Although the skin is an effective barrier to the entry of contaminants, it is possible for small amounts to penetrate and enter the body. The two approaches for estimating contaminant absorption through the skin recommended by the EPA for Superfund sites (EPA 1989) are adopted here. One applies to contact with contaminated water and the other to contact with contaminated soil. For both scenarios, the contact rate does not have an easily described, intuitive physical meaning as it did for ingestion and inhalation, where it was the volume of water or mass of food taken into the body every hour or every day. For dermal contact, the medium does not enter the body and the contact rate incorporates either water or soil contact with the skin and contaminant penetration of the skin. It has units of water volume or soil mass per unit time which when multiplied by concentration yields the rate of contaminant entry into the body. Unlike the intake calculations for ingestion and inhalation, which ultimately yield estimates of potential chemical dose, dermal absorption calculations ultimately yield estimates of applied chemical dose.
The equation for contact rate due to absorption from water is
where kp is dermal permeability constant [L/T], Askin the area of skin exposed [L2], and td the daily exposure duration [T/T]. The physical interpretation of the permeability constant is contaminant penetration rate per unit area of skin per unit concentration, and the expanded units are[(M(c)/T)/L2]/[M(c)/L3]. Values of the permeability constants for selected contaminants are given in Table 9.2. The typical scenarios for dermal absorption due to water contact are showering, bathing, and swimming where the total body is exposed. EPA recommends conservative, health-protective default values for the various parameters (EPA 2004). Default skin areas are 18,000 cm2 for adults and 6600 cm2 for children. Default values for exposure duration for showering, bathing, and swimming are given in Table 9.1.
For contaminated soil, contact rate is calculated by
where aF is the soil-to-skin adherence factor [M(s)/L2], fabs the dermal absorption fraction, and ve the exposure frequency [event/T]. The soil-to-skin adherence factor
TABLE 9.2 Dermal Permeability Constants for Water | |
Contaminant |
kp (cm/h) |
Aldrein/dieldrin |
1.4 x 10-3 |
Arsenic |
a1.0 x 10-3 |
Benzene |
1.5 x 10-2 |
Cadmium |
a1.0 x 10-3 |
Carbon tetrachloride |
1.6 x 10-2 |
Cesium |
a1.0 x 10-3 |
Chlordane |
3.4 x 10-2 |
Chloroform |
6.8 x 10-3 |
Chromium (+ 6) |
2 x 10-3 |
Chromium (+3) |
1 x 10-3 |
Cobalt |
4 x 10-4 |
DDD |
1.8 x 10-1 |
DDE |
1.6 x 10-1 |
DDT |
2.7 x 10-1 |
Hexachlrorocyclohexanes (lindane) |
1.1 x 10-2 |
Iodine |
a1.0 x 10-3 |
Lead |
1 x 10-4 |
Mercury (+2) |
1 x 10-3 |
Mercury vapor |
0.24 |
Methyl mercury |
1 x 10-3 |
Methylene chloride |
3.5 x 10-3 |
Plutonium |
a1.0 x 10-3 |
Polychlorinated biphenyls |
7.5 x 10-1 |
Radium |
a1.0 x 10-3 |
Strontium |
a1.0 x 10-3 |
Tetrachloroethylene (PCE) |
3.3 x 10-2 |
Trichloroethylene (TCE) |
1.2 x 10-2 |
Uranium |
a1.0 x 10-3 |
Vinyl chloride |
5.6 x 10-3 |
Source: EPA 2004. |
a The default value for inorganics is 1 x 10 3.
a The default value for inorganics is 1 x 10 3.
is the mass of soil that can adhere to a unit area. It is weighted by the fraction of body surface area that is exposed and thus depends on the type of activity that is responsible for contact with soil. Values vary widely and include geometric mean values of 0.01 mg/cm2 for children indoors and adult grounds keepers, 0.1 mg/cm2 for adults playing soccer, 0.3 mg/cm2 for archaeologists, and over 20 mg/cm2 for children playing in mud. Health-protective default values recommended by EPA (EPA 2004) are 0.07 mg/cm2 for adults and 0.2 mg/cm2 for children, which represent conservative values for typical individuals experiencing a variety of activities. The value for adults is the median value for gardening, a high-end activity, and the value for children is the 95th percentile for children playing at a day care center, a central tendency activity. Dermal absorption fraction is the fraction of contaminant absorbed through the skin. Values for selected contaminants are given in Table 9.3.
Contaminant |
fabs |
Arsenic |
0.03 |
Cadmium |
0.0001 |
Chlordane |
0.04 |
2,4-Dichlorophenoxyacetic acid |
0.05 |
DDT |
0.03 |
TCDD and other dioxins |
0.03 |
If soil organic content > 10% |
0.001 |
Benzo[a]pyrene and other PAHs |
0.13 |
Arolcors 1254/1242 and other PCBs |
0.14 |
Pentachlorophenol |
0.25 |
Semivolatile organic compounds |
0.1 |
Source: EPA 2004. |
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