Molecule of the Month: Little killers that have been around for a long time: John Emsley queries whether dioxins really are the world's deadliest chemicals

DIOXINS are often said to be the world's deadliest chemicals, with a millionth of a gram enough to kill a guinea pig. A month ago Greenpeace issued a report, Dioxin Factories, which blamed European PVC manufacturers for emitting dioxins. It speculated that enough dioxin was produced each year to give everyone in the world more than the limit set by the World Health Organisation (WHO).

Dioxins are organochlorine derivatives and over the past 20 years they have been found in weedkillers, water supplies, paper products, even breast milk. In all cases the cause appeared to be industrial use of chlorine.

Less well-publicised has been the discovery of dioxins in soil samples collected in Britain 150 years ago, and in sediments from Japan that were deposited more than 8,000 years ago. We now realise that dioxins are not new; they have been around since the time of the dinosaurs, or whenever there first were forest fires. However, it was an accident at a chemical works near Seveso in Italy that alerted people to them. On 10 July 1976, an explosion scattered over the town a white powder containing about 3,000g of dioxin. For days children played in the curious 'snow' before the population was evacuated. But despite all the publicity about hazards and dioxin's toxicity, no one died.

When the US Environmental Protection Agency (EPA) tested dioxin for its ability to cause cancer, it was startled to find that it was the most carcinogenic compound it had ever encountered. However, when a group of German workers who had been exposed to excess dioxin after an accident in 1954 were monitored 37 years later, most were still alive and well. Of those who had died, the incidence of cancer was only slightly higher than expected.

As the techniques of chemical analysis improved it became possible to detect smaller and smaller amounts of dioxin until we can now measure as little as a million-billionth of a gram. We can show that dioxins are produced quite naturally when we light a bonfire, smoke a cigarette or even strike a match. The safe daily dose set by the WHO for humans is as low as one-thousandth of a billionth of a gram, so it would take a mere two grams to give everybody in the world this amount every day for a year.

There is no doubt that dioxins are dangerous, but how dangerous is a matter of debate. On the one hand, there are environmental researchers who believe the background levels are already too high and may be affecting human health and fertility. On the other, the chemicals industry is taking great care to eliminate the dioxins it produces and prevent them escaping. 'Contrary to what the Greenpeace report says, the amount of dioxins emitted by the PVC industry in the UK is minute,' says Dr John Baldwin, environmental affairs manager of Hydro Polymers, the UK subsidiary of Norsk Hydro, whose plants in Norway and Sweden the report criticised.

Thirty years ago dioxins were identified as contaminants in the manufacture of weedkillers. Apart from a research chemist who once made 20g of dioxin, no one set out deliberately to make them, but they are produced in significant quantities as byproducts in chlorination processes. The closing of plants that once manufactured weedkillers, and controls at other plants using chlorine, have reduced dioxin emissions to a few grams, until today the largest emitters are thought to be municipal waste incinerators. Crematoria also emit some.

Nevertheless, the dioxins that contaminated the environment in the past remain, and some end up in our food. Dr Paul Johnston and Ruth Stringer, of the Greenpeace Laboratory at Exeter University, have been researching the dispersal of dioxins throughout the environment. They argue that the WHO and UK limits for tolerable daily doses, which are set at a thousandth of a billionth of a gram, should be brought into line with those set by the EPA. This recommends a limit of less than a millionth of a billionth of a gram, although this is below the level at which we already imbibe dioxins.

'To protect human health, food purity and land values, and prevent further contamination of the wider environment, it is vital that highly contaminated sites be cleaned up,' say Dr Johnston and Ms Stringer in a paper soon to be published. They even suggest that one way of doing this would be to extract the dioxins from polluted soil by using liquefied carbon dioxide.

The world's leading analyst of dioxins is Professor Christoffer Rappe, of the University of Umea in Sweden, whose skill is such that he can identify all 210 closely related chemicals that come into this category, of which only 17 are toxic. Earlier analyses lumped them all together and so gave cause for concern, whereas the real danger was actually very small. In 1991 Professor Rappe discovered dioxins in soil samples gathered at Rothamsted, Hertfordshire, more than a century ago, long before chlorine-based chemical industries came into existence. His work shows dioxin levels in soils have increased from an earlier 30 to today's 90 picograms per gram (parts per 1,000 billion), and this can be blamed mainly on the chemicals industry, although car exhausts and furnaces have played a part.

At the turn of the century, industry began to use chlorine to manufacture safer solvents and antiseptics, and from about 1950 it also made chlorinated plastics such as PVC and organochlorine weedkillers. These were used as Agent Orange in the Vietnam war, and for eight years were sprayed to kill trees and expose Viet Cong troops. About 50 million litres of herbicide were deployed, carrying with it 150,000g of dioxins. Today, a chemical plant making PVC typically emits about one gram of dioxin a year. A municipal incinerator may produce up to 750g, but these can be contained. The dioxin scares are nearly over.

The author is science writer in residence at the Department of Chemistry, Imperial College, London.

(Photograph omitted)