1,3-butadiene is a genotoxic carcinogen for which there is no known safe level of exposure (ENDS Report 238, p 8 ). The Government has proposed an air quality standard (AQS) of 1ppb expressed as a running annual mean, to be achieved by 2005. In general, vehicles are by far the main source of the pollutant - although high levels on Teesside have been traced to emissions from ship unloading (ENDS Report 250, pp 6-7 ).
However, emission inventories compiled to assist in local air quality management are revealing that in some areas industrial sources can be major contributors to levels of pollutants normally associated with traffic. One inventory, prepared by RSK Environment and the London Research Centre, showed that around 20% of 1,3-butadiene emissions on Merseyside stem from a single point source - Shell's Stanlow oil refinery.1Shell uses 1,3-butadiene as a feedstock in a proprietary process for production of sulfolane. Process emissions of the chemical have been reduced significantly since Shell applied for integrated pollution control (IPC) authorisation in 1994, and venting during offloading of road tankers is now the main source. Emissions have amounted to around 35 tonnes in each of the past three years.
Neston and Ellesmere Port Borough Council monitors air quality close to the plant but does not test for 1,3-butadiene. The nearest site which does so is four miles away at Speke, across the Mersey. The running annual mean level at this site is currently 0.19ppb, well within the AQS, though hourly peaks of up to 28ppb have been recorded.
Shell modelled dispersion of 1,3-butadiene as part of its IPC application. Assuming an annual release of up to 100 tonnes, it concluded that a peak annual average concentration of 4ppb would occur on the refinery site. The maximum level off-site was put at around 0.5ppb. This is within the AQS, but excludes the typical background level for the pollutant of some 0.2ppb. In the absence of monitoring data it is possible, though unlikely, that the AQS is exceeded off site.
A Shell spokesman said: "We conduct regular occupational health checks for exposure on site, and these demonstrate that regulatory and company requirements are not exceeded." The firm says that improvements agreed with the Environment Agency should reduce emissions by 90% by the end of 2001.