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.

Copyright © 2000 Canadian Association of Physicians for the Environment
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