Unlike its earlier recommendations for air quality standards, EPAQS' latest proposal has been issued as a draft. The move reflects a recent review of the Panel by the Department of Environment, Transport and the Regions which concluded that EPAQS must carry out wider consultation and report annually on its work. The DETR also accepted industry calls for greater transparency when translating EPAQS' recommendations into national air quality standards and objectives.
PAHs are a large group of organic hydrocarbons which are released into the air by incomplete combustion. They are found as complex mixtures both in the vapour phase and bound to particles. EPAQS has proposed that levels of benzo[a]pyrene (B[a]P), the best characterised PAH, should be used as a marker for the total carcinogenic activity of this mixture.
Of the 16 main PAHs found in ambient air, three - including B[a]P - are potent animal carcinogens and classed as probable human carcinogens. The others are regarded as possible carcinogens but not classifiable owing to lack of data.
High exposures to PAHs have been linked to cancers of the lung, skin and possibly bladder. EPAQS considered a range of studies of workers in the coke production and aluminium industries. It also looked at several European epidemiological studies which point to an association between urban air pollution and excess risk of lung cancer.
EPAQS says that occupational studies have found no effects below average workplace exposures of 0.25-2.5µg/m 3 of B[a]P. It takes the lower end of this range and proposes that a safety factor of 1,000 be applied to reflect lifetime exposure, variations in sensitivity among the wider population and uncertainty in the occupational data.
As a result, EPAQS proposes an air quality standard of 0.25ng/m 3 B[a]P as an annual average. At this level, the Panel believes that risk to the population "would be so small as to be undetectable."
The proposed standard appears to be widely exceeded in urban areas, where traffic is generally the dominant source of PAHs. The DETR currently operates seven PAH monitoring sites. In 1997, the three urban background sites in London, Manchester and Middlesbrough recorded annual average B[a]P levels of 0.6, 0.8 and 0.5ng/m 3, respectively. In contrast, rural background levels were 0.2ng/m 3.
EPAQS says that urban B[a]P levels have probably declined ten-fold since the era of widespread domestic coal burning. However, data from the DETR's urban sites suggests that levels of most PAHs have increased from a low-point in the mid-1990s - perhaps reflecting higher emissions from the growing number of diesel vehicles.
EPAQS "strongly recommend[s] that the number of PAH monitoring sites in the UK should be increased, particularly to include areas influenced by industrial emissions." Coke production and the aluminium industry are particularly likely to lead to elevated PAH concentrations - as highlighted by an emissions inventory compiled by the National Environmental Technology Centre (see table ).
The inventory excludes many potentially significant sources such as production and use of bitumen, foundries, non-ferrous metals, waste oil combustion, minerals processes and fires. The aluminium industry stands out as a major source - although the Aluminium Federation was unable to tell ENDS of any PAH monitoring near its members' sites.
According to NETCEN, the aluminium sector's emissions are dominated by anode baking, which released 860 tonnes of PAHs and 4 tonnes of B[a]P in 1997. However, emissions from anode baking fell by 40-60% from the 1996 level as pollution control measures took effect - and NETCEN expects B[a]P releases in 1998 to drop to around one tonne. In 1996, emissions from the UK's four aluminium smelters stood at 190 tonnes of total PAHs and 1.3 tonnes of B[a]P.