7.4 Risk assessment methodology for conventional and alternative  (Page 8/17)

PEF: Particulate emission factor (m ³ /kg) – 1.36 x 10 ⁹ m ³ /kg per ( EPA 2002a )

VF: Soil-to-air volatilization factor (m ³ /kg – chemical-specific)

BW: Body weight (kg)

AT: Averaging time (days) – (ED*365 d/y for noncarcinogens; 70 y*365 d/y for carcinogens)

CF: Conversion factor – 10 ^-6 kg/mg

In deterministic risk assessment, ADD and LADD estimates are performed for a reasonable maximum exposure scenario (RME) and a central tendency exposure scenario (CTE), resulting in a range. EPA's reliance on the concept of RME for estimating risks is based on a conservative but plausible exposure scenario (which is defined to be the 90 ^th to 95 ^th percentile exposure, signifying that fewer than five percent to 10 percent of the population would be expected to experience higher risk levels), and has been scientifically challenged over the years. For example, Burmaster and Harris ( 1993 ) showed that the use of EPA recommended default exposure parameter values resulted in exposure and risk estimates well in excess of the 99 ^th percentile due to multiplication of three upper-bound values (i.e. 95 ^th percentiles) for IR, EF, and ED. The authors argued that this leads to hazardous waste site cleanup decisions based on health risks that virtually no one in the surrounding population would be expected to experience. They advised the EPA to endorse and promote the use of probabilistic methods (e.g. Monte-Carlo simulations ) as a way to supplement or replace current risk assessment methods, in order to overcome the problem of "compounded conservatism" and enable calculation of risks using a more statistically defensible estimate of the RME. In probabilistic risk assessment, the input parameters are characterized by their unique probability distribution. The EPA's Exposure Factors Program provides information on development of exposure parameter distributions in support of probabilistic distributions and can be accessed via: (External Link) .

The values of the exposure parameters corresponding to RME or CTE scenarios are often compiled from EPA's Exposure Factors Handbook (EFH):

• General Exposure Factors Handbook ( EPA, 1997 ) provides exposure assessors with data needed on standard factors to calculate human exposure to toxic chemicals as part of risk assessments. These factors include: drinking water consumption, soil ingestion, inhalation rates, dermal factors including skin area and soil adherence factors, consumption of fruits and vegetables, fish, meats, dairy products, homegrown foods, breast milk intake, human activity factors, consumer product use, and residential characteristics. Recommended values are for the general population and also for various segments of the population who may have characteristics different from the general population. The most recent version of the EFH can be accessed via (External Link) .
• The Children-Specific EFH ( EPA, 2002b ) provides a summary of the available and up-to-date statistical data on various factors assessing child exposures, which can be accessed via (External Link) .