Page 160 - Materials Chemistry, Second Edition

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indirect (as a result of exposure to a media in which the pollutants arrive by
transport for another media where the emission takes place). Thus, all derived
exposure levels should be representative of the exposure situation they describe.
The duration and frequency of exposure, routes of exposure, human habits and
practices, as well as technological processes, need to be considered. Furthermore,
the spatial scale of exposure (e.g., personal, local, regional levels) must be taken
into account.
The quantitative process of estimating exposure is straightforward. With the
exception of the inhalation pathway, exposure is normally estimated as the rate of
pollutant contact per unit of body weight:

⋅
⋅
Dose ≡ Concentration Contact Rate Frequency (4.1)
Body weight
where Dose is the rate of exposure, Concentration is the level of pollutant in a
particular environmental media, Contact rate is the amount (per time) of the media
contacted, Frequency is a measure of how often (and over what period) exposure
occurs, and Body weight is the weight of the individual.
For some exposure routes, the individual term of doses may include multiple
parameters. For example, in estimating dermal pollutant intake during swimming,
the contact rate is calculated as the product of (1) the surface area of the skin, (2)
a chemically specific permeability, and (3) the density of water.
Exposure parameters are generally selected as a mix of typical and high-end
values to afford an overall conservative bias. Although situation-specific values are
always preferable, they are seldom available and often impractical to develop.
Default values have been established for many parameters and some conventions
have been yielded. For example, an average adult body weight of 70 kg is routinely
used in dose calculations. Moreover, exposure profiles are subject to considerable
discretion; the difficulty of exposure assessment is to choose a combination of
assumptions that satisfies the aim of the assessment and is appropriate for the
populations of interest. Implications of parameter variability and uncertainty are
difficult to test with deterministic methods; probabilistic techniques such as those
described in Chapter 5 can directly incorporate these aspects.
Risk assessments contain numerous uncertainties that are typically compensated
for by conservative assumptions designed to bias risk estimates high. Recently, the
philosophy has shifted toward the use of less conservatism. Most risk assessments
conducted in the late 1980s were centered on extreme situations such as a maximally
exposed individual (MEI). An MEI was built to receive (in theory) a level of exposure
not likely to be exceeded by any person, a level that would be extremely improbable.
More recent guidance, however, has recommended the use of reasonable maximum
exposure (RME) scenarios that attempt to work out plausible, high-end exposure
estimates. In reality, the difference between MEI and RME scenarios may be one
of semantics because concepts such as plausible, maximum and high-end are too
often subjective. Psychologically, however, the shift from MEI to RME implies a
movement from the unlikely to the plausible and assigns a greater sense of realism
to the risk estimates.

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