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The health threat from dioxin is greater than suspected and even at trace amounts - close to everyday background exposure - the chemical may be harmful to human immune and reproductive systems, says a draft study by the Environmental Protection Agency.

The 2,000-page report to be released this week is expected to heighten demand for more stringent control on dioxin releases into the air and water. It may also increase pressure on industry to phase out the use of chlorine compounds in everything from bleaching paper to making plastics.The new reassessment by EPA scientists of dioxin's toxicity acknowledges that there is still much to be learned about the chemical's complex health effects on the human body. And the study itself must still be reviewed by the agency's Science Advisory Board before becoming final next year.

But, according to documents obtained Sunday by The Associated Press, the draft study reaffirms that dioxin, which for years has been believed to be a carcinogen, in fact causes cancer in humans at significant exposure levels, and likely has other serious health effects at even very low levels.

The study furthermore says there is evidence that background levels of dioxin, or levels to which most humans already are exposed, could be sufficient to cause adverse health effects in some individuals.

EPA spokesman David Cohen had no comment on the study and said the draft report would become public Tuesday.

Dioxins are highly toxic chemical compounds that are produced as byproducts in combustion and in certain chemical and industrial processes involving chlorine. Although produced in relatively small quantities, dioxins for years have been considered among the most toxic pollutants.

The EPA study, which has been in development for three years and involved some 100 scientists both in and outside the agency, is an attempt to reassess the health dangers of dioxin in order to map out future regulations.