Air quality standards proposed for 1,3-butadiene and CO

Industrial workers exposed to 1,3-butadiene have been shown to have a slightly increased risk of lymphoma or leukaemia. EPAQS felt it unlikely that excess risk would be detectable at exposures below around 1,000ppb. To arrive at its proposed AQS, the panel repeated the approach it took with benzene and reduced this level by a factor of one hundred to allow for lifetime exposures and to protect any sensitive individuals in the general population.

A further ten-fold safety margin was then added to take into account the weakness of the epidemiological data. The report also notes that recent laboratory studies have provided "good evidence" that 1,3-butadiene is a genotoxic carcinogen, for which there is no absolutely safe level of exposure.

EPAQS has recommended that the 1ppb standard should be reviewed within five years "in the light of any additional human data and the experience of improved pollution control." The report notes that levels have probably increased in recent years as a result of the increased use of higher olefins in petrol, but suggests that this trend will be reversed by the introduction of catalytic converters.

At present, data on 1,3-butadiene levels in the UK are sparse. The chemical is monitored at nine sites in the Department of the Environment's national monitoring network, and a further three will be added by March 1995. Figures gathered to date suggest that urban baseline levels are typically 0.2-0.7ppb, with rural concentrations well below 0.1ppb. At these levels, says EPAQS, the risks to the general population "must be exceedingly small".

However, some doubt hangs over EPAQS' conclusion that "it is unlikely that annual average 1,3-butadiene concentrations will exceed 1ppb close to the most heavily trafficked roads in the UK." Monitoring by local authorities suggests that the DoE's sites may underestimate typical urban levels of benzene, which correlate closely to levels of 1,3-butadiene (see p 3 ).

The main health concern with outdoor carbon monoxide pollution is a reduction in the oxygen-carrying capacity of the blood that may increase the risk of problems in individuals with heart disease. EPAQS' aim was to limit the exposure of susceptible individuals to levels at which harm is thought unlikely to occur.

Eighteen DoE monitoring sites currently test for carbon monoxide. Most urban sites detected exceedances of the proposed AQS on at least one day between 1990 and 1993. Sites in west London have recorded exceedances on up to five days per year. In contrast, the WHO 1-hour guideline of 25ppm appears to be rarely, if ever, exceeded.

Some 87% of the UK's carbon monoxide emissions come from petrol-engined vehicles, and emissions have increased by 50% since 1970. A downward trend is now beginning as new cars fitted with catalytic convertors enter the fleet.