The article on dioxins (AJ 10.1.02) correctly points out the tendency in the media to stir up anxiety in the public by presenting arguments with very little scientific basis. Dioxins are a popular target for such attacks and great care should be taken to ensure that the facts are rigorously presented.
We should condone dangerous practices, but a rational assessment should replace sensationalism. It should be recognised that dioxins are endemic in the natural environment. The regulations governing their emission from industrial processes are extremely strict.
For example, emissions from incinerator chimneys are required by law to be 10(-13) and in practice are generally 10 times smaller. It is difficult for the average person in the street to appreciate such small values.
However, the contamination is 10,000 times less than that in the purest material used by the electronics industry to make computer processors. They require that contaminants be less than one thousandth of a millionth, ie 10(-9)). The achievement of such a high purity is a triumph of technology, and I reiterate Pettit's statement that more dioxins are emitted by bonfires on 5 November than by all the incinerators in a whole year.
The role of dioxins in ash and construction materials is also an issue. Here I must point out that technology exists to destroy these and other organic compounds in the ash by converting them to small molecules such as carbon dioxide. Please note that the additional carbon dioxide would only be a few kg per year and would thus be quite negligible as a contributor to global warming.
This merely illustrates my point about rational/quantitative arguments.
At Sheffield, we coordinate a national dioxin network which addresses these matters at regular scientific meetings which also embrace the economic aspects so that the inevitable cost to the public of solid, liquid and gaseous effluent treatment processes will be appropriately and fully justified.
Professor Jim Swithenbank, department of chemical and process engineering, University of Sheffield