to account for the many
individual experimental and environmental factors which act to
alter acute laboratory toxicity test values away from field
effect levels. For convenience, however, we have avoided great
complexity by grouping these numerous influences under only three
categories. They are comparative toxicology, chronicity, and
laboratory-to-field differences.
Readily available evidence for the values used in each
assessment factor will constitute the bulk of the remainder of
this technical support document.
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Figure #1 illustrates the ralationships between the data
which are intended by the given assessment factors.
1000
SINGLE ACUTE or QSAR
100
LOWEST ACUTE
MATC
10
FIELD EFFECT LEVEL
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Figure #2 illustrates these linkages with a more
sophisticated model which pictures how the idealized
relationships between the various forms of data are viewed
statistically. The figure makes use of the log dose-probit
response model which so often fits the data.
Experimental evidence exists linking each of these tests
statistically. The frequency of one test result (say the single
LC^Q) being a certain amount higher than another test result (say
the lowest of several LC50 values) can be derived from available
data on a wide array of test chemicals.
Before proceeding with experimental evidence of these
relationships, an historical review of regulatory use of
assessment factors to represent these relationships is in
order. Such perspective should provide an indication of the
degree to which regulatory use has been made of this concept.
B. Historical Review of Assessment Factors
Previous use of factors in the absence of specific chemical
data is well established. In the human health field, for
example, Dourson and Stara (in press) have reviewed the extensive
history of the use of the numbers 10, 100, and 1000 in regulatory
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Figure 2. Relative Effect Level Model
most likely
single LC-50
field effects
concern
leve
10-fold
V
1
10-fold 10-fold
Y
10
• f
100
1000
Relative Concentrations
16a
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history. One example, that of the 100-fold uncertainty factor,
is attributed variously to Lehman and Fitzhugh (1954), Bigwood
(1973), and Vettorazzi (1976 and 1980).
•
Although the specific areas of uncertainty described by these
authors differ somewhat, they can be viewed as due to intra- or
interspecies variability. It has also been suggested that two
10-fold uncertainty factors, one for each type of variability, be
used to describe the 100-fold factor in some instances (Bigwood,
1973; Klassen and Doull, 1980; Food Safety Council, 1982).
The FDA expanded their approach when chronic data were
unavailable. In cases where only subchronic animal NOELs (no
observed effect levels) or NOAEL (no observed adverse effect
levels) were available for only two species, the FDA recommended
a factor of 1000 instead of 100. The additional 10-fold was due
ostensibly to the added uncertainty when estimating an ADI
(acceptable daily intake) from adequate short-term toxicity data
(Kokoski, 1976). If subchronic data were available for only one
species, a 2000-fold uncertainty factor was recommended (Shibko,
1981). The National Academy of Sciences (NAS, 1977) has
recommended a similar approach for drinking water. Finally, the
USEPA (1980) has
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previously used the MAS reasoning for pollutants in ambient
waters, under the consent decree. Figure 3, from Dourson and
Stara (1983), illustrates a human health use of uncertainty
factors.
The record of use of uncertainty factors does not stop with
human health. The Research and Development Department of the
Procter and Gamble Company (Duthie, 1979) proposed that "Testing
for aquatic effects is indicated for any chemical that is
expected to occur regularly or continuously in surface waters at
concentrations greater than 0.001 mg/1..." This 0.001 mg/1 "is
calculated as resulting from the disposal of a material that is
uniformly distributed across the U.S. in quantities of 1 million
pounds/year." The proposal continues that "the same
concentration could be obtained by the release of much smaller
quantities in a narrowly bounded locality." The proposal further
states that the LCSOs of several species of fish should be
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Figure 3 Guidelines for the Use of Uncertainty (Safety) Factors
(1) Use a 10-fold factor when extrapolating from valid
experimental results from studies on prolonged ingestion by
man. This 10-fold factor protects the sensitive members of
human population estimated from data garnered on average
healthy individuals
(2) Use a 100-fold factor when extrapolating from valid results
of long-term feeding studies on experimental animals with
results of studies of human ingestion not available or scanty
(e.g., acute exposure only). This represents an additional
10-fold uncertainty factor in extrapolating data from the
average animal to the average man.
(3) Use a 1000-fold factor when extrapolating from less than
chronic results on experimental animals with no useful long
term or acute human data. This represents an additional 10-
fold uncertainty factor in extrapolating from less than
chronic to chronic exposure.
(4) Use an additional uncertainty factor of between 1 and 10
depending on the sensitivity of the adverse effect when
deriving an ADI from LOAEL. This uncertainty factor drops
the LOAEL into the range of a NOAEL.
FROM: Dourson and Stara (in press).
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greater than 100 times the concentration to exist in surface
waters if concerns for hazard are to be dropped. Beck et al.
(1980) and Beck et al. (1981) proposed essentially the same
concepts but laid them out in greater detail.
Kimerle et al. (1978) of Monsanto Industrial Chemicals
Company presented hazard evaluation criteria as follows: If the
screening LC50 is between 1 and l/1000th the estimated
environmental concentration (EEC), continue testing. If the
lowest of additional acutes are more than l/500th the EEC,
continue testing. If the short-term chronic (estimated MATC) is
more than l/50th the EEC or the long-term chronic (measured MATC)
is more than l/20th the EEC, continue testing. If the impact on
ecosystems in field studies is minor, continue testing. If the
ecosystem is impaired, no use or restricted use of the chemical
was proposed.
The American Institute of Biological Sciences (unpublished)
reported, to EPA, their Task Group recommendation that
"...experimentation has shown, however that there sometimes
exists a direct relationship between acute and chronic [aquatic]
t ox i c i ty
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of chemicals to aquatic organisms and that the acute LC50 can be
used, within certain limits to estimate the MATC." AIBS
described the ratio as generally between 2 and .10,000, but felt
that 100 was a useable figure. '
The European standard-setting organization EIFAC and the FAO
suggested that concentrations which are at least four orders of
magnitude lower than the 96-hr LC50 from a standard acute
toxicity test are surely harmless to fish populations. Calamari
et al. (198-) in quoting this figure (i.e., 10,000) calls it too
*
restrictive.
The Organization for Economic Cooperation and Development
proposed that further confirmatory testing take place if the NOEC
(similar to our MATC) for Daphnia, fish, or algae is not greater
than 10 times the environmental concentration (OECD, 1980).
Finally, OTS contracted with Life System, Inc. to bring
together the top environmental toxicology scientists and chemists
frcn the United States to a series of testing trigger workshops
in 1982 and 1983. The formal report of this workshop to EPA
contained the recommendation that if the EC50 or LC50 from a
single aquatic species, divided by 1000, equalled or exceeded the
environmental exposure, acute tests with three or more species
should be initiated. Also proposed by the workshop for use by
EPA was that "chronic" tests (fish early-life stage, invertebrate
life cycle) be initiated if the environmental exposure would be
equal to or greater than the acute effect level for the most
sensitive of three or more [aquatic] animal species.
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Data In Support of Assessment Factors
With all these authors and respected authoritative groups
proposing and using factors of 10, 100, and 1000, two questions
arise. First, why have all these people come to the same
conclusion? Second, why are the numbers the magnitude that they
are? Let us next investigate the available experimental evidence
to see what justification exists for these numbers.
What are these variables which can so greatly influence the
outcome of ecotoxicity studies? They must be taken into account
if test results are used to project possible field population
effects. Crosby et al. (1966) reported on the importance of the
variables which they noticed to significantly alter ecotoxicity
figures derived in aquatic bioassays with Daphnia magna. One
might expect these variables* to affect other aquatic test
results as well. The list included age and instar, temperature
(400), light intensity, light quality and periodicity, food
quantity and type, water quality (100), pH (61) (Huey et al.,
1982), oxygen-carbon dioxide content of the test water, test
animal population density, test duration, length of pre-test
fasting period, criteria for detection of toxicity (6),
randomization and selection of individual test organisms, purity
of the toxicant, and physical form of the toxicant (2). If these
* Factors in parentheses after each variable given above are
extreme ratios found. Though these extremes seldom occur,
knowledge of them should give one caution when decisions to
forego further, or any, testing are made.
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and other variables can influence the outcome of an ecotoxicity
test in the laboratory, it can be expected that the same
variables can likewise influence the outcome of chemical
exposures in the field. A thorough discussion of this can be
found in Tucker and Leitzke (1979).
The fact that many modern-day aquatic testing procedures have
been standardized to some extent should work to reduce the
likelihood of seeing extreme variability in the toxicity
values. But it is not to be concluded that nature has been
standardized. These variables will not disappear under field
exposure conditions. Nor are the data available for evaluating
PMN chemicals always derived from state-of-the-art testing
protcols and procedures
Since there are entirely too many factors which may alter
effect levels to take all of them individually into careful
account everytime a testing decision is made, it is proposed to
group all the variables under just three headings for
convenience. They are species-to-species sensitivity related
variables, chronicity related variables, and laboratory-to-field
related variables.
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Species-To-Species Variation
This section deals with readily available information on
acute toxicity to aquatic organisms which shows that an
assessment factor of about 10 can be used to account for species
to species variation, on average. That is to say, given any
single LC-50 for an aquatic species, dividing by an assessment
factor of 10 will provide a figure which is likely to encompass
the most sensitive species about 50% of the time. This
assessment factor, therefore, is applied when there is only a
single LC50 for aquatic species.
a. The "Cluster Concept"
Don Mount of the EPA Duluth Environmental Research
Laboratory* has said, "We can see that [aquatic] species
sensitivity distributes itself in a rather consistent way for
most chemicals.... The distribution resembles a log normal one.
We can then take the approach of sampling this distribution in
order to predict the range about the mean." He goes on to say,
"This is in reality our objective. Each species we test then is
not representating any other species but is one estimate of
sensitivity and with several such estimates then of the overall
range of sensitivity for all species." Dr. Mount concludes: with
•{the data in Table I of th« consent
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species - rainbow trout, fathead, bluegill, D. magna and Gammarus
(included 2 species) - is a large part of the range of all
species tested for most chemicals." This idea has been called
the "cluster concept."
Specifically, Dr. Mount is arguing that a limited set of
aquatic toxicity data for several fish and invertebrates is
representative enough to provide a good indication of the most
sensitive aquatic species. This concept is useful since we
cannot reasonably require testing on all aquatic species which
might be exposed to a chemical. Please note also that Dr.
Mount's use of the cluster concept includes only fish and
invertebrates; EEB has included algae in the species cluster to
be considered, although we do not have hard data to support this
approach.
b. Analysis of Data
Work by comparative ecotoxicologists initially focused on
identifying relationships between specific species reactions to
chemicals. This relationships was typically expressed as a range
of toxicity values, from a numerically low value (most
toxic/sensitive'species) to a numerically high value (least
toxic/insensitive specie*). For example, although Tiot backed fey
any presented data, Kenaga (1978) reported that a Daphnia LC50
would lie within ± 3.26 orders of magnitude (i.e., the Daphnia
LC50 divided or multiplied by about 200) of the LC50 of the same
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chemical for the rainbow trout, 95% of the time for all chemical
groups together. He similarly reported that the various species
of saltwater fish LC50 values lie within ± 0.83 orders of
magnitude (i.e., a single saltwater fish LC50 multiplied or
divided by about 7) 95% of the time for chlorinated hydrocarbon
insecticides. Most other aquatic species correlations he made
also tended to fall between ± 1 and 3 orders of magnitude, 95% of
the time (i.e., a single LC50 multiplied or divided by 10 or
10UO). This approach is not useful for regulatory purposes
because we are not interested in how insensitive other species
may be compared to a given LC-50, but in how sensitive other
species may be.
Statistical Consultants Inc.* under contract to OTS, reported
results of a similar nature in a way more useful for regulatory
purposes. They based their results upon large numbers of fish
toxicity figures of a type (tests, species, chemicals, etc.)
expected to be used under TSCA. They determined the factors by
which one would have to divide any specified fish LC50 in order
to arrive at the lowest of the LC50 values for any of eleven
other fish species for which they had toxicity data. The result
was that a divisor of 48.9 would
Personal communication
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include the most sensitive of these eleven species 95% of the
time. Likewise, 19.6 would include the most sensitive of the
fish 90% of the time, and 2.4 would include the most sensitive
50% of the time. A total of 920 LC50 values were used in these
computations. However, these data were solely on one taxonomic
group of aquatic organisms, i.e., fishes. In reality, we must
protect a far more diverse group of organisms than just bony
fish.
In one exercise, we have used data from the Handbook of Acute
Toxicity of Chemicals to Fish and Aquatic Invertebrates.
(Johnson & Finley, 1980). The Handbook contains 1587 acute tests
with 271 chemicals on 28 species of fish and 30 species of
aquatic invertebrates. However, there were only 67 chemicals
for which data was presented for 6 or more species. For these
chemicals the median overall range of sensitivity of the tested
organisms was 450. Now if one takes the view of Dr. Mount quoted
earlier, that the distribution of species sensivity is log
normal, Figure #4 would well represent the distribution of LC50
values.
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Figure 4
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Thus if the lowest LC50 value were set at 1 ppb, the highest
LC50 value would be projected to be 450 ppb. Owing to the log
normal nature of this curve, the highest frequency LC50 would be
calculated to lie at 21.12 ppb. If one assumes therefore that
the most likely LC50 to be reviewed by EPA is 21 ppb and the
lowest is 1 ppb, a divisor of 21 is indicated as the 50th
percentile when projecting lowest LC50 from a "typical" LC50.
To summarize, we have presented data (See Table 1) to show
that an assessment factor of about 10 can be used to estimate,
from a single LC50, a figure which encompasses the LC50 for the
most sensitive aquatic species. As can be seen from the data in
Table 1, our assessment factor of 10 is a ballpark figure;
assessment factors derived from actual data can vary depending on
the number and kinds of both species and chemicals tested.
Table 1. Assessment Factors for Species to Species Variation
Assessment
Species Chemicals Factor Confidence Ref.
Fish all groups 10 50% this
invertebrates paper
algae
Fish only
(11 species)
Fish/
invertebrates
(6 or more
species)
TSCA-type 48.9 95% statistical
chemicals 19.6 90% consultants
2.4 50% (pers. comm.)
67 21 50% EEB analysis
chemicals of data in
Johnson &
Finley (1980)
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Acute - To - Chronic Toxicity Ratio
Data which are readily available have been reviewed in the section
that follows to evaluate the level of support for an additional factor
of 10 to account for chronicity and related variables.
Any assessment factors based upon acute toxicity data alone must
take into account the fact that longer exposures or longer response
times may greatly reduce the dosage level at which toxic effects
occur. The degree to which this happens is called "chronicity." A
measure of chronicity which we commonly use is that of the LC50
divided by the MATC. This acute-to-chronic ratio (ACR) contains other
elements in addition to pure chronicity. For example, a 50% effect is
^
denoted by the LC50 while a "just significant effect" level is denoted
by the MATC. Also, the MATC can be based upon effects such as
reproduction which can be more sensitive to a chemical than is
mortality.
The assessment factors used by EEB are based, among other things,
upon an MATC which is derived from less than full length
chronic tests. We utilize early life stage tests to derive MATC
values, for example. While several life stages are implied in this
MATC, it is not based on full-length chronic testing. It would need
to be divided by about two, on an average, based on 34 chemicals to
project the "real" chronic MATC values from full life-cycle studies
(McKim, 1977).
30
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Goodman (in press) found chronicity ratios arrayed between 2-and
3300-fold for estuarine fish. The most sensitive effects were
reported as reduced survival, growth, or fecundity. Call et al.
(1983) reported that the acute LC50 was 17,551 times higher than the
MATC for propanil on fathead minnow. We have seen few ratios as high
as this one.
Although we have, on occasions seen very large acute-to chronic
(LC50/MATC = ACR) ratios, this does not mean they are always large.
EEB staff have from time to time performed analyses upon readily
available data in limited efforts to determine the statistical
relationships between the array of acute-to-chronic (ACR) ratios for
various species.
•
One series of computations (Ells, personal communication) resulted
in our determination of the approximate aquatic chronicity ratio which
is exceeded in tests with 50% of the chemicals. More data would be
required to reliably establish the ACR which is exceeded just 5% of
the time. Using a set of 29 available ACRs on fathead minnows for
organic chemicals having acute LC50 values in excess of 1 ppm, the
median chronicity ratio was found to be 7. This means if LC50 values
for fathead minnows for these chemicals were divided by 7, half of the
MATCs would be found below the dividend and half would be found
above. On this same data set, a divisor of 28 would have reduced the
LC50 to a value below which about 5% of the MATC values would lie.
Stated another way, one could have 95% confidence that dividing an
LC50 by 28 in this data set would produce a concentration below which
the true MATC would not be found in more than 5% of the cases.
31
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When additional data on fathead minnows was added to this data
set, the median chronicity ratio dropped very slightly to 6.4, but the
95th percentile rose dramatically to 300! When acute to chronic
ratios for many other species were added to this data set to make a
total of 219 cases, the median chronicity ratio was 6.4, but the 95th
percentile was 500. This data set included much the same mix of
species found in OTS's ecotoxicity guidelines: fathead minnows,
sheepshead minnows, rainbow trout, brook trout, Daphnia maqna, and
mysid shrimp.
Further data analyses on the same data set indicated to us that
restricting the chemicals to just organics (omitting metals and
inorganics) would bring the 50th and 95th percentile ratios down
somewhat, in this instance to 5.2 and 180, respectively.
Contract statistical studies are underway by Statistical
Consultants Incorporated (SCI) to refine the acute to chronic ratio
estimates by use of additional data sets. We have received interim
phone reports as follows: An analysis has been done on a data set for
fish containing ACRs for 95 chemicals including metals, pesticides,
and industrial chemicals. The median ACR was reported as 8.46. This
means that half of the tested chemicals produced MATC values below the
acute LC50 divided by 8.46. Five percent of these chemicals produced
MATC values below l/875th of the acute LC50. This ratio comes to
1/522 if only chemicals having LC50 values below 1 ppm are considered.
32
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In summary, a factor of about 10 'seems likely to be a useful
factor to project "typical" chronicity (See Table 2). No doubt many
more special relationships will arise as we and SCI continue to obtain
and evaluate increasing amounts of data. These will improve our
analyses and could refine the statistical relationships between
various toxicity test results. We will continue to test certain
surrogate species in the laboratory and apply the results to untested
or untestable species. The real proof of these methodologies will be
the degree to which these assessment factors will flag the appropriate
chemicals for adequate testing/ upon which results sound risk
assessments can be made. A specific available test of these
assessment factors, then, is to see what would have happened had we
applied these factors to chemicals for which we already know the field
effect levels. The next section is an attempt to review these
laboratory to field ratios.
33
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Table 2.
Assessment Factors for Acute-to Chronic Ratio
Assessment
Chemicals Factor Confidence Ref.
all groups
Speci
es
fish
Invertebrates
algae
Fathead
Minnow
Fathead
mi nnow
Fathead Minn.
Sheep she ad
Minnow
Rainbow &
brook trout
D. magna,
mys ids
Same as
above
Fish
10
29 chemicals 7
28
unkn
219
unk. #:
same as
above excl.
metals &
inorganics
95
(includes
metals,
pesticides,
industrial
chems.)
6.4
300
6.4
500
5.2
180
8.46
875
50% this paper
50% Ells
95% (pers. comm.)
50% EEB
95% analysis
50% EEB
95% analysis
50% EEB
95% analysis
50% Statist.
95% consultant
(pers. comm. )
Laboratory - To - Field Comparisons
This section deals with some of the readily available field effect
levels and compares these to the levels projected by the EEB
assessment factors. Due to limited field data these comparisons can
at present only be anecdotal rather than statistical in nature.
34
-------
Some may question the need to divide an acute LC50 for, say, a
fish by a number as great as 1000 to arrive at a level above which EEB
would assume concern for adverse environmental effects in the field.
The technical staff of EEB, using their best scientific judgement,
felt that levels above 1000th of any .single LC50 value represented too
great a risk of field effects. This was based upon experience with
laboratory and field effects levels, plus a sense that while no
particular level of sureness has been made into a policy, any less
than 95% sureness would not be acceptable. Even at 95%, one in twenty
environmentally hazardous PMN chemicals would accidently be allowed
through the review process without testing or regulation.
How well did the staff do? Available data are reviewed below;
much more data exist and, with adequate resources, could be included.
In 1969, standard application of malathion used in mosquito
control resulted in an immediate concentration of 2.0-3.2 ug/1 in a
saline marsh with rapid reduction thereafter. If one had the goldfish
acute LC50 of 10,700 ug/1 from Johnson and Finley (1980), it would
have been easy to assume that this level (2-3 ug/1) was safe since
this LC50 is about 3300 to 5300 times higher than the highest field
exposure. It would have been easy, but it would have been wrong,
because 14 to 80% of the valuable brown shrimp and white shrimp were
killed in the marsh (Conte & Parker, 1975). But some would say
goldfish are insensitive. Had the available LC50 been for carp it
would have been 6590 ug/1. Had the available LC50 been for an OTS
guideline recommended species, such as the fathead minnow, the LC50
35
-------
would be 8650 ug/1. This is a good example of how the EEB assessment
factor of 1000, large as it may seem still would not be sufficiently
protective. Other examples follow.
Pimental (1971) discusses data by Oliver et al. (1966) showing how
algal productivity was reduced in aquatic habitats by as little as 2
Ib/A of cacodylic acid. This translates to less than 2 mg/1, and
likely less than 1 mg/1. The LC50 for mosquito fish and taillight
shiners is about 1000 mg/1.
Wellborn (1969) reported an LC50 of 315 mg/1 for diquat on striped
bass. But Tatum and Blackburn (1962) had already reported that 0.5
mg/1 adversely affected plankton in outdoor ponds.
Bottom-dwelling organisms of a great variety were reduced by 50%
or more following applications of 0.5 to 10.0 mg/1, of simazine
(Walker, 1962). On the other hand, the laboratory LC50 of simazine
for bluegills (a test species recommended by OTS) is 118 mg/1
(Bohmont, 1967).
The LC50 of copper sulfate to bluegills is 45 mg/1 at 8 grains
water hardness (McKee & Wolf, 1963). In contrast 0.05-0.08 mg/1
applied to ponds had demonstrated effects upon a great variety of pond
animals and plants. In this instance an assessment factor of about
900 would have been required to protect the aquatic environment.
Since the LC5Q for bluegill drops to 0.6 mg/1 when water hardness is
only 1 grain, an assessment factor of only 12 might have sufficed.
36
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But which LC50 would have been the first or only one submitted if this
were a PMN?
Farm ponds with 2.8 mg/1 silvex (K + salt) showed disruption of
the benthic community and rapid decreases in several species. The
bluegill LC50 for the same material is 100 mg/1 (Hughes & Davis,
1965).
Treatment of outdoor ponds with 2.39 ug/1 Dursban produced a 46%
kill of bass and a 55% kill of bluegills. A treatment of 0.97 ug/1
produced 10% kill of bass in the ponds (Macek et al., 1972). By way
of comparison, the LC5Q of Dursban for channel catfish, a common test
species, is 280 ug/1 (Johnson & Finley, 1980).
In the previous paragraphs we have noted several examples where
assessment factors of at least 236, 289, 630, 900, 1000, or even 5300
would have been needed (See Table 3). To be sure, lower factors might
sometimes suffice, but we have no means as yet of telling when this
would be the case for any PMN chemical when we have but a single
aquatic LC50 as our basis.
37
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Table 3.
Species
Fish
Inverte-
brates
algae
Chemicals
Malathion
Cacodylic
acid
Diquat
Simazine
Copper
sulfate
Assessment Factors for LC-50 to Field Effect Levels
Chemicals Assessment Factor Confidence Ref.
all groups
Test
species
Goldfish
Carp
Fathead
Minnow
Mosquito
fish &
tail-light
shiners
Striped
Bass
Bluegill
Bluegill
1000 95%
Lab Field
LC50 Species
10,700 ug/L Brown &
6,590 ug/L White
8,650 ug/L Shrimp
this
EEB's
Effects &
Effect Level
14-80%
mortality
at
paper
Calculated
Assess, factor
3343-5350
2059-3295
2703-4325
2.0-3.2 ug/L
both algae
§1000 reduced
mg/1 at
-------
(Chart continue)
Silvex Bluegill
Dursban
100 mg/L Fann pond
benth ic
species
2.8 mg/L
disruption or
rapid popul.
decrease at
Channel
Catfish
280 ug/L Bass and 46% bass kill
Bluegill 55% bluegill
kill at 2.39
ug/1;
10% bass kill
at 0.77 ug/L
35.7
117 &
288
Finley
Hughes &
Davis (1965)
Macek et
(1977) &
Johnson &
(1980)
al
-------
Based upon expert judgement and experience, the use of an
assessment factor of 1000 seems reasonable to project concern
levels from any single LC50. If additional resources become
available, we should focus additional study on the degree to
which laboratory results can be predictive of field effect
levels. More collection and analysis of past field data (such as
that of the US Fish and Wildlife Service), more actual field
studies, and more terrestrial studies are needed to refine our
projections from laboratory to field. But we do now have a
working model.
QSAR-Calculated LC-SOs
In the absence of other data, LC-50s for aquatic species can
be calculated from the log P of certain compounds (alchols,
ke tones, ethers, alkyl halides, and benzenes). Veith et al.
(1983). examined 54 industrial chemicals from the TSCA Inventory
which fall into the above chemical classes. QSAR - based LCSOs
were calculated for these chemicals and compared to actual
(measured) LCSOs for the same compounds. They concluded, "All of
the predicted values were within one order of magnitude of the
measured values, with a range for observed/predicted ratios from
0.12 for 3,3-dimethyl-2-butanone to 4.22 for 2-propanol."
The authors also noted, "Predicting the mode-of-action of
chemicals remains a difficult task and the errors associated with
selecting the wrong structure-toxicity mechanism may be greater
40
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than the standard error of the estimate associated with any
•
individual QSAR. We recommend that the equation presented herein
be used only for [alcohols, ketones, ethers, alkyl halides, and
benzenes] or for chemicals for which there is evidence that- they
or analogous structures are narcotic."
Since the QSAR-calculated LCSOs for these classes of
chemicals is correct within one order of magnitude when compared
to measured LCSOs, EEB assumes that QSAR-calculated LCSOs for
chemicals in the above classes may be treated for practical
*
purposes as if they were measured LCSOs. Therefore, in using a
QSAR-calculated LC50 to determine concern levels for field
concentrations, we will apply the assessment factor for single,
measured LCSOs (i.e., 1000). It should also be noted that QSAR-
\
calculated LCSOs are minimum values, and that actual toxicity may
be greater if other, or additional, modes of action than narcosis
are operating.
Summary
EEB has developed a series of assessment factors to estimate
concern levels for field concentrations of a chemical when only
limited data on that chemical are available. These assessment
factors are intended to account for the different levels of
certainty associated with variously-derived toxicity figures (See
Table 4).
41
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Table 4. EEB Assessment Factors
Toxicity Derivation Assessment
method Factor
QSAR or Acute LC50 1000
with single species
Acute LC50 with several 100
species (use value for most
sensitive species)
Chronic toxicity value 10
(MATC)
Field test value 1
We have presented anecdotal data to support an assessment
factor of 1000 for estimating concern levels for field
concentrations from a single LC50. We have also presented more
quantitative data to support a factor of 10 between a single
"typical" LC50 and the LC50 for the most sensitive species (i.e.,
the single LC50 divided by 10). Since the assessment factor for
single LC50 to field level is 1000, and since the factor between
single and multiple LCSOs is 10, it follows that the assessment
factor to estimate field concern levels from multiple LCSOs is
100.
42
-------
We have also presented quantitative data to support an
additional factor of 10 to account for variables between the most
sensitive of multiple LCSOs and the MATC (chronic toxicity
value). Since the assessment factor for multiple LCSOs to field
concern level is 100, and since the factor between multiple LCSOs
and the MATC is 10,- it follows that the assessment factor to
estimate field concern levels from the MATC is 10.
We have also presented data (cited in Veith et al., 1983) to
show that QSAR-calculated LCSOs for specified classes of
chemicals are correct within an order of magnitude, compared to
the measured LCSOs. Therefore we assume that, for practical
purposes, QSAR-calcluated LCSOs may be treated as if they were
measured LCSOs for a single species, and that an assessment
factor of 1000 should be used to estimate field concern levels.
43
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Literature Cited
American Institute of Biological Sciences, Aquatic Hazards of Pesticides Task
Group. Unpublished. Report to the Environmental Protection Agency:
Criteria and rationale for decision making in aquatic hazard evaluation.
U.S. E.P.A, Off. Pest. Progr., Contract No. 68-01-2457.
Beck, L. W., A. W. Maki, N. R. Artman, and E. R. Wilson. 1980. Outline and
criteria for evaluating the safety of new detergent chemicals, p. VII-
1-12. In: Environmental risk assessment — An integrated approach. First
Ann. Meet. Soc. Environ. Toxicol. Chem., November 23-25, 1980.
Beck, L. W., A. W. Maki, N. R. Artman, and E. R. Wilson. 1981. Outline and
criteria for evaluating the safety of new chemicals. Reg. Tox. Pharm.
1:19-58.
Bigwcod, E. J. 1973. The acceptable daily intake of food additives. C.R.C.
Crit. Rev. Toxicol. June, 41-93.
Bohmont, B. L. 1967. Toxicity of herbicides to livestock, fish, honeybees, and
wildlife. 20th West. Weed Cont. Conf. , Proc. 21:25-27.
Butler, P. A. 1964. Coimercial fishery inestigations: Effects of DDT in a
tidal marsh, p. 14-15. In: Pesticide-wildlife studies, 1963: A review
of Fish and Wildlife Service investigations during the calender year.
U.S.D.I., Fish Wildl. Serv., Circ. 199.
Cairns, J., Jr., K. L. Dickson, and A. W. Maki (eds.). 1978. Estimating the
hazard of chemical substances to aquatic life. Amer. Soc. Test. Mat.,
ASTM Spec. Tech. Publ. 657. 278 p.
Calamari, D., R. Da Gasso, S. Galassi, A. Provini, and M. Vighi. 198 .
Estimating the hazard of eight amines on aquatic life, p. 329-341.
Source is an unidentified book.
Call, D. J., L. T. Brooks, R. J. Kent, M. L. Knuth, C. Anderson, and C.
Moriarity. 1983. Toxicity, bioconcentration, and metabolism of the
herbicide propanil (31, 4'-dichloropropionanilide) in freshwater fish.
Arch. Environ. Contain. Toxicol. 12(2)sl75-182.
Conte, F. S. and J. C. Parker. 1975. Effect of aerially-applied malathion
on juvenile brown and white shrimp Penaeus aztecus and P. setiferus.
Trans. Amer. Fish. Soc. 104(4):793-799. ~
Crosby, D. G., R. K. Tucker, and N. Aharonson. 1966. The detection of acute
toxicity with Daphnia magna. Food Cosmet. Toxicol. 4:503-514.
Dourson, M. L. and J. F. Stara. In press. Regulatory history and experimental
support of uncertainty (safety) factors. Reg. Toxicol. Pharmacol. 15 p.
Duthie, J. R. Prepublication. Outline and criteria for evaluating the safety
of new chemicals. Proctor and Gamble Company, Res. Dev. Dep., Cincinati,
Ohio. 37 p.
-------
Food Safety Council. 1982. A proposed food safety evaluation process. The
Nutrition Foundation, Inc. Washington, D. C.
Goodman, L. R. In press. Chronic toxicity of organophosphorus pesticides to
estuarine fish (Abstract). Presented at the Symposium on organophosphorus
pesticides in the marine environment, June 7-9, 1981, Duke Univ. Mar. Lab.
Harp, G. L. and R. S. Campbell. 1964. Effects of the herbicide silvex on
benthos of a farm pond. J. Wildl. Managem. 28(2):308-317.
Hays, H. W. 1969. Uses, regulations, and registration of chemicals used in
fish culture and management, 6 p. Int Fish and Chemicals: A symposium
on registration and 'clearance of chemicals for fish culture and fishery
management. A symposium arranged by the Bureau of Sport Fisheries and
Wildlife for the 99th Annual Meeting of American Fisheries Society, New
Orleans, Louisiana, September 12, 1969.
Houser, A. 1963. Loss of weight of sunfish following aquatic vegetation
control using the herbicide silvex. Proc. Okla. Acad. Sci. 43:232-237.
Huey, D. W., M. C. Wcoten, L. A. Freeman, and T. L. Beitinger. 1982. Effect of
pH and chloride on nitrile-induced lethality in bluegill (Lepomis
macrochirus). Bull. Environ. Contain. Toxicol. 28(1):3-6.
Hughes, J. S. and J. T. Davis. 1965. Comparative toxicity to bluegill sunfish
of granular and liquid herbicides. S. E. Assoc. Game Fish Conn., Ann.
Conf., Proc. 16:319-323.
Johnson, W. W. and M. T. Finley. 1980. Handbook of acute toxicity of chemicals
to fish and aquatic invertebrates. U.S.D.I., Fish Wildl. Serv., Res. Publ.
137. 98 p.
Kenaga, E. E. 1978. Test organisms and methods useful for early assessment of
acute toxicity of chemicals. Environ. Sci. Technol. 12(11):1322-1329.
Kimmerle, R. A., W. E. Gledhill, and G. J. Levinskas. 1978. Environmental
safety assessment of new materials, p. 132-146. In; Cairns, J., Jr., K.
L. Dickson, and A. W. Maki (eds.). Estimating the hazard of chemical
substances to aquatic life. Amer. See. Test. Mat., ASTM Spec. Tech. Publ.
657.
Klassen, C. D. and J. Doull. 1980. Evaluation of safety: Toxicological evalu-
ation, p. 26. In; C. D. Klassen and M. 0. Amdur. (eds.). Toxicology.
McMillan, New York.
Kokoski, C. J. 1976. Written testimony of Charles J. Kokoski, Oep. Heal. Educ.
Welf., Food Drug Admin., Docket No. 76N-0070, Washington, D. C.
Lehman, A.J. and 0. G. Fitzhugh. 1954. 100-Fold margin of safety. Assoc. Food
Drug Off. U. S. 0. Bull. 18:33-35.
•
Life Systems, Inc. 1982. Surrogate species workshop. Workshop report. Life
Systems, Inc., Cleveland, OH, U.S. Environmental Protection Agency
Contract No. 68-01-6554, LSI TR-507-36B.
-------
Macek, K. J., D. F. Walsh, J. W. Hogan, and D. D. Holz. 1972. Toxicity of the
insecticide Dursban* to fish and aquatic invertebrates in ponds. Trans
Amer. Fish. Soc. 101(3):420-427.
McKee, J. E. and H. W. Wolf. 1963. Water quality criteria. Calif. St. Wat.
Qual. Contr. Board, Publ., No. 3-A.
McKim, J. M. 1977. Evaluation of tests with early life stages of fish for
predicting long-term toxicity. J. Fish. Res. Board Can. 34(8):1148-1154.
Mount, D. Unpublished. Aquatic surrogates. Presented by J. G. Eaton at the EPA,
OTS Workshop on Surrogates Species in May, 1983 in Washington, D.C. 3 p.
National Academy of Sciences. 1977. Drinking water and health. Nat. Acad.
Sci., Washington, D. C.
O.E.C.D., Chemicals Testing Programne Ecotoxicology Group. 1980. Report on the
assessment of potential environmental effects of chemicals: The effects on
organisms other than man and on ecosystems. Vol. 1. O.E.C.D., Div.
Technol. Soc. TNO Dep. Biol. Study and Infor. Cent. TNO Environ. Res.,
Delft, Netherlands.
Oliver, K. H., G. H. Parsons, and C. T. Huffstetler. 1966. An ecological study
on the effects of certain concentrations of cacodylic aid on selected fauna
and flora. U. S. Air Force, Air Force Systems Conmand, Air Proving Ground
Center, Eglin Air Force Base, Florida. 25 p.
Pimental, D. 1971. Ecological effects of pesticides on non-target species.
Exec. Off. President, Off. Sci. Technol. 220 p.
Schmidt-Bleek, F. 1980. Regulation of chemicals within the European Economic
Community and the Organization for Economic Cooperation and Development:
Efforts for international harmonization in the control of chemicals, p.
56-63. In; Environmental risk assessment — An integrated approach. First,
Ann. Meet. Soc. Environ. Toxicol. Chem., November 23-25, 1980.
Shibko, S. 1981. Memorandum to M. L. Dourson, U.S. E.P.A., Cincinati, Ohio,
December 24, 1981. Food and Drug Admin., Washington, D. C.
Tatum, W. M. and R. D. Blackburn. 1962. Preliminary study of effects of diquat
on the natural bottom fauna and plankton in two subtropical ponds. S. E.
Assoc. Game Fish Comm. Ann. Conf., Proc. 16:301-307.
Tucker, R. K. and J. S. Leitzke. 1979. Comparative toxicology of insecticides
for vertebrate wildlife and fish. Pharmac. Theor. 6:167-220.
U.S. Environmental Protection Agengy. Unpublished. Environmental effects of
regulatory concern under TSCA. U. S. E.P.A., Off. Tox. Subst., Health and
Environmental Review Division, Environmental Effects Branch. December 2,
1983.
U.S. Environmental Protection Agengy. Unpublished. Testing triggers workshop:
Workshop report. U.S. E.P.A., OTS, Workshop on Testing Triggers held in
July and September, 1982 Washington, D.C. TR-507-47C. Project 1247 for
Contract No. 68-01-6554, Work Assignment NO. 05.
-------
U'.S. Environmental Protection Agengy. 1980. Guidelines and methodology used in
the preparation of health effects assessment chapters of the consent decree
water quality criteria. Fed. Reg. 45:79347-79357.
Veith, G. P., D. J. Call, and L. T. Brooks. 1983. Structure-toxicity relation-
ships for the fathead minnow, Pimephales promelas; narcotic industrial
chemicals. Can. J. Fish. Aquat. Sci. 40:743-748.
Vettorazzi, G. 1976. Safety factors and their application in the toxicological
evaluation, p. 207-223. In: The evaluation of toxicological data for the
protection of public health. Pergamon Press, Oxford.
Walker, C. R. 1962. Toxicological effects of herbicides on the fish environ-
ment. Ann. Air Wat. Poll. Conf., Proc. 8:17-34.
Wellborn, T. L., Jr. 1969. The toxicity of nine therapeutic and herbicidal
compounds to striped bass. Progr. Fish-Cult. 31(l):27-32.
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