Refined Avian Risk Assessment for Chlorpyrifos in the United States - Reviews of Environmental Contamination and Toxicology - page 202

Table 7 Sources of uncertainty in the avian risk assessment for fl owable chlorpyrifos
Area Source of uncertainty Action and infl uence on risk estimates

Exposure
scenarios

Not possible to assess all scenarios.
Potential to miss high risk scenarios.

High-use crops were assessed assuming maximum application rates and minimum re-treatment
intervals. Exposure scenarios likely included upper bound risks posed by CPY to birds.

Routes of
exposure

Focus of assessment on ingestion of food
and water. Could be other important
routes of exposure.

Dermal contact, inhalation and preening unlikely to be important based on results of limited studies.
Refi ned models are lacking for these routes of exposure. Evaluations of model performance against
fi eld studies indicated that LiquidPARAM is closely or over-estimating risk to birds. Thus, it is
unlikely that routes of exposure other than ingestion of food and water are highly signifi cant.

Risk to
non-focal
bird
species

The refi ned risk assessment considered
exposure to 15 focal species. Thus,
there is a possibility that bird species
not considered in this assessment are at
risk on or near CPY-treated fi elds.

The focal species were selected because of their affi nity for agricultural areas where fl owable CPY is
used. This group of species is more likely to be exposed to CPY than would most other bird species.
Furthermore, they span a range of sizes and taxonomic groups. Thus, there is a low degree of
uncertainty associated with overlooking bird species at risk. Additionally, the focal species likely
bracket all of the bird species potentially at risk due to CPY application.

Proportion
time birds
on fi elds

Information is lacking to fully characterize
between-fi eld, between-individual and
between-time steps variability in
proportion of time birds forage on
treated fi elds. Also assumed that the
proportion of time birds spend on
treated fi elds equates to the proportion
of their diet that comes from treated
fi elds.

Original data for proportion of time birds spend in treated fi elds ( PT ) were obtained for corn and alfalfa
and analyzed to determine between-fi eld variability for each of the focal species. Without radio-tracking
data, however, it was not possible to quantify between-individual and between-time steps variability
for any of the focal species. Given the state of knowledge for between-individuals variability,
uncertainty was maximized by parameterizing the within-fi eld PT distributions to range from 0 to 1,
with the best estimate being the randomly chosen most likely PT from the between-fi elds distribution.
No variation was assumed between time steps, a reasonable assumption for nesting birds, but not
pre-migratory or migratory birds. No data exist to determine whether proportion of time birds spend on
treated fi elds equates to the proportion of their diet that comes from treated fi elds. The various
shortcomings with the PT variable could ultimately lead to under- or over-estimation of risk. The
results of the evaluation of model performance and the fl owable CPY fi eld studies suggest that the risk
estimates produced by LiquidPARAM are reasonable or over-estimated.

Quality and
quantity
of toxicity
studies

Toxicity studies for CPY were conducted
over several decades using a variety of
protocols. Quantity of data is limited
for bird species particularly for chronic
studies. Gavage method used in acute
oral studies does not replicate feeding
patterns in the fi eld.

Toxicity studies underwent data quality review. Only data of suffi cient quality were used to derive
dose-response curves, the species sensitivity distribution and chronic effects metrics. Additional
chronic toxicity studies on species other than the mallard ( Anas platyrhynchos ) and northern
bobwhite ( Colinus virginianus ) would reduce uncertainty in the chronic risk assessment.