Sethoxydim Risk Assessment

for the environmental fate models is substantially complicated by the number of sethoxydim 
metabolites that may be generated in the environment in additional to those discussed above.  For 
example, Koskinen et al. (1994) have identified eight soil metabolites in which the 
cyclohexen-1-one ring remains intact.  Similar complex sets of metabolites have been identified in 
anaerobic aquatic metabolism (Shiotani 1990a), anaerobic soil metabolism (Soeda and Shiotani 
1989), hydrolysis (Soeda and Shiotani  1988a) and photolysis (Huber 1981; Soeda and Shiotani 
1988b; Soeda and Shiotani 1988c). 
Ideally, monitored values or modeled estimates of the concentrations of each metabolite in 
environmental media such as water would be used along with RfD’s or similar estimates for each 
metabolite to assess and characterize risk.  This approach cannot be taken for sethoxydim and its 
metabolites, however, because toxicity data are not available for many of the metabolites of 
sethoxydim and the available data are not adequate for estimating concentrations of each 
metabolite in water or other environmental media. 
As an alternative, the assumption, analogous to that used by U.S. EPA, is made that the 
metabolites of sethoxydim have a toxicity equivalent to that of sethoxydim itself.  Based on the 
albeit limited acute toxicity data discussed above, this appears to be a reasonable and perhaps 
conservative assumption.  As a consequence of this assumption, parameters used in the 
application of the GLEAMS and related models, such as halftimes in water and soil, are based on 
rates of conversion from sethoxydim to carbon dioxide - i.e., complete degradation - rather than 
on the rate of disappearance of sethoxydim itself - i.e., the conversion of sethoxydim to 
metabolites of sethoxydim.  These longer halftimes are used in the environmental fate models 
applied in Section 3.2.3.4.2. 
3.1.9.3.  Inerts –  As indicated in Section 2, Poast contains a substantial amount of petroleum 
solvent (74%) that includes naphthalene (7% of the total).  This material has been classified by the 
U.S. EPA  as “solvent naphtha (petroleum) heavy aromatic” which is listed by the U.S. EPA/OPP 
(1998b) as a potentially toxic agent with a high priority for testing.  There is a large and complex 
literature on the toxicity of naphthalene and petroleum solvents in general (e.g., ATSDR 1997) 
and a detailed review of this literature is beyond the scope of the current document.  Nonetheless, 
the primary effect of naphthalene and petroleum solvents involves CNS depression and other signs 
of neurotoxicity that are similar to the effects seen in animals exposed to Poast as well as 
sethoxydim. 
At least for oral and dermal exposures, however, the quantitative significance of the petroleum in 
Poast does not appear to be substantial.  As discussed in Sections 3.1.2. and 3.1.7, the toxicity of 
Poast and sethoxydim appear to be comparable after oral and dermal exposures.  For inhalation 
exposures (Section 3.1.8), however, there is at least some evidence that Poast may be more toxic 
and cause qualitatively different toxic effects consistent with the presence of petroleum solvent. 
Thus, the potential effect of the petroleum solvent in Poast is considered qualitatively and 
quantitatively in this risk assessment for potential human health effects (see Section 3.4).  As 
detailed in the ecological risk assessment (Section 4), there is ample evidence that Poast is much 
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more toxic to aquatic species than sethoxydim, suggestive of the role of the petroleum solvent in 
Poast. 
3.1.10. Toxicological Interactions.  No studies have been encountered on the toxicologic 
interactions of sethoxydim or Poast in humans or experimental mammals.  In the absence of a 
better understanding of the mechanisms of the toxic action of sethoxydim, there is no basis for 
speculating as to the nature and direction of potential toxicologic interactions with other 
herbicides or other compounds in general. 
3.2.  EXPOSURE ASSESSMENT 
3.2.1.  Overview.  There are no occupational exposure studies in the available literature that are 
associated with the application of sethoxydim.  Consequently, worker exposure rates are 
estimated from an empirical relationship between absorbed dose per kilogram of body weight and 
the amount of  chemical handled in worker exposure studies on nine different pesticides.  Separate 
exposure assessments are given for broadcast ground spray (low boom spray) and backpack 
applications. 
For both types of applications, central estimates of worker exposure are similar: about 0.007 
mg/kg/day for broadcast ground spray and 0.004 mg/kg/day for backpack applications.  The 
upper limits of the exposure estimates are about 0.06 mg/kg/day for broadcast ground spray and 
0.03 mg/kg/day for backpack applications. 
Except in the case of accidental exposures, the levels of sethoxydim to which the general public 
might be exposed should be far less than the levels for workers.  Longer-term exposure scenarios 
for the general public lead to central estimates of  daily doses in the range of about 0.0000002 to 
0.0002 mg/kg/day with upper limits of exposure in the range of 0.000007 to 0.003  mg/kg/day. 
While these exposure scenarios are intended to be conservative, they are nonetheless plausible. 
Accidental exposure scenarios result in central estimates of exposure of up to 0.2 mg/kg/day and 
upper ranges of exposure up to 0.77 mg/kg/day.  All of the accidental exposure scenarios involve 
relatively brief periods of exposure, and most should be regarded as extreme. 
3.2.2.  Workers.  A summary of the exposure assessments for workers is presented in Table 3-1. 
Two types of exposure assessments are considered: general and accidental/incidental.  The term 
general exposure assessment is used to designate those exposures that involve estimates of 
absorbed dose based on the handling of a specified amount of a chemical during specific types of 
applications.  The accidental/incidental exposure scenarios involve specific types of events that 
could occur during any type of application.  Details regarding all of these exposure assessments 
are presented in the worksheets that accompany this risk assessment, as indicated in Table 3-1. 
3.2.2.1.  General Exposures  – Details of the calculations used in the worker exposure 
assessments are given in Worksheets C01a (backpack) and C01b (boom spray).  No worker 
exposure studies with sethoxydim were found in the literature.  Worker exposure rates are 
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