Sethoxydim Risk Assessment

the current risk assessment for short-term exposures to children and adults.  For adults, the 
applicability of the FQPA uncertainty factor is questionable but, as discussed further in Section 
3.4, the use of this factor has no impact on the risk characterization. 
The LOAEL used to define the NOAEL for the chronic RfD involved only slight decreases in 
hematologic parameters in dogs.  These are characterized by U.S. EPA as equivocal anemia (U.S. 
EPA/OPP 1998a) or mild anemia (U.S. EPA/IRIS 1989).  While there were enzymatic changes 
indicative of liver damage (Appendix 1), no liver pathology was noted and the dogs evidenced no 
overt signs of toxicity.  Thus, excursions above the chronic RfD would not be expected to result 
in frank adverse effects and modest excursions would probably not result in clinically significant 
effects. 
3.4. RISK CHARACTERIZATION 
3.4.1. Overview.  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. 
Thus, sethoxydim does not seem likely to pose any substantial risk to human health.  This 
conclusion is consistent with the recent evaluation of sethoxydim by the U.S. EPA/OPP (1998a) 
in which margins of exposure were calculated to be over 100 for acute exposure and over 1000 
for chronic exposure. 
The only reservation associated with this assessment of sethoxydim is the same reservation 
associated with any risk assessment in which no plausible hazards can be identified: Absolute 
safety cannot be proven and the absence of risk can never be demonstrated.  No chemical, 
including sethoxydim, is studied for all possible effects.  Furthermore, using data from laboratory 
animals to estimate hazard or the lack of hazard to humans is an uncertain process.  Prudence 
dictates that normal and reasonable care should be taken in the handling of this or any other 
chemical.  Notwithstanding these reservations, the use of sethoxydim in Forest Service programs 
does not pose any identifiable hazard to workers or members of the general public. 
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Although the U.S. EPA does not classify sethoxydim as an irritant to the skin and eyes, other 
reports not addressed by U.S. EPA suggest that skin and eye irritation can result from exposure 
to relatively high levels of  Poast.  From a practical perspective, eye or skin irritation is likely to 
be the only overt effect as a consequence of mishandling sethoxydim.  These effects can be 
minimized or avoided by prudent industrial hygiene practices during the handling of the 
compound. 
3.4.2. Workers.  A quantitative summary of the risk characterization for workers is presented in 
Table 3-6.  The quantitative risk characterization is expressed as the hazard quotient.  For general 
exposures, the hazard index is calculated as the estimated doses from Table 3-1 divided by the 
chronic RfD of 0.09 mg/kg/day.  For accidental exposure scenarios, the estimated doses are 
divided by the acute RfD of 0.6 mg/kg.  Documentation for both of these RfD’s is provided in 
Section 3.3.2. 
Given the very low hazard quotients for accidental exposure, the risk characterization is 
reasonably unambiguous.  None of the accidental exposure scenarios approach a level of concern. 
While the accidental exposure scenarios are not the most severe one might imagine (e.g., 
complete immersion of the worker or contamination of the entire body surface for a prolonged 
period of time) they are representative of reasonable accidental exposures.  Given that the highest 
hazard quotient for any accidental exposure scenario - i.e., 0.08 as the upper range of the hazard 
quotient for wearing contaminated gloves for one hour - is a factor of 12.5 lower than the level of 
concern, substantially more severe and less plausible scenarios would be required to suggest a 
potential for systemic toxic effects.  As discussed in Section 3.2, confidence in this assessment is 
diminished by the lack of information regarding the dermal absorption kinetics of sethoxydim in 
humans as well as the lack of information on the effects of additives in Poast on the dermal 
absorption of sethoxydim.  Nonetheless, the statistical uncertainties in the estimated dermal 
absorption rates, both zero-order and first-order, are incorporated into the exposure assessment 
and risk characterization.  Again, these estimates would have to be in error by a factor of greater 
than 12.5 in order for the basic characterization of risk to change. 
Similarly, the hazard quotients for backpack and low boom broadcast applications are below a 
level of concern by a factor of 3 or more for upper limits and factors of over 10 for central 
estimates.  As with the accidental exposures, there are uncertainties in these exposure 
assessments; however, given the low hazard quotients, these uncertainties do not impact 
substantially the characterization of risk. 
As discussed in Section 3.1.6, sethoxydim can cause mild irritation to the skin and eyes. 
Quantitative risk assessments for irritation are not derived; however, from a practical perspective, 
eye or skin irritation is likely to be the only overt effect as a consequence of mishandling 
sethoxydim.  These effects can be minimized or avoided by prudent industrial hygiene practices 
during the handling of the compound. 
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