Dioxin confirmed as a human carcinogen.

Nearly everyone in the world carries traces of dioxin. Some chemical-plant workers, however, have carried 1,000 times as much dioxin as other people. Such high exposures to dioxin in the workplace pose a cancer risk, a new federal analysis concludes.

Researchers at the National Institute for Occupational Safety and Health (NIOSH) in Cincinnati studied 5,132 chemical workers from 12 U.S. facilities. All these plants have produced chemicals contaminated with TCDD, the most potent dioxin.

People who sustained high exposures over the years were 1.6 times as likely to die of cancer as coworkers with low exposures, the new study finds. In fact, the high-risk workers had exposures "similar to the [dioxin] levels used in animal studies," report epidemiologist Kyle Steenland and his NIOSH coworkers in the May 5 JOURNAL OF THE NATIONAL CANCER INSTITUTE.

Although the team found signs of excess cancers among these workers almost a decade ago, at that time the researchers were unable to correlate dioxin dose with disease. They recently completed exposure estimates for 70 percent of the workers by considering each person's specific job, how long he or she was exposed, and in what plant the person worked.

What the new calculations indicate, Steenland told SCIENCE NEWS, "is that there is a dose response--we see a trend of more disease with more exposure." Such a relationship, he explains, "increases our confidence that the observed association is real."

While an excess cancer risk was apparent only among the most highly exposed workers in this study, Steenland says this does not prove that there is no risk for people with low exposures.

In their work 10 years ago, the NIOSH scientists sampled blood from 250 of the chemical-plant workers. Knowing how long dioxin tends to stay in the body and when each worker's occupational exposure started and ended, "we were able to back-extrapolate what their [workplace] dioxin levels would have been," Steenland says.

Whereas blood concentrations of dioxin in the general population are typically just 6 to 8 parts per trillion, the NIOSH assays retrospectively calculated an average of about 2,000 ppt in the 250 workers' serum during their job exposure.

The most heavily exposed chemicalplant workers in the new study may have had substantially higher concentrations, Steenland notes. The scientists haven't calculated blood-dioxin concentrations for the larger group.

On the basis of earlier NIOSH analysis of chemical-plant workers and studies of a few other populations receiving heavy exposures (SN: 9/4/93, p. 149), the International Agency for Research on Cancer (IARC) in Lyon, France, designated dioxin a human carcinogen 2 years ago. Steenland says that the new analysis of U.S. workers suggests that IARC made a good call.

In an editorial accompanying the latest NIOSH findings, Robert N. Hoover of the National Cancer Institute in Bethesda, Md., agrees that "TCDD should be considered a human carcinogen." He also concurs with Steenland that it's difficult to determine whether dioxin exposures typical of the general population increase cancer risks, especially since there are almost no unexposed groups.

Ordinarily, to learn more about lowdose risks, he notes, scientists would call for further research on people with high exposure to the chemical. Such research "is unlikely," Hoover says. Virtually all the populations that had been heavily exposed "have been studied, and fortunately, high-level exposures no longer occur," he adds. Indeed, people's average blood concentrations of dioxin have fallen to a quarter of what they were 25 years ago.