Specifying the equation to calculate dose

Specifying the equation to calculate dose

After selecting and describing the exposure pathways, the exposure assessment

must calculate the intake of the contaminant of concern (in milligrams contaminant

per kilogram body weight per day). This is the dose of contaminants that enters the

human body through the gastrointestinal tract following consumption of

contaminated seafood. USEPA guidance describes this dose as the Average Daily

Potential Dose (ADDpot). This is a central calculation in the human health

assessment because it integrates all the elements of the exposure assessment. For

the assessment of risk associated with contaminated dredged materials, it should be

calculated for each of the individual fish species that are ingested by each receptor

described in the exposure scenarios.

Figure 11 shows the elements in this equation and their sources. The

quantification of this exposure is expressed as the product of the exposure point

concentration and various exposure factors:

EPC Abs IR FI EF ED

ADD pot (mg / kg / day ) =

(6)

BWavg AT

where

EPC

Exposure point concentration in seafood influenced by the

dredged material disposal site (mg/kg)

Abs

Fraction of contaminant absorbed from the seafood through the

gastrointestinal tract

Ingestion rate (kg/meal)

Fraction of seafood ingested from contaminated source

(unitless)

Frequency of potential exposure events, total annual seafood

meals ingested (meals/year)

Duration of the exposure period (years)

BWavg =

Average body weight of receptor (kg)

Averaging time (days)

According to USEPA guidance (USEPA/OERR 1992a,b), the EPC and the

exposure factors in this equation should represent reasonable maximum exposures

(RME). The RME is a plausible estimate of the individual risk for those individuals

at the upper end of the risk distribution. Under the reasonable maximum exposure

case, a combination of 50th- and 90th-percentile values of exposure factors should be

used for intake rates, fraction of seafood diet harvested from the disposal site,

exposure frequency, and exposure duration. Table 3 summarizes the factors risk

assessors need to consider when determining default values or directly measuring

values for this calculation.

Chapter 4 Human Health Risk Assessment