Risks to Aquatic Organisms from Use of Chlorpyrifos in the United States - Reviews of Environmental Contamination and Toxicology - page 125

Fig. 2 Conceptual model for effects of chlorpyrifos on aquatic organisms in surface waters. The
weights of the arrows indicate importance of the pathway of exposure

hypotheses (Fig. 2 ). Fish and amphibians are less sensitive to direct effects than
crustaceans and insects but they could be affected indirectly via alterations of the
food web (Fig. 2 ). As discussed above, exposure via the food chain was not consid-
ered in this ERA.

2.5 Analysis Plan

The previous assessment of risks of CPY in surface waters of the U.S. (Giesy et al.
1999 ) began with a lower-tier deterministic characterization of risk quotients (RQs,
also referred to as hazard quotients, HQs) and then advanced through several tiers
of refi nement to a probabilistic assessment of risks based on comparisons of SSDs
to distributions of measured concentrations of CPY in surface waters. Because
lower tiers are designed to be conservative and to be applied when few data are
available, they are not applicable to CPY, for which there is a wealth of data. Thus,
the risk assessment was focused on the upper, more refi ned, tiers. SSDs were used
to characterize acute effects, and these distributions were compared to concentrations
predicted by simulation models and concentrations measured in surface waters.