Sethoxydim Risk Assessment

from 0.09375 lb sethoxydim/acre to 0.375 lb sethoxydim/acre.  For simplicity, all application rates 
cited in this risk assessment are referenced simply as lb/acre rather than lb a.i./acre or lb a.e./acre. 
Unless otherwise specified, all such designations refer to lb a.i./acre or lb sethoxydim/acre.  For 
this risk assessment, the lower and upper limits of the application rate are taken as 0.09375 lb/acre 
to 0.375 lb/acre, respectively, based on the lower and upper limits of the labeled rates.  Based on 
the most recent use statistics from the Forest Service, the central estimate of the application rate 
is taken as 0.3 lbs/acre. 
Poast as well as many of the other commercial formulations of sethoxydim are used extensively in 
agriculture.  Based on the most recent use statistics encountered in the literature, over 1,000,000 
lbs of sethoxydim are applied to crops annually, primarily to soybeans and cotton in the mid-west. 
By comparison, the uses of sethoxydim by the Forest Service are trivial - i.e., a total of 3.8 lbs in 
1999. 
HUMAN HEALTH RISK ASSESSMENT 
Hazard Identification –  Reported gavage LD
50
 values for sethoxydim range from about 3000 to 
6000 mg/kg in rats and 5600 to 6500 mg/kg in mice.  The oral LD
50
 in dogs is 2500-5000 mg/kg 
but the method of administration involved capsules rather gavage exposures and thus the results 
cannot be directly compared to those in rats and mice.  The acute oral LD
50
 of the formulated 
product, Poast, is comparable to that of sethoxydim – i.e., 4390 to 5000 mg Poast/kg.  For both 
sethoxydim and Poast, the primary signs of acute poisoning in  mice, rats, and dogs are consistent 
with neurological effects: lacrimation, salivation, incontinence,  ataxia, tremors, and convulsions. 
The available data on sethoxydim are sufficient to define NOAELs for systemic toxic effects from 
both acute and chronic exposures.  Sethoxydim has been tested for and does not appear to cause 
carcinogenicity, birth defects, or other reproductive effects. 
Poast contains a substantial amount of petroleum solvent (74%) that includes naphthalene (7% of 
the solvent).  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.  While sethoxydim is rapidly degraded in the environment, some of the 
degradation products are much more persistent and this pattern is quantitatively considered in the 
risk assessment. 
Based on standard studies required for pesticide registration, Poast may cause skin and eye 
irritation. Concentrations of sethoxydim in the air that would be much higher than any plausible 
concentrations in human exposure scenarios have been associated with lung congestion in rats. 
The potential inhalation toxicity of sethoxydim is not of substantial concern to this risk assessment 
because of the implausibility of inhalation exposure involving  high concentrations of this 
compound. 
Exposure Assessment – There are no occupational exposure studies in the available literature that 
are associated with the application of sethoxydim.  Consequently, worker exposure rates are 
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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. 
Dose-Response Assessment – The Office of Pesticide Programs of the U.S. EPA has derived both 
an acute and chronic RfD for sethoxydim.  The chronic RfD of 0.09 mg/kg/day based on a 
NOAEL of 9 mg/kg/day for a 1-year feed study in dogs and an uncertainty factor of 100.  This 
uncertainty factor includes 10 for extrapolating from animals to humans and 10 for extrapolating 
to sensitive individuals within the human population.  The acute RfD is 0.6 mg/kg/day based on a 
NOAEL in rabbits of 180 mg/kg/day and an uncertainty factor of 300.  The uncertainty factor for 
the acute RfD includes the same two components as the uncertainty factor for the chronic RfD as 
well as an FQPA (Food Quality Protection Act) uncertainty factor of 3 for the possible increased 
sensitivity of children to sethoxydim. 
Risk Characterization – None of the exposure scenarios for workers result in levels that exceed 
the RfD.  For members of the general public, none of the longer term exposure scenarios exceed 
the chronic RfD and the only acute exposure scenario that exceeds the acute RfD involves an 
accidental spill into a small pond. 
Based on central estimates of longer term exposure for workers and the general public, the levels 
of exposure will be below the RfD by factors of about 25 (backpack workers) to about 50,000 
(contaminated fish for members of the general public).  Even for accidental exposures, the upper 
limits of the exposure estimates are below the RfD by factors of about 10 to over 100 except for 
the consumption of contaminated water by a child after an accidental spill.  As detailed in the 
exposure assessment, the accidental spill scenario should be regarded as extreme.  Nonetheless, 
this assessment does suggest that measures should be taken to limit exposure in the event of a 
large spill.  Such measures would be routinely taken by the Forest Service after any spill into 
ambient water. 
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