What is dioxin?
Dioxin is the common name used to refer to the chemical 2,3,7,8-tetrachlorodibenzo-p-dioxin or TCDD. In addition to dioxin itself there are other compounds, such as the polychlorinated dibenzodioxins (PCDDs), polychlorinated dibenzofurans (PCDFs) and some polychlorinated biphenyls (PCBs), that have similar structures and activity as dioxin. These are often commonly referred to as dioxin-like compounds or "dioxins".
How are dioxins formed?
Dioxins are chemical contaminants that have no commercial usefulness by themselves. They are formed during combustion processes, such as waste incineration, forest fires and backyard trash burning, and during manufacturing processes such as herbicide manufacture and paper manufacture. e.g. dioxin was a contaminant of the herbicide Agent Orange used as a defoliant by U.S. forces in Vietnam.
Why are dioxins of concern?
People are constantly exposed to dioxins through ingestion of dioxins that are present at low levels as environmental contaminants in food. Although they are at low levels in food , some dioxins are very slowly removed from the body and therefore they accumulate in our fat tissue. In laboratory animals, dioxins are highly toxic, cause cancer, and alter reproductive, developmental and immune function.
What are the health effects of dioxin on humans?
Studies have shown that dioxin exposure at high levels in exposed chemical workers leads to an increase in cancer. Other studies in highly exposed people show that dioxin exposure can lead to reproductive and developmental problems, increased heart disease and increased diabetes. Dioxins ability to cause birth defects (teratogenicity) has not been established in humans but studies in mice have shown that dioxin and similar chemicals can produce congenital defects.
In general the effects of dioxin on humans were only observed in populations that were highly exposed. The effect of the long term low level exposure that is normally experienced by the general population is not known. The long-term effects of dioxin exposure on human immunity, reproduction and development, and other organs and systems remain focal points for ongoing research, as are the molecular and cellular mechanisms by which dioxin causes these health effects.
What are the biological mechanisms of dioxin's toxicity?
The way in which dioxin affects cells is similar in some way to the way in which hormones such as estrogen work. Dioxin enters a cell and binds to a protein present in cells known as the Ah receptor. The receptor when bound to dioxin can then bind to DNA and alter the expression of some genes. This can lead to alterations in the level of specific proteins and enzymes in the cell. While it is not known exactly how changes in the levels of these different proteins cause the toxicity of dioxin, it is believed by most scientists that the initial binding of dioxin the Ah receptor is the first step.
The health effects of dioxin on humans are the subject of ongoing studies at a number of research centers including the National Institute of Environmental Health Sciences in Research Triangle Park, N.C. In addition, NIEHS-supported researchers have studied the health effects of dioxin for more than 20 years and their efforts are reported in a large number of articles published in the scientific literature.
Dioxin and Cancer
The ability of dioxin to cause cancer (carcinogenicity) in laboratory animals is well established. The way in which dioxin cause cancer in animals and humans however is not fully understood, nor is it known if other dioxin-like compounds cause cancer. Humans are exposed to mixtures of dioxins and regulatory agencies such as the EPA regulate dioxin-like compounds together. Ongoing research at NIEHS is being conducted to examine the carcinogenicity of different dioxins, mixtures of these compounds, and the biological mechanisms responsible.
Dioxin and Agent Orange
Researchers are examining the effect of long term exposure to dioxin as a result of exposure to a Agent Orange, a defoliant used by U.S. forces in Vietnam. These studies have shown an elevation in diabetes in serviceman exposed to dioxin contaminated Agent Orange
Dioxin and Endometriosis
Studies have shown that dioxin exposure can increases the occurrence of endometriosis in monkeys exposed to low levels of dioxins. NIEHS grantees are extending these studies to examine the consequence of long-term exposure to dioxins on the incidence of endometriosis in women exposed to dioxin during a chemical accident in Seveso, Italy.
Dioxin and Immune function
One of the primary toxic effects of dioxin in laboratory animals is immune suppression, which results in decreased resistance to infectious agents and some cancers. The mechanisms and relationship between altered host resistance and immune dysfunction is complex, poorly defined, but extremely important to understanding health effects. NIEHS researchers are examining the mechanisms of immune suppression. Other studies are examining alterations in immune cell function in several human populations exposed to dioxin at both high levels and at low levels similar to that seen in the general US population.
Human response to Dioxin
The increased knowledge of the changes occurring in cells after exposure to dioxin allow us to examine if those effects are occurring in humans. Ongoing studies using state of the art molecular techniques are examining the effect of dioxins on the levels of specific genes in humans exposed long term to both high and low levels of dioxins. These studies will determine if there are individuals that are more responsive to dioxin and if these differences indicate that they are at higher risk of developing health problems.
Who at NIEHS can give me more specific information about dioxin and its health effects?
All NIEHS staff scientists may be reached at this postal mailing address:
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Dioxin data reviewed for currency on April 24, 2001
Additional Information:
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