Environmental Health Criteria 214

HUMAN EXPOSURE ASSESSMENT
Environmental Protection Agency 1992 report entitled Dermal
Exposure: Applications and Principles, reviews the literature
regarding methods for estimating soil and water contact (US EPA,
1992b). Hawley (1985) has used data from previous studies and
professional judgement to develop assumptions for use in estimating
outdoor soil contact time, but these estimates do not account for
indoor exposure such as soil tracked into the house or for exposure to
contaminants that reside primarily in indoor dust (e.g., pesticides)
(US EPA, 1992b). The US EPA report cites Tarshis (1981) and James &
Knuiman (1987) as sources of data on the frequency with which people
shower and bathe. Few data are available on swimming (US EPA, 1992b)
which could be important because of elevated chloroform concentrations
found within air just above the pool-water surface, or other
contaminants which can be swallowed or dermally absorbed from lakes or
river waters.
Linking activities with measurements of dermal exposure,
researchers are testing innovative approaches to assessing skin
contact with contaminated surfaces. For instance, Fenske et al.
(1986a,b) applied non-toxic fluorescent tracers to lawns in lieu of
insecticides; after participants engaged in a standard set of play
activities, video imaging was used to ascertain the amount of tracer
on the exposed skin. The degree of soil adherence to skin is a closely
related issue and has been examined by several researchers (Driver et
al., 1989; Finley et al., 1994a; Kissel et al., 1996).
5.3 Potential limitations
Time-activity data can enhance an understanding of sources and
behaviours important in assessing exposures. Inferences can be drawn
from simulations, case studies or even studies using large randomized
designs. However, all users of time-activity data should be aware of
its limitations for assessing human exposure to environmental
contaminants.
The feasibility of collecting time-activity data is often limited
by the burden which such studies impose on participating individuals.
The data collection requires constant, or regular, attention to the
fact that the subjects are participating in the study, that they have
to remember about all activities and to fill in the diaries. This is
often inconvenient and takes respondent's time. Collection of the data
by an observer, which often is a method of choice in studies involving
children, may be of limited feasibility owing to the restricted access
of the observer to the subject under study and because typical
activities may possibly be modified by the fact of being under
observation.
5.3.1 Activity representativeness
One of the uses of time-activity data is to allow
characterization of the distribution of exposure for a given
geographic, demographic or socioeconomic segment of the population.
However, the study protocol may call for certain types of days or
individuals to be excluded (e.g., travel that takes the participant
away from the home for more than the 24-h or 48-h sampling period may
lead to disqualification). Although standard techniques such as
weighting and imputation can be used to treat non-response, these
methods assume that refusal to participate is random and there is
information about the non-respondents (Kalton & Kasprzyk, 1986). In
the case of time-activity studies, however, once contacted, people may
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