Environmental Health Criteria 214

HUMAN EXPOSURE ASSESSMENT
Probabilistic exposure models may be run in one or two
dimensions. One-dimensional models estimate either variability among
exposures to individuals or uncertainty about a single exposure
metric; for example, the mean 8-h average carbon monoxide exposure for
individuals in a specific area. Two-dimensional simulation models
may be used to estimate both population distributions (i.e.,
inter-individual variability) and uncertainty about the population
distribution. The IAEA (1989) has suggested a Monte Carlo simulation
approach for conducting two-dimensional simulations. In the first
phase, a single realization is obtained from the distribution of each
uncertain parameter. In the second phase, repeated realizations are
obtained from the variable parameters. The entire process of a single
sampling from the uncertain parameters, followed by repeated sampling
from the variable parameters, is referred to as a simulation. A
single model run consists of generating k simulations each composed
of i iterations, which produces a family of k predicted
distributions of population exposures. Prediction uncertainty is
represented by the distribution of individual estimates for a specific
percentile or summary statistic among the family of population
distributions. In this way, the type of plot shown in Fig. 23 contains
probabilistic information on estimates of both inter-individual
variability in the prediction end-point, and uncertainty about any
specific percentile of the population distribution.
6.7 A generalized dose model
The magnitude of exposure (dose) is the amount of agent available
at human exchange boundaries (skin, lungs, gastrointestinal tract)
where absorption takes place over a specified period of time.
Depending upon boundary assumptions, a number of dose questions may be
derived. The average daily dose (ADD) is one of the most useful
approaches, and is applied for exposure to non-carcinogenic compounds
(for carcinogens, lifetime average daily dose, LADD, is often
employed). The ADD is calculated by averaging the potential dose
( D pot ) over body weight and the appropriate averaging exposure time:
ADD = total potential dose/body weight × averaging time,
where the potential dose is a product of contaminant concentration
(C) in the exposure medium contacting the body, intake rate (IR)
and exposure duration (ED):
total potential dose = C × IR × ED.
The intake rate refers the rates of inhalation, ingestion or dermal
contact depending on the route of exposure.
The concentrations in air, water and soil used for an exposure
assessment are those measured or estimated to be available in these
environmental media at the nearest receptor point to the source (e.g.,
soil or groundwater at a hazardous waste site). When an environmental
concentration is assumed constant over a long time period, the
population-averaged potential dose (for ingestion or inhalation
pathways) or absorbed dose (for dermal contact) is expressed as an
average daily dose (ADD) in mg kg -1 day -1 :
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