Environmental Health Criteria 214

HUMAN EXPOSURE ASSESSMENT
Dermal absorption of contaminants in residential water sources
may occur during bathing and other forms of washing or cleaning. There
are three principal mechanisms by which molecules can transverse the
skin and enter the body: passive transfer or diffusion, facilitated
diffusion and active transport. Passive diffusion is the mechanism
most commonly expressed in dermal exposure models. The rate of passive
diffusion is a function of the concentration gradient of the
contaminant on the surface of the skin and in the tissue immediately
below the skin and the ease with which a molecule of the contaminant
can move through the lipophilic interior of the skin membrane. Ease of
passage is a function of the partition coefficient of the contaminant
(e.g., the octanol-water partition coefficient, K ow ), molecular
size, the degree of ionization and the porosity of the skin. Porosity
of the skin to VOCs present in drinking-water treated with chlorine
has been shown to be temperature dependent (Gordon et al., 1998).
Table 18. Lognormal distributions of water intake by age group in
the USA. Source: Roseberry & Burmaster (1992)
Age group Geometric mean (ml/day) Geometric standard deviation
Drinking and cooking water intake
< 1 year 267 1.85
2-11 620 1.65
12-20 786 1.72
21-65 1122 1.63
> 65 1198 1.62
Total water intake (direct + intrinsic)
< 1 year 1074 1.34
2-11 1316 1.40
12-20 1790 1.41
21-65 1926 1.49
> 65 1965 1.50
Inhalation exposures to VOCs transferred from water to air could
be as great as, or even greater than, exposures from ingestion.
Inhalation pathways include contaminants transferred to the air from
showers, baths, toilets, dishwashers, washing machines and cooking.
Several models have been proposed to explain the mass-transfer
process; in particular, a time-dependent, three-compartment model for
residential exposure (McKone et al., 1987). The three compartments
used in such a model are the shower/bath stall, the bathroom and the
remaining residential volume. Factors that affect the projected
exposure are chemical mass-transfer rates from water to air,
compartment volumes, air-exchange rates and human occupancy factors.
6.4.3 Surface waters
The transport of contaminants in surface waters is determined by
two factors: the rate of physical transport in the water system and
the chemical reactivity. Physical transport processes are dependent to
a large extent on the type of water body under consideration (e.g.,
oceans, seas, estuaries, lakes, rivers or wetlands). Schnoor (1981)
and Schnoor & MacAvoy (1981) have summarized important issues related
to surface water transport. At low concentrations, contaminants in
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