Industrial burning can produce dioxins

Dioxins are common pollutants - produced as the result of many industrial processes.

They have been found in breast milk, and can cross the placenta between mother and unborn child.

Doctors believe that exposure to low levels of these chemicals during critical periods of development can have harmful effects, particularly involving fertility - but this is hard to prove in humans, and they do not know why it happens.

A team of researchers from the University of Tokyo found that dioxins produced an effect similar to that of steroid hormones such as oestrogen.

This hormone's role is to control the growth and function of reproductive tissues, and studies in rats have already suggested that dioxins disrupt this process.

Hormone action

Oestrogen works normally by binding to a "receptor" on the outer surface of the cell, triggering effects within it.

However, the Japanese team found that, even in the absence of oestrogen, dioxins could bind to another receptor, and trigger similar effects inside the cell, activating genes that are normally associated with oestrogen.

In mice whose ovaries had been removed, with the result that they lacked any oestrogen of their own, the dioxins produced an effect that mimicked the hormone.

Other research offers clues to some of the other health problems for which dioxins have been blamed.

Wider problems

When researchers engineered mice to lack the "alternative receptor" which binds to dioxins, the mice, evidence emerged which suggested that this receptor played a role in the immune system, the liver, ovaries and other organs.

The latest research, published in the journal Nature, not only offers an explanation for the "hormone disrupting" effects noticed in other studies - but may even offer ways for scientists to counteract dioxin exposure in future.

Professor Malcolm Parker, and Dr Jan Brosens from the Institute of Reproductive and Developmental Biology at Imperial College, London, wrote in an accompanying commentary that it was still difficult to prove a link between dioxins and human disease.

"Part of the problem is that many of the toxic effects attributed to dioxins - such as impaired immunity, fertility and cognition - are relatively subtle, multifactorial, difficult to diagnose and quantify, and often take years to appear."