Science and sensibility

Science and sensibility

Friday, September 10, 2004

Taranaki dioxin report

A report(PDF) released yesterday shows that people who lived close to Ivan Watkins-Dow's chemical plant in Taranaki between 1962 and 1987 have elevated levels of the dioxin 2,3,7,8-Tetrachlorodibenzo-p-dioxin (mercifully shortened to TCDD for the rest of this post) in their blood. TCDD is a known carcinogen that was produced as a by product of the plant’s manufacturing of the herbicide 2,4,5-T. The report also shows that the risk of exposure to TCDD stopped in 1987 when the plant stopped producing 2,4,5T. So, how were these conclusions drawn and what does it all mean to those affected by the dioxin?

The chemical that caused all the trouble

The authors of the report took blood samples from 24 residents thought to have been exposed to dioxin from the chemical plant. Each of these samples were analysed to find the level of TCDD in the participants blood. At the same time they measured the level of TCDD in the soil surrounding the plant and asked the study’s participants to complete a questionnaire estimating their exposure to the plant’s emissions by describing how far away from the plant they lived and for how long. As was expected there were considerably higher than background levels of soil TCDD in the area surrounding the plant. When the results from the blood samples came back they showed that the average level of TCDD in residents was three times the average levels of New Zealanders in a more general study undertaken in 1997. Additionally 9 of the 24 had a level of TCDD more than three standard deviations higher than the general population’s average. Elevations in serum TCDD increased linearly as a function of soil TCDD concentration at the participant’s house and duration of residence at that house. There were two pieces of good news for people currently living under the shadow of the plant. The relationship between TCCD concentration and duration of exposure breaks down for after 1987 – when the plant stopped producing 2,4,5T. This strongly suggests that all the exposure was from inhalation of TCDD in the air as a result of the plant’s activity – and that there is no risk of exposure from the residual soil contamination. The other source of cold comfort was the participants in the study had lower than the national average level of other dioxins in their blood. Since all the dioxin including TCDD are tought to act on the sameprotein in mammalian cells (see below) it is the so called "total equivalent" dose of dioxn that is important in estimating the health risk dioxins present to a patient.

The real question facing everyone now is “what next?” The question of compensation and who’s liable to pay it is certainly beyond my small brief, but I can give a quick run down on some of the known effects of TCDD in what is pretentiously referred to as “the literature.” When TCDD and other dioxins enter a cell they bind to a receptor molecule called the Ah receptor. No one is clear what the Ah receptor is meant to do, but other proteins with a similar sequence are known to be involved in regulating development and cell division.Regardless of the receptor's intended purpose when a dioxin binds to it the receptor “turns a switch” inside the cell leading to changes in the genes are being used inside the cell and changes in the cell’s usual cycle of growth and reproduction. The upshot of all of that activity is summed up unequivocally by the USA’s National Institute of Health in their report on TCAA(PDF)

“[TCAA] is known to be a human Carcinogen [their emphasis] based on sufficient evidence of carcinogenicity from studies in humans.

That designation comes from a variety of sources. In rats TCDD exposure can be shown to cause cancer in a “dose dependant” fashion – that is to say the more TCDD a rat is exposed to the more likely it is to develop cancer. As well as this a study following people living or working in areas that have high levels of TCDD have been shown to get cancer more readily than people not exposed to it. More spefically TCDD and other dioxins appear to be very good at "promoting" the biological irregularities that can lead to cancer once they crop up, but not very good at causing or "intiating " those irregularaties.

There is certainly no doubt that there is no safe level of exposure to dioxins, and it is up to the government to assess whether the elevated levels of TCDD expereinced by those living near the Ivan Watkins-Dow chemical plant raise their chances of devloping cancer significantly enough to warrent compensation. The individual residents themselves can take some heart at their comparative lack of other dioxins and at leastbe aware of the increased risks they face.

Posted by David Winter 5:51 pm

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