Sethoxydim Risk Assessment

assessments, the summaries from Bryceland et al. (1997) are used.  In most cases, this did not 
have a substantial impact on the risk assessment.  Specific examples are discussed in various 
sections of this risk assessment as appropriate. 
The human health and ecological risk assessments presented in this document are not, and are not 
intended to be, comprehensive summaries of all of the available information.  The information 
presented in the appendices and the discussions in chapters 2, 3, and 4 of the risk assessment are 
intended to be detailed enough to support a review of the risk analyses; however, they are not 
intended to be as detailed as the information generally presented in Chemical Background 
documents or other comprehensive reviews. 
For the most part, the risk assessment methods used in this document are similar to those used in 
risk assessments previously conducted for the Forest Service as well as risk assessments 
conducted by other government agencies.  Details regarding the specific methods used to prepare 
the human health risk assessment are provided in SERA (2000). 
Risk assessments are usually expressed with numbers; however, the numbers are far from exact. 
Variability and  uncertainty may be dominant factors in any risk assessment, and these factors 
should be expressed.  Within the context of a risk assessment, the terms variability and 
uncertainty signify different conditions. 
Variability reflects the knowledge of how things may change.  Variability may take several forms. 
For this risk assessment, three types of variability are distinguished: statistical, situational, and 
arbitrary.  Statistical variability reflects, at least, apparently random patterns in data.  For 
example, various types of estimates used in this risk assessment involve relationships of certain 
physical properties to certain biological properties.  In such cases, best or maximum likelihood 
estimates can be calculated as well as upper and lower confidence intervals that reflect the 
statistical variability in the relationships.  Situational variability describes variations depending on 
known circumstances.  For example, the application rate or the applied concentration of a 
herbicide will vary according to local conditions and goals.  As discussed in the following section, 
the limits on this variability are known and there is some information to indicate what the 
variations are.  In other words, situational variability is not random.  Arbitrary variability, as the 
name implies, represents an attempt to describe changes that cannot be characterized statistically 
or by a given set of conditions that cannot be well defined.  This type of variability dominates 
some spill scenarios involving either a spill of a chemical on to the surface of the skin or a spill of 
a chemical into water.  In either case, exposure depends on the amount of chemical spilled and the 
area of skin or volume of water that is contaminated. 
Variability reflects a knowledge or at least an explicit assumption about how things may change, 
while uncertainty reflects a lack of knowledge.  For example, the focus of the human health 
dose-response assessment is an estimation of an “acceptable” or “no adverse effect” dose that will 
not be associated with adverse human health effects.  For sethoxydim and for most other 
chemicals, however, this estimation regarding human health must be based on data from 
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experimental animal studies, which cover only a limited number of effects.  Generally, judgment is 
the basis for the methods used to make the assessment.  Although the judgments may reflect a 
consensus (i.e., be used by many groups in a reasonably consistent manner), the resulting 
estimations of risk cannot be proven analytically.  In other words, the estimates regarding risk 
involve uncertainty.  The primary functional distinction between variability and uncertainty is that 
variability is expressed quantitatively, while uncertainty is generally expressed qualitatively. 
In considering different forms of variability, almost no risk estimate presented in this document is 
given as a single number.  Usually, risk is expressed as a central estimate and a range, which is 
sometimes very large.  Because of the need to encompass many different types of exposure as 
well as the need to express the uncertainties in the assessment, this risk assessment involves 
numerous calculations. 
Most of the calculations are relatively simple, and the very simple calculations are included in the 
body of the document.  Some of the calculations, however, are  cumbersome.  For those 
calculations, a set of worksheets is included as an attachment to the risk assessment.  The 
worksheets provide the detail for the estimates cited in the body of the document.  The 
worksheets are divided into the following sections: general data and assumptions, chemical 
specific data and assumptions, exposure assessments for workers, exposure assessments for the 
general public, and exposure assessments for effects on nontarget organisms.  The worksheets are 
included at the end of this risk assessment and further documentation for these worksheets are 
included as Attachment 1.  As detailed in Attachment 1, two versions of the worksheets are 
available: one in a word processing format and one in a spreadsheet format.  The worksheets that 
are in the spreadsheet format are used only as a check of the worksheets that are in the word 
processing format.  Both sets of worksheets are provided with the hard-text copy of this risk 
assessment as well as with the electronic version of the risk assessment.  Documentation for the 
use of these worksheets is provided in a separate document that also accompanies this risk 
assessment (SERA 2001). 
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