Environmental Engineering Reference
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variation of the stratum corneum thickness by anatomic region (Marzulli, 1962, cited in USEPA,
1992a). Dermal permeability in humans is relatively invariant by skin age (Wester et al., 1985).
Much of the experimental data for dermal exposure factors are obtained from tests on laboratory
animals such as rabbits and guinea pigs. Intraspecies variability therefore becomes an important
factor for estimating human dermal exposure. Toxicologists and risk assessors have developed a
weight-of-evidence scoring system to normalize the myriad sources of variability and uncertainty
in dermal toxicological data. The absence of available dose-response relationships for dermal con-
tact makes for considerable uncertainty in estimates of risk from dermal exposure (USEPA,
1992a).
The experimentally measured permeability coefi cient
*
for 1,4-dioxane in aqueous media is
estimated to be 4
10
−4
), measured
in vitro
on human skin. Permeability coef-
i cients were measured by using diffusion cells that (1) did not fully mimic the capacity of the
circulatory system to remove penetrated chemicals and maintain a negligible contaminant concen-
tration under the skin and (2) did not estimate incidental metabolism (USEPA, 1992a). The
in vitro
value agrees well with an empirical estimation technique, which uses molecular weight, octanol-
water partitioning coefi cient, and several statistically derived empirical factors to estimate the
permeability coefi cient value. The empirically estimated permeability coefi cient for 1,4-dioxane
is 3.6
×
10
−4
cm/h (
±
0.36
×
×
10
−4
(USEPA, 1992a). For comparison, the average dermal permeability coefi cient of water
is 1.55
10
−3
cm/h (Bronaugh and Stewart, 1986; USEPA, 1992a).
No publications provide measured 1,4-dioxane exposure data from the use of shampoos, baby
lotions, hand dish-washing liquids, or other sundries known to contain 1,4-dioxane as a by-product
of manufacturing (ECB, 2002). Vacuum stripping used in the production of soaps and shampoos
now effectively reduces 1,4-dioxane in shampoos, dish-washing liquids, and baby lotions to less
than the regulated 10 ppm level. The European Chemicals Bureau nevertheless modeled exposure
to 1,4-dioxane from use of these products, assuming elevated concentrations at levels measured in
shampoos, soaps, and lotions in the 1980s.
Table 6.21
summarizes the assumptions used to model
dermal exposure to 1,4-dioxane in sundries.
The 1,4-dioxane concentrations in products listed in Table 6.21 correspond to measured con-
centrations from the 1980s and early 1990s (see Table 2.9). Recent surveys of 1,4-dioxane concen-
trations in shampoos and other sundries document a substantial reduction in most products (Fuh
et al., 2005; Campaign for Safe Cosmetics, 2007). Both current and past dermal exposure studies
for contact with 1,4-dioxane from shampoos, baby lotions, and dish-washing soaps conclude a
margin of safety ranging from 6000 to 1,000,000 (ECB, 2002). The margin of safety is obtained
by dividing the European Chemicals Bureau value for the calculated 1,4-dioxane dermal NOAEL
[2000 mg/(kg d)] by the calculated cumulative worst-case uptake, that is, 0.0009 mg of 1,4-diox-
ane per kilogram body weight per day. Dermal NOAELs for 1,4-dioxane have been determined
for male Wistar rats [8300 mg/(kg d)], guinea pigs [143 mg/(kg d)], and rabbits [57 mg/(kg d)]
(ATSDR, 2004).
If incidental workplace exposure to 1,4-dioxane is assumed, the dermal uptake [
D
sk
, in mg/(kg
d)] can be calculated as a relationship of the following dermal exposure parameters (NICNAS,
1998):
×
WSAEF
¥¥ ¥ ¥
D
=
,
(6.8)
sk
BW
where
W
is the weight fraction of 1,4-dioxane in the substance applied to the skin (i.e., 0.01
1%),
S
is the skin absorption rate [i.e., 0.3 mg/(cm
2
h)],
A
is the skin surface area exposed (i.e., hands
=
*
Permeability coefi cient (
K
p,s
) is a l ux value, normalized for concentration, that represents the rate at which the chemical
penetrates the skin (cm/h) (USEPA, 1992a).
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