Dermal lead absorption

When exposed to mixtures, chemicals in the exposure medium may affect each other s uptake by humans in a manner that is analogous to some of the bioavailability effects outlined here for environmental species. This was, for instance, shown for the neurotoxicity of EPN (O-ethyl-O-4-nitrophenyl phenylphosphono-thionate), which was enhanced by aliphatic hydrocarbons due in part to increased dermal absorption (Abou-Donia et al. 1985). It was also shown that dietary zinc inhibits some aspects of lead toxicity, which could in part be explained by decreasing dietary lead absorption (Cerklewski and Forbes 1976). Other examples of interactions of chemicals at the uptake phase in humans, which may in part be related to bioavailability interactions, are summarized in Table 1.3. 

In this study, 4.4 mg of lead equivalent was applied to the skin under a covered wax/plastic patch on the forearms of human subjects of the applied dose, 1.3 mg of lead was not recovered from skin washings. The amount that actually remained in (or on) the skin and the mass balance of the fate of this lead was not determined it may have been absorbed or eliminated from the skin by exfoliation of epidermal cells. Thus, while this study provides evidence for dermal absorption of lead, it did not quantity the fraction of applied dose that was absorbed. The quantitative significance of the dermal absorption pathway as a contributor to lead body burden remains an uncertainty. The wax/plastic patch provided a means by which the lead compounds could permeate or adhere to the skin. The effect of concentration in aqueous solution may cause skin abrasion through enhanced acidity since the lead ion is acidic. Abraded skin is known to promote subsequent higher lead penetration. 

Limited information was located regarding dermal absorption of inorganic lead in animals. An early study reported that lead acetate was absorbed from the clipped skin of rats, as determined by an increase in the concentration of lead in the kidneys relative to controls (Laug and Kunze 1948). It was further shown in that study that mechanical injury to the skin significantly increased the penetration of lead and that the penetration of lead from lead arsenate was significantly less than from lead acetate. 

Recycling of printer circuit boards is deemed as the most important source of heavy metals to the ambient environment. These heavy metals may be entering into human body from various exposure routes such as ingestion, inhalation, and dermal absorption. Exposure to high levels of heavy metals can lead to acute and chronic toxicity, such as damage to central and peripheral nervous systems, blood composition, lungs, kidneys, liver, and even death [14],... 

Human exposure to endrin may occur by ingestion, inhalation, or by dermal contact. Dermal absorption can be significant. Gastrointestinal absorption is enhanced by dietary fats. While not highly volatile, endrin-laden aerosols or dust particles can be trapped in respiratory mucus and swallowed, leading to gastrointestinal absorption. 

For acute smdy data (oral LD50 values compared with inhalation LC50 values), a wide variation was observed for extrapolation from the oral route to the inhalation route suggesting that such an extrapolation is extremely difficult and should not be undertaken. ECETOC also noted that, in most cases, it can be assumed that the dermal route leads to less absorption of a substance than the oral route, because of the skin barrier, and recommended that this should be assessed on a case-by-case basis. 

As an alternative to the assumption of a one-time exposure for 1,000 h at the time of facility closure, permanent occupancy of a disposal site following loss of institutional control could be assumed (see Section 7.1.3.4). The assumption of chronic lifetime exposure would affect the analysis for hazardous chemicals that induce deterministic effects only if estimated intakes due to additional pathways, such as consumption of contaminated vegetables or other foodstuffs produced on the site, were significant. Based on the results for lead in Table 7.8, an intake rate from additional pathways of about 50 percent of the assumed intake rate by soil ingestion, inhalation, and dermal absorption would be sufficient to increase the deterministic risk index above unity. The importance of additional pathways was not investigated in this analysis, but they clearly would warrant consideration. The increase in exposure time during permanent occupancy does not otherwise affect the analysis for chemicals that induce deterministic effects, provided RfDs are appropriate for chronic exposure, because chronic RfDs incorporate an assumption that the levels of contaminants in body organs relative to the intake rate (dose) are at steady state. 

Ingestion of contaminants is the primary exposure pathway for drinking-water. Dermal absorption and inhalation of contaminants during bathing are other common pathways. When contaminated surface waters serve as recreational areas for children, accidental ingestion (water or sediment) and dermal contact become additional pathways for exposure. Finally, aquatic organisms can bioaccumulate contaminants in surface waters, which can lead to dietary exposure through the food-chain. 

Some chromium(VI) compounds, such as, chromium trioxide (chromic acid), potassium dichromate, potassium chromate, sodium dichromate, and sodium chromate, are very caustic and can cause bums upon dermal contact. These burns can facilitate the absorption of the compound and lead to systemic toxicity. 

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