Risks to Aquatic Organisms from Use of Chlorpyrifos in the United States - Reviews of Environmental Contamination and Toxicology - page 118

risks. If the criteria for lower tiers do not indicate risk, further refi nements of the risk
assessment are not needed. This was not the case in the earlier assessment where
lower-tier risk criteria were exceeded and potential hazards to aquatic organisms
were identifi ed (Giesy et al. 1999 ). However, these potential hazards were not con-
sistent with the lack of incident reports, such as fi sh kills, attributable to use of chlor-
pyrifos in agriculture (Giesy et al. 1999 ). Refi nement of the earlier ERA by the use
of Species Sensitivity Distributions (SSDs) and measured concentrations of CPY in
surface waters showed that, in almost all locations in the U.S., risks associated with
use of CPY in agriculture were either negligible or de minimis (Giesy et al. 1999 ).
Since the 1999 ERA for CPY was written, there have been refi nements in the
process of risk assessment and additional data have become available for toxicity and
exposures in surface waters. In addition, there have been changes in the labeled uses
of CPY (Solomon et al. 2014 ); most notably, removal of termite control and residen-
tial uses of CPY in 2001. The former uses involved large rates of application (with
attendant potential for large environmental exposures), and the changes in the labels
signifi cantly reduced exposures from relatively uncontrolled uses in urban environ-
ments (Banks et al. 2005 ; Phillips et al. 2007 ). Availability of additional data and
changes in use patterns prompted the reassessment of risks to aquatic organisms from
use of CPY in agriculture in the U.S., the results of which are presented here. Since
lower-tier assessment had already indicated risk for CPY in surface waters (Giesy
et al. 1999 ), the lower tiers were omitted from this ERA. The current assessment
focused on a refi ned approach that employed SSDs and results of community-level
studies in microcosms and mesocosms (“cosms”) as points of departure for toxicity,
and refi ned modeling of concentrations in surface waters (Williams et al. 2014 , in this
volume) to characterize exposures. Concentrations of CPY measured in surface
waters were used as a check on the estimates of exposures predicted by use of simula-
tion models (Williams et al. 2014 ) and as another line of evidence in the ERA.

2 Problem Formulation for Risk Assessment

Risk assessments, particularly ERAs, use a formal process of problem formulation
(PF) to narrow the focus of the assessment to address key questions and, from these,
develop risk hypotheses (USEPA 1998 ). Several components of the PF have been
addressed in detail in companion papers and will only be summarized here.

2.1 Exposures to Chlorpyrifos

A conceptual model for exposures to CPY (Fig. 1 ) was constructed from environ-
mental properties data that are presented in the companion papers (Mackay et al.
2014 ; Solomon et al. 2014 ; Williams et al. 2014 ). As several studies have noted,
urban uses were a signifi cant source of historical exposures to CPY in surface water