Sethoxydim Risk Assessment

immersing the hands in a solution.  In either case, the concentration of the chemical in solution 
that is in contact with the surface of the skin and the resulting dermal absorption rate are 
essentially constant. 
For both scenarios (the hand immersion and the contaminated glove), the assumption of 
zero-order absorption kinetics is appropriate.  Following the general recommendations of U.S. 
EPA/ORD (1992), Fick's first law is used to estimate dermal exposure. 
Exposure scenarios involving chemical spills on to the skin are characterized by a spill on to the 
lower legs as well as a spill on to the hands.  In these scenarios, it is assumed that a solution of the 
chemical is spilled on to a given surface area of skin and that a certain amount of the chemical 
adheres to the skin.  The absorbed dose is then calculated as the product of the amount of the 
chemical on the surface of the skin (i.e., the amount of liquid per unit surface area multiplied by 
the surface area of the skin over which the spill occurs and the concentration of the chemical in 
the liquid) the first-order absorption rate, and the duration of exposure.  For both scenarios, it is 
assumed that the contaminated skin is effectively cleaned after 1 hour.  As with the exposure 
assessments based on Fick's first law, this product (mg of absorbed dose) is divided by body 
weight (kg) to yield an estimated dose in units of mg chemical/kg body weight.  The specific 
equation used in these exposure assessments is taken from SERA (2000). 
3.2.3.  General Public. 
3.2.3.1. General Considerations –  Under normal circumstances, members of the general public 
should not be exposed to substantial levels of sethoxydim as a result of Forest Service activities. 
Nonetheless, any number of exposure scenarios can be constructed for the general public, 
depending on various assumptions regarding application rates, dispersion, canopy interception, 
and human activity.  Several highly conservative scenarios are developed for this risk assessment. 
The two types of exposure scenarios developed for the general public include acute exposure and 
longer-term or chronic exposure.  Given the limited use of sethoxydim by the Forest Service – 
e.g., a total of 3.8 lbs in 1999 – chronic exposures to significant amounts of sethoxydim from 
Forest Service programs is highly implausible.  Nonetheless, as detailed below, a number of 
standard exposure scenarios involving substantially greater amounts of sethoxydim are covered in 
this risk assessment in the event that the Forest Service may consider increasing the use of this 
compound. 
Most of the acute exposure scenarios are accidental.  They assume that an individual is exposed to 
the compound either during or shortly after its application.  Specific scenarios are developed for 
direct spray, dermal contact with contaminated vegetation, as well as the consumption of 
contaminated fruit, water, and fish.  Most of these scenarios should be regarded as extreme, some 
to the point of limited plausibility.  The longer-term or chronic exposure scenarios parallel the 
acute exposure scenarios for the consumption of contaminated fruit, water, and fish but are based 
on estimated levels of exposure for longer periods after application. 
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The exposure scenarios developed for the general public are summarized in Table 3-2, and the 
details regarding the assumptions and calculations involved in these exposure assessments are 
provided in worksheets D01-D09.  The remainder of this section focuses on a qualitative 
description of the data supporting each of the assessments. 
3.2.3.2.  Direct Spray  -- Direct sprays involving ground applications are modeled in a manner 
similar to accidental spills for workers (see Section 3.2.2.2.).  In other words, it is assumed that 
the individual is sprayed with a solution containing the compound and that an amount of the 
compound remains on the skin and is absorbed by first-order kinetics.  As with the similar worker 
exposure scenarios, the first-order absorption kinetics are estimated from the empirical 
relationship of first-order absorption rate coefficients to molecular weight and octanol-water 
partition coefficients, as defined in worksheet A07a. 
For direct spray scenarios, it is assumed that during a ground application, a naked child is sprayed 
directly with sethoxydim.  The scenario also assumes that the child is completely covered (that is, 
100% of the surface area of the body is exposed), which makes this an extremely conservative 
exposure scenario that is likely to represent the upper limits of plausible exposure (Worksheet 
D01a).  An additional set of scenarios are included involving a young woman who is accidentally 
sprayed over the feet and legs (Worksheet D01b).  For each of these scenarios, some assumptions 
are made regarding the surface area of the skin and body weight.  These assumptions are detailed 
and referenced in Worksheet A03. 
Because sethoxydim is only applied by backpack or low boom spray, none of these acute 
exposure scenarios are plausible.  They are included in the current risk assessment to 
accommodate the possible expansion of the use and application methods of sethoxydim in Forest 
Service operations. 
3.2.3.3.  Dermal Exposure from Contaminated Vegetation  -- In this exposure scenario, it is 
assumed that the herbicide is sprayed at a given application rate and that an individual comes in 
contact with sprayed vegetation or other contaminated surfaces at some period after the spray 
operation. 
For these exposure scenarios, some estimates of dislodgeable residue and the rate of transfer from 
the contaminated vegetation to the surface of the skin must be available.  No such data are 
directly available for sethoxydim, and the estimation methods of Durkin et al. (1995) are used as 
defined in worksheet D02.  Other estimates used in this exposure scenario involve estimates of 
body weight, skin surface area, and first-order dermal absorption rates.  The estimates of body 
weight and surface area are detailed in Worksheet A03 and the estimated first-order dermal 
absorption rate is detailed in Worksheet B03. 
As discussed in Section 3.1.7, no experimental studies are available on the dermal absorption rate 
of sethoxydim and estimates of both zero-order and first-order dermal absorption rates are based 
on empirical relationships of the molecular weight and octanol-water partition coefficient 
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