Dermal Absorption as an Exposure Route
Compared to other body structures, the skin is unique in being
perpetually exposed to the environment.15,16,17
The rate of dermal absorption of a substance is proportional to
both the concentration of the substance and the surface area over
which it is applied. The wider the contact area and the more concentrated
the substance, the greater will be the absorption.
The thickness of the skin, especially the stratum corneum, also
determines the degree to which substances are absorbed. Thicker
skin is a greater barrier to passage of foreign substances. Depending
on skin thickness, there can also be variability in absorption of
a given substance by different regions of the body. For example,
hydrocortisone is absorbed over 50-times greater by genital skin
versus the skin of the palms.18
Damage to skin, both through disease or direct environmental influence,
can also alter the barrier properties of skin and enhance absorption
of substances. Even something as innocuous as the removal of outer
layers of skin with cellophane tape can apparently dramatically
increase dermal absorption.19
Occlusion of the area of skin in contact with a substance also
serves to enhance absorption. This phenomenon explains why dressings
are placed over topically applied medications in clinical practice.
Occlusion serves to increase hydration and temperature of skin and
can also enhance injury to skin from hazardous substances, thereby
increasing absorption secondarily as noted above. For this reason,
it is recommended that if insect repellants are used on children
they only be applied to naked skin or to clothing, but never to
skin that will be covered by clothing.
Vehicle composition will also determine the absorption of an agent.
Lipids and lipid-soluble substances absorb more readily. Agents
that have lower solubility with the vehicle will absorb more easily
into skin. “In general, chemicals that travel quickly through the
skin have low molecular weights, no electrostatic charge and easily
dissolve in fat.”20 Finally, if the vehicle’s properties are such that it damages
skin, this may secondarily increase dermal absorption as well.
Dermal absorption is also a route of exposure to contaminants that
are found in soil, air and water. For example, exposure may occur
via bathing, showering or immersion in contaminated natural swimming
water and, therefore, through direct skin absorption. There has
been considerable study concerning the exposure in bath, shower
and chlorinated pool water to chlorine disinfection by-products.
Inhalation and dermal absorption are the two most likely routes
of exposure to certain of these substances.21 Trihalomethanes represent one class of these agents that
have been linked to increased risk for spontaneous abortion and
for developing bladder and colon cancer.
Health Canada researchers estimated that skin exposure to polycyclic
aromatic hydrocarbons (PAHs) via Great Lakes water sediment might
in some extreme circumstances be equally important to the oral exposure
route. They conclude that there is the potential for toxicologically
significant absorption of contaminants found in water if skin has
prolonged exposure and particularly if it has sunburn damage.22
Such a conclusion has important implications for children’s exposures
since they are likely to spend more time in swimming water, will
absorb more relative to their body weight and are reliant on adults
to ensure adequate protection against sun damage that would compromise
the barrier property of their skin.
Recreational water contaminated with high levels of E.coli
and other fecal coliform bacteria can produce skin rashes and eye,
ear or throat irritation from direct contact.23
Such health effects have been taken into consideration in
the development of recreational water quality guidelines. If a beach
or body of water is determined to have unacceptably high levels
of microorganisms, the local medical officer of health has the authority
to post warnings to the public of the probable health risks.