The effect of dioxins on human health is the subject of much controversy. Several epidemiological studies of exposed workers have reported excesses of some rare cancers, notably soft tissue sarcomas and lymphomas.
Earlier this year, a Department of Health advisory committee suggested that it would be "prudent" to regard TCDD, the most toxic dioxin, as a possible human carcinogen. More recently, a draft reassessment of dioxin health risks by the US Environmental Protection Agency (EPA) confirmed its characterisation of dioxins as probable human carcinogens (ENDS Report 236, pp 21-24 ).
In the UK, the most clearly defined group of workers exposed to dioxins are those employed at Coalite's Bolsover site. Production of 2,4,5-T, a herbicide which was heavily contaminated with TCDD, began at the site in 1965. In 1968, an explosion on the plant killed a duty chemist, and in the subsequent clean-up 79 workers developed chloracne, a skin complaint caused by dioxin exposure. Production resumed in 1970, but was halted in 1976 after an accident at a similar plant at Seveso in Italy caused widespread dioxin contamination.
Recent tests by Greenpeace found high dioxin levels in the blood of some Coalite workers. The highest concentrations were found in workers involved in the clean-up operation after the explosion (ENDS Report 236, p 22 ).
The HSE has now completed a ten-year study of the mortality of workers employed at the factory between 1965 and 1976. The researchers obtained data on causes of death among a population of 673 workers, 381 of whom were employed at the time of the 1968 incident.
According to John Osman, an HSE medical adviser, "the study showed no convincing evidence of an increased mortality risk - whether from cancer or from other causes of death - arising from the 2,4,5-T process, or following the explosion." By the end of 1989, 126 of the study group had died compared to an expected mortality of 128.2.
Indeed, the study found no deaths from malignant lymphoma or soft tissue sarcoma, the cancers most commonly linked to dioxins. However, the HSE issued an important caution. "Because of the relatively small size of the workforce," it said, "the study had only limited power to detect risks of the magnitude that other studies have suggested." Only 85 workers in the study had developed chloracne and could therefore be assumed to have been heavily exposed to dioxins, and only eight of these had died by 1989.
Car accidents were the only cause of death to show a statistically significant excess. Deaths from stomach cancer were 86% higher than expected in the overall group, and more than four times higher among those workers who developed chloracne. However, the small numbers involved made these results subject to very wide confidence intervals, and the HSE points out that other studies have tentatively ruled out a link between dioxins and stomach cancer. An excess of deaths from lung cancer was considered to be not statistically significant.
Seventeen workers in the study group had died of cerebrovascular disease, or strokes - a figure that was 71% more than expected and on the borderline of statistical significance. The HSE noted that this observation - and the overall mortality statistics for Coalite staff - runs counter to the "healthy worker effect" often seen in occupational exposure studies.
A study by German chemical manufacturer BASF also found that mortality among workers exposed to dioxins by a 1954 incident at a 2,4,5-T plant was similar to that of the population at large. However, BASF's study - with a longer latency period and larger population - found a significant increase in rare cancers among workers who had developed chloracne.
Furthermore, a recent follow-up study by BASF found significant increases in non-fatal illnesses among the TCDD-exposed group (ENDS Report 236, p 22 ). Again, morbidity was particularly elevated in the chloracne sub-group.
The failure of the HSE's study to find convincing data to link dioxin exposure to mortality may alleviate Coalite's dioxin headache, but is unlikely to cure it. In November, HM Inspectorate of Pollution announced its intention to prosecute the firm for dioxin emissions from an incinerator which are alleged to have contaminated the locality (ENDS Report 238, pp 4-5 ).
Furthermore, thanks to the EPA's draft reassessment, the focus of concern has shifted from the carcinogenic effects of dioxins to their possible impact on reproduction, development and hormonal and immune response. Many of these effects fall outside the scope of the HSE's study.