The debate about the human health effects of dioxin continues, even as the levels of dioxin measured in people have continued to decline from their peak 30 to 40 years ago. Increasingly, scientific studies have shown that humans are less sensitive to the effects of dioxin than some laboratory animals.
While Dow acknowledges that studies have shown that dioxin (specifically, 2,3,7,8-TCDD) can cause a variety of health effects in laboratory animals exposed to high levels of dioxin normally not encountered in the environment, we believe that there is a lack of evidence of humans having experienced similar health effects. The absence of health effects is evident in workers highly exposed to TCDD who were involved in trichlorophenol exposure or in accidental high level environmental exposures that occurred in the past. The only consistently demonstrated health effect in these highly exposed populations has been chloracne — a temporary skin condition which can be severe in some cases. There is limited evidence that high level 2,3,7,8-TCDD exposure in people can cause a temporary increase in liver enzymes suggestive of mild, reversible liver injury. Two recent studies from Seveso suggest high level TCDD exposure in young males or in women having children, has resulted in apparent declines in sperm number or small changes in thyroid hormone. The results from these Seveso studies are somewhat equivocal with the apparent effects in either the normal clinical range or reflecting screening level results that are not direct evidence of toxicity. The accumulating scientific evidence shows humans to be less responsive to TCDD than sensitive laboratory animals such as rats. Overall, the conclusion that high levels of TCDD exposure does not produce the same effects observed in laboratory animals is supported by the findings of Dow’s studies on its own exposed employees and the collective evaluation of human health data. The other sixteen dioxin compounds with some of TCDD’s effects are less toxic than 2,3,7,8-TCDD.
A key consideration in evaluating potential health risk is exposure. According to the U.S. Environmental Protection Agency (EPA), the primary way people in the general population are exposed today to dioxin is through eating meat, fish and dairy products that have been exposed to background levels of dioxins and furans in the soil, on vegetation, in water, and in some commercial animal feeds. Over time, we can accumulate very small amounts of dioxin in the fatty tissues of our bodies. Dioxin levels in people of all ages in the U.S. continue to decline and are seven-fold lower now than they were 30 years ago through a combination of government regulations and voluntary industry efforts. In particular, children born today will accumulate far lower levels of dioxin than those born 30 years ago. As dietary background levels continue to decline, the margin of exposure (or difference between higher exposures required to cause the most sensitive effect in laboratory animals and exposures to the general public) will increase, thus further reducing the potential risk of dioxin effects. Adding to the enlarging margin of exposure is the consideration of lower sensitivity of humans to dioxin contributing to risk reductions of even greater magnitudes. However people will always have dioxin levels in their bodies, because it is generated through a number of human activities involving combustion, and natural sources such as forest fires.
In 2006, the University of Michigan reported the results of a two-year study of dioxin and furan and PCB exposure in the Midland-Michigan area and along the Tittabawassee River downstream from Dow manufacturing operations. Dow provided a “no strings attached” grant to the University of Michigan to conduct a human dioxin exposure study. To review the findings, please go to http://www.sph.umich.edu/dioxin/. The results of this study show no relationship between furan soil levels and serum blood dioxin levels in people living along the Tittabawassee River and in Midland. This finding should be reassuring and demonstrates that contact with contaminated soil does not contribute significantly to dioxin concentrations in humans. “The most important factors related to levels of dioxins in people’s blood are age, diet and body type,” (according to the University of Michigan study). The lack of a relationship between soil exposure and increase in blood dioxin levels is supported by a U.S. EPA study in West Virginia and a study of PCB-exposed residents of Anniston Alabama conducted by the Agency for Toxic Substance and Disease Registry. A health study on dioxin in Toshima area of Kita City, Tokyo, carried out in 2006, reported similar findings.
For more information on human levels of dioxin, please see the U.S. Centers for Disease Control and Prevention (CDC)’s Fourth National Report on Human Exposure to Environmental Chemicals.