L09 Avoidance of dioxins in the environment

Aim: To show that the term dioxin represents a group of substances, how dioxins are formed and to what extent they are present in the environment.

The term dioxin is used for all polychlorodibenzo-paradioxins (PCDD) and polychlorodibenzofuranes (PCDF).
More than 210 different dioxins are known and the most poisonous of these is 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin.
The toxicity of all dioxins is therefore expressed with reference to 2, 3, 7, 8-TCDD in toxic equivalents thereof (TEQ). Exposure to dioxins can cause chloracne, birth defects and in severe cases death. It also acts as a defoliant.

Dioxins are formed during the combustion of chlorinecontaining species, e.g. aromatic compounds, soot, printed circuit boards and wood which has been treated with pentachlorophenol,at temperatures between 250 and 400°C and in the presence of heavy metals such as copper.

Dioxins are apolar, insoluble in water and only slightly volatile. They are stable up to fairly high temperatures and are only thermally decomposed at temperatures above 750°C.

 

   Dioxins are formed in incinerators operating at temperatures below 750°C. Modern incinerators, however, mainly operate at sufficiently high temperatures to avoid dioxin formation. The formation of dioxin during emission of the smoke is prevented by rapidly cooling the smoke to below 250°C.

In more modern incinerators less than 2 ng TEQ per m3 of smoke is produced, but older installations produce up to 10 ng TEQ per m3. However, purification of the smoke is still necessary to reduce the dioxin to an acceptable level i.e. 0.1 ng TEQ per m3.

This is achieved by removing the dust from the smoke using an electrofilter and then passing the smoke over activated charcoal or a catalyst to remove most of the remaining dioxins. When a catalyst is used this is normally a mixture of metal oxides which is heated to 200 to 300°C. At this temperature the dioxins react with the oxygen in the air to form CO2, H2O and HCl.

Major sources of dioxins are the burning of garden rubbish, combustion engines, metallurgy factories, household fires and paper mills.

Up till now there has been no evidence of harmful effects from dioxins at levels of 0.1 ng TEQ/ m3, but steps are being taken to reduce the concentration in the environment still further. These include a ban on the use of printed circuit boards and the closure or modernisation of older types of incinerators.

The dioxin level in Great Britain is much lower than in neighbouring European countries making the dioxin level in milk, for example ,acceptable. Dioxins are easily adsorbed by soil and dioxins on grass are consumed by cows and therefore are present in milk consumed by humans.
In Belgium ,for example, where the dioxin level is higher than in Great Britain, the concentration in milk is much higher although not regarded as dangerous.