Dioxin via skin: a hazard at low doses?

Dioxin via skin: A hazard at low doses?

How well skin bars entry to toxic chemicals can depend on several factors, such as body site and hairiness (SN: 6/25/88, p.407). U.S. toxicologists now find that two other factors are important for dioxins and furans: dose and age.

The lower the dose, the more easily dioins and furans penetrate skin, report researchers at the National Institute of Environmental Health Sciences, in Research Triangle Park, N.C. For example, 40 percent of 2,3,7,8-TCDD -- the most toxic dioxin -- passes through a rat's skin when given at a dose of 0.3 microgram per kilogram of body weight. Absorption falls to less than half that at TCDD doses between 32 and 320 micrograms per kilogram, they note in the January TOXICOLOGY AND APPLIED PHARMACOLOGY. The furans tested show a similar trend.

Structural similarity offered no reliable gauge of how a chemical will move. In skin penetrability and accumulation in liver and fat, the four-chlorine dioxin more resembled the five-chlorine furans (PeCDFs) than TCDD's four-chlorine furan analog -- 2,3,7,8-tetrachlorodibenzo furan (TCDF). At 30 micrograms per kilogram, 50 percent of the TCDF was absorbed while only 18 to 34 percent of the others entered skin. Linda S. Birnbaum, who directed the work, suspects TCDF's higher absorption results from its greater solubility and her team's finding that so much more of it (56 percent) was metabolized by the skin and cleared from animals than was TCDD (10 percent), 1PeCDF (32 percent) or 4PeCDF (2 percent).

These data suggest systemic toxicity from acute dermal exposure to these chemicals "is unlikely," the researchers report. Chronic low-dose exposure is another matter. That's where "you're going to have the potential to build up a body burden," Birnbaum says.

In a related study reported this week at the Society of Toxicology meeting in Atlanta, Birnbaum described a 70 percent reduction in TCDD absorption through skin -- from 16 percent to just 5 percent -- as she moved from rats 3 months old (young adults) to those 9 months (middle aged) or older. Since human data suggest baby skin would be much more permeable, she says the new findings will compel her to repeat the study with neonatal to adolescent animals.