7.4 Risk assessment methodology for conventional and alternative  (Page 7/17)

In any exposure assessment, the risk scientists ask a number of questions to hypothesize the exposure scenario pertaining to environmental pollution affecting the population or a sub-group. Some of these are:

• What is the source of pollution at the site? (e.g. underground storage tank leak, emissions from an industrial plant, surface water run-off from agricultural fields)
• Which environmental compartments are likely to be contaminated? (i.e. air, water, sediment, soil, plants, animals, fish)
• What are the chemicals of concern (COC) originating from the pollution source?
• What are the fate and transport properties of these chemicals that may inform the aging of the pollution in the environment over time and resultant chemical signature in each environmental medium?
• Who is exposed? (e.g. children, elderly, asthmatics, general population, workers)
• How many people are exposed?
• Where are people exposed? (e.g. home, workplace, outside environment, retirement communities, schools)
• How are people exposed? (i.e. exposure pathway – inhalation, dermal contact or ingestion)
• How often are people exposed? (i.e. exposure frequency)
• How long are people exposed? (i.e. exposure duration)

Answers to these questions frame the problem at hand. In the next step, a number of exposure parameters are integrated into an estimate of daily dose received by an exposed individual via each exposure route (ingestion, dermal contact or skin absorption, and inhalation). The magnitude of human exposures, in general, is dependent on COC concentration in soil, exposure parameters describing human physiology (e.g. soil ingestion rate, body weight), and population-specific parameters describing exposure behavior (exposure frequency, duration). When evaluating subchronic or chronic exposures to noncarcinogenic chemicals, dose is averaged over the period of exposure, termed "Average Daily Dose" (ADD). However, for carcinogens, dose is averaged over an entire lifetime (i.e. 70 years), thus referred to as "Lifetime Average Daily Dose" (LADD). Both ADD and LADD represent normalized exposure rate in the units of mg of chemical per kg body weight per day (mg/kg-day). The ADD for noncarcinogenic COCs and LADD for carcinogenic COCs are estimated for four most commonly studied exposure pathways in EPA risk assessments, particularly for hazardous waste sites, as shown below ( Erdal, 2007 ):

Soil Ingestion: $L(\text{ADD}{)}_o=\frac{C_s{\text{xIR}}_o\text{xEFxEDxCF}}{\text{BWxAT}}$

Dermal Contact: $L(\text{ADD}{)}_d=\frac{C_s\text{xSAxAFxABSxEVxEFxEDxCF}}{\text{BWxAT}}$

Inhalation of Particulates: $L(\text{ADD}{)}_{\text{ip}}=\frac{C_s{\text{xIR}}_i\text{xEFxEDx}\left(\frac{1}{\text{PEF}}\right)}{\text{BWxAT}}$

Inhalation of Volatiles: $L(\text{ADD}{)}_{\text{iv}}=\frac{C_s{\text{xIR}}_i\text{xEFxEDx}\left(\frac{1}{\text{VF}}\right)}{\text{BWxAT}}$

Where:

C _s : Exposure Concentration (i.e., 95th Upper Confidence Limit on the Mean) of COC in soil (mg/kg) – (chemical-specific; can be estimated using EPA 2004b )

IR _o : Ingestion rate of soil (mg/d)

IR _i : Inhalation rate (m ³ /d)

SA: Skin surface area (cm ² )

AF: Soil-to-skin adherence factor (mg/cm ² )

ABS: Dermal absorption fraction (unitless – chemical-specific)

EV: Event frequency (events/d)

EF: Exposure frequency (d/y)

ED: Exposure duration (y)