2,3,7,8-Tetrachlorodibenzo-p-dioxin

Polychlorinated dibenzo-p-dioxins (PCDDs; known colloquially and inaccurately as dioxins) are subject to the European Waste Incineration Directive, which puts strict limits on emissions to air. Incineration is controlled to minimise their production, and the flue gas is treated post-combustion. The resulting toxic fly ash must be handled as hazardous waste.

Emissions of dioxins and furans from an incinerator typical of those currently operating in the UK (230,000 tonnes per year) are approximately equivalent to emissions from accidental fires in a town the size of Milton Keynes (population 230,000). That is, emissions from the Exeter Incinerator will be equivalent to half the emissions from accidental fires in Exeter.

The structure of dioxins comprises two benzene rings (six carbon atoms) joined by two oxygen atoms. Chlorine atoms may be attached to this structure at any of positions 1–4 and 6–9 in the above picture, which gives 75 flavours. Hydrogen atoms are attached to the remaining positions.

Dioxins are commonly regarded as highly toxic compounds that are environmental pollutants and persistent organic pollutants. Of the 75 flavours, the seven below are considered toxic by the World Health Organization (WHO). 2,3,7,8-Tetrachlorodibenzo-p-dioxin* became known as a contaminant in Agent Orange, and is the most toxic of all. It is therefore designated the reference molecule for rating toxicity.

For more information, see the Wikipedia articles about polychlorinated dibenzodioxins and specifically 2,3,7,8-Tetrachlorodibenzo-p-dioxin.

*The ‘p’ stands for ‘para’, indicating the oxygen atoms are opposite each other. The oxygen atoms could be next to each other, which would be indicated by ‘o’ for ‘ortho’, but this molecular configuration is unstable.