Environmental Engineering Reference
In-Depth Information
Other critical design factors in dermal absorption studies are particle size
fractions and soil loading. Typically only particles less than 150
mindiam-
eter are expected to adhere to skin, and for dry soils a more reasonable cutoff
may be 65
μ
m or less (Choate et al.
2006
). Consequently, studies will be more
representative of expected exposure conditions if only the fine particle fraction is
tested. Characterizing the particle size distribution is also a critical step in planning
the soil loading to be used in a study. When the soil loading exceeds a monolayer
of particles on the skin, it has been shown that the absorption fraction declines.
Therefore, studies conducted using high soil loadings may underestimate dermal
absorption at soil loadings more typical of those on people at contaminated sites.
Spalt et al. (
2008
) described many of these issues in a recent critical review, and
offered design recommendations for site-specific studies.
US EPA (
2004b
) provided default dermal absorption fractions for ten
compounds, including arsenic (0.03), cadmium (0.001), chlordane (0.04),
2,4-dichlorophenoxyacetic acid (0.05), DDT (0.03), TCDD and other dioxins (0.03
or if soil organic content exceeds 10%, then 0.001), lindane (0.04), benzo(a)pyrene
and other PAHs (0.13), PCBs (0.14), and pentachlorophenol (0.25). None of
these default values are based on studies that used weathered soils representative
of contaminated sites. The differences, however, can be substantial. For exam-
ple, a recent study of dermal absorption of arsenic from weathered soils found
that in contrast to US EPA's default value of 0.03, absorption was negligible
(Lowney et al.
2007
). The US EPA (
2004b
) recommended default value for other
semi-volatile contaminants is 0.01. According to US EPA (
2004b
) volatile contam-
inants and other inorganic contaminants do not need to be evaluated for dermal
absorption.
μ
11.7.4.2 Soil Adherence Factors
The adherence factor describes the amount of soil that sticks to the skin. It is
expressed in terms of soil mass per unit surface area (mg/cm
2
). Soil adherence
to skin is influenced by the properties of the soil, the part of the body exposed,
and the activities of the exposed individual. Empirical data on adherence factors
is summarized by US EPA (
2004b
). Soil adherence studies have examined the
adherence of soil to various body parts of children and adults engaged in a vari-
ety of activities. Adherence factors recommended by US EPA (
2004b
) are weighted
averages based on body part- and activity-specific adherence factors. Updated rec-
ommendations and more detailed documentation are provided in the
Child-Specific
Exposure Factors Handbook
(US EPA
2008
) and the
Exposure Factors Handbook
2009 Update
(US EPA
2009
).
For a child resident, US EPA (
2004b
) recommended default soil adherence fac-
tors of 0.04 and 0.2 mg/cm
2
for central tendency estimates and reasonable maximum
exposure cases, respectively. The central tendency value is a geometric mean derived
for daycare children playing indoors and outdoors and for children playing in dry
soil. The reasonable maximum exposure value is a geometric mean derived for
children playing in wet soil.
Search WWH ::
Custom Search