Sethoxydim Risk Assessment

expressed in units of mg of absorbed dose per kilogram of body weight per pound of chemical 
handled.  These exposure rates are based on worker exposure studies on nine different pesticides 
with molecular weights ranging from 221 to 416 and log K
ow
 values at pH 7 ranging from -0.75 to 
6.50.  The estimated exposure rates are based on estimated absorbed doses in workers as well as 
the amounts of the chemical handled by the workers (SERA 2000).  As summarized in Table 2-1 
of this risk assessment, the molecular weight of sethoxydim is 327.5 g/mole and the log  K
ow
 at 
pH 7 is about 1.65.  Both of these values are within the range of values used in the empirical 
relationships for worker exposure.  As described in SERA (2000), the ranges of estimated 
occupational exposure rates vary substantially among individuals and groups, (i.e., by a factor of 
50 for backpack applicators and a factor of 100 for mechanical ground sprayers).  While this adds 
substantial uncertainty to the exposure assessment, this variability has little practical impact on 
this risk assessment because even the upper limits of exposure are below levels of concern 
(Section 3.4.2). 
For the worker exposure assessments, the number of acres treated per hour is taken from previous 
USDA risk assessments (USDA 1989a,b,c). The number of hours worked per day is expressed as 
a range, the lower end of which is based on an 8-hour work day with 1 hour at each end of the 
work day spent in activities that do not involve herbicide exposure.  The upper end of the range, 8 
hours per day, is based on an extended (10-hour) work day, allowing for 1 hour at each end of the 
work day to be spent in activities that do not involve herbicide exposure. 
It is recognized that the use of 6 hours as the lower range of time spent per day applying 
herbicides is not a true lower limit.  It is conceivable and perhaps common for workers to spend 
much less time in the actual application of a herbicide if they are engaged in other 
activities.  Thus, using 6 hours can be regarded as conservative.  In the absence of any published 
or otherwise documented work practice statistics to support the use of a lower limit, this 
conservative approach is used. 
The range of acres treated per hour and hours worked per day is used to calculate a range for the 
number of acres treated per day.  For this calculation as well as others in this section involving the 
multiplication of ranges, the lower end of the resulting range is the product of the lower end of 
one range and the lower end of the other range.  Similarly, the upper end of the resulting range is 
the product of the upper end of one range and the upper end of the other range.  This approach is 
taken to encompass as broadly as possible the range of potential exposures. 
The central estimate of the acres treated per day is taken as the arithmetic average of the range. 
Because of the relatively narrow limits of the ranges for backpack and boom spray workers, the 
use of the arithmetic mean rather than some other measure of central tendency, like the geometric 
mean, has no marked effect on the risk assessment. 
The range of application rates and the typical application rate are taken directly from the program 
description (see Section 2.4). 
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As detailed in worksheets C01a (directed foliar) and C01b (broadcast foliar), the central estimate 
of the amount handled per day is calculated as the product of the central estimates of the acres 
treated per day and the application rate.  The ranges for the amounts handled per day are 
calculated as the product of the range of acres treated per day and the range of application rates. 
Similarly, the central estimate of the daily absorbed dose is calculated as the product of the central 
estimate of the exposure rate and the central estimate of the amount handled per day.  The ranges 
of the daily absorbed dose are calculated as the range of exposure rates and the ranges for the 
amounts handled per day.  The lower and upper limits are similarly calculated using the lower and 
upper ranges of the amount handled, acres treated per day, and worker exposure rate. 
3.2.2.2.  Accidental Exposures  -- Typical occupational exposures may involve multiple routes of 
exposure (i.e., oral, dermal, and inhalation); nonetheless, dermal exposure is generally the 
predominant route for herbicide applicators (van Hemmen 1992).  Typical multi-route exposures 
are encompassed by the methods used in Section 3.2.2.1 on general exposures.  Accidental 
exposures, on the other hand, are most likely to involve splashing a solution of a herbicide into the 
eyes or to involve various dermal exposure scenarios. 
Poast can cause irritant effects in the skin and eyes (see Section 3.1.6).  The available literature 
does not include quantitative methods for characterizing exposure or responses associated with 
splashing a solution of a chemical into the eyes; furthermore, there appear to be no  reasonable 
approaches to modeling this type of exposure scenario quantitatively.  Consequently, accidental 
exposure scenarios of this type are considered qualitatively in the risk characterization 
(Section 3.4). 
There are various methods for estimating absorbed doses associated with accidental dermal 
exposure (U.S. EPA/ORD 1992, SERA 2000).  Two general types of exposure are modeled: 
those involving direct contact with a solution of the herbicide and those associated with accidental 
spills of the herbicide onto the surface of the skin.  Any number of specific exposure scenarios 
could be developed for direct contact or accidental spills by varying the amount or concentration 
of the chemical on or in contact with the surface of the skin and by varying the surface area of the 
skin that is contaminated. 
For this risk assessment, two exposure scenarios are developed for each of the two types of 
dermal exposure, and the estimated absorbed dose for each scenario is expressed in units of mg 
chemical/kg body weight.  As specified in Table 3-1, the details of these exposure estimates are 
presented in the worksheets appended to this risk assessment. 
Exposure scenarios involving direct contact with solutions of the chemical are characterized by 
immersion of the hands for 1 minute or wearing contaminated gloves for 1 hour.  Generally, it is 
not reasonable to assume that the hands or any other part of a worker will be immersed in a 
solution of a herbicide for any period of time.  Notwithstanding this assertion, contamination of 
gloves or other clothing is quite plausible.  For these exposure scenarios, the key element is the 
assumption that wearing gloves grossly contaminated with a chemical solution is equivalent to 
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