Measurement and Monitoring of Benzene

9. The value of any standard depends upon the methods and strategy of measurement. Until recently it was necessary to collect samples of air in evacuated steel flasks or tubes containing absorbent material and take these to a laboratory for analysis by gas chromatography- - a labour intensive and time consuming process. In recent years automatic gas chromatographs have become available. These instruments can be sited in remote places, do not require to be attended, and can carry out and report hourly analyses on a continuous basis. This has meant that it is now practicable to obtain information on benzene (and other chemical) concentrations in the air from multiple sites and to make this available to the public. The frequency of measurement also allows much more accurate information on benzene concentrations to be obtained than was possible in the past.
10. The Department of Environment is now establishing a network of 12 sites around the country where continuous monitoring of benzene (measuring to an accuracy of ±10% and a precision of 0.1 ppb) and other hydrocarbon compounds will take place. A site has operated in central London (roadside) since mid-1991 and in Middlesbrough from early 1992. Other sites have since been established in east London, Belfast, Edinburgh, Birmingham and Cardiff and sites in Leeds and Bristol are expected to be operational by the Spring of 1994.
11. In spite of the difficulties associated with measurement of benzene in the past, some information is available from a number of sites. In central London, at a roadside site (where concentrations would be expected to be highest), measurements have been made since the late 1970s. These measurements have shown evidence of a downward trend over a period when traffic movements and fuel consumption have increased significantly. Since the early 1980s measures have been taken to reduce progressively vehicle exhaust hydrocarbon emissions. These measures, which have largely involved combustion modification, are believed to have accounted for the decrease in benzene levels observed in central London.
12. In comparison, at a rural site, Harwell in Oxfordshire, concentrations have fluctuated between about 1.7 ppb and 0.4 ppb, expressed as a running three month average. Figure I shows a slight downward trend in these concentrations since 1986, probably resulting from increasingly stringent European legislation with respect to vehicle emissions. The figure also shows the marked increase in benzene concentrations in the winter, a feature observed at most urban and rural sites for primary pollutants such as benzene, due to cold still weather which usually occurs at this time of year. However, measurements made in two other rural sites in southern England have shown rises in annual average concentrations of benzene between 1989 and 1991, reaching between 0.9 ppb and 1 ppb in the latter year.
13. From the foregoing, it will be clear that, in general, benzene concentrations in the outdoor air are highest adjacent to busy urban streets and lowest in remote rural locations, and reflect traffic density. Figure 2 shows concentrations averaged over monthly periods in 1991-2 in central London and Middlesbrough. These generally ranged between 1 ppb and 4 ppb, but were as high as 13.6 ppb in December 1991 during a pollution episode in London, when heavy traffic coincided with very cold, still weather. Thus, the predicted increases in traffic in the United Kingdom might be expected to cause an increase in concentrations of benzene in the air over the next few years. However, recent legislation requiring the introduction of catalytic converters on new cars and introducing a check on exhaust emissions as part of the annual MOT test is expected to more than counterbalance the effect of predicted increases in traffic over the next 15-20 years; it is therefore realistic to set standards for airborne benzene concentrations that would require a lowering of present ambient concentrations.
14. The Panel, in deciding upon a recommended Standard, have considered the time period over which benzene measurements should be made. As is discussed in the next section, benzene has the potential to cause leukaemia, a type of cancer of the blood-forming tissues of the bone marrow. At concentrations occurring in the ambient atmosphere, benzene does not have short-term, or acute, effects. The risk of leukaemia in industrial workers, exposed to much higher concentrations of benzene, has been related to their calculated lifetime exposure - the more benzene they had been exposed to, the greater the risk.
15. The Panel therefore consider it appropriate to recommend a Standard in terms of a running annual average concentration², which is likely to best reflect the integrated exposure of the population. The Standard is set on the basis of possible health effects; it is intended that techniques for its monitoring be consistent with those of the Department of the Environment's Enhanced Urban Network. A long term average will tend to obscure individual high concentrations recorded on a daily basis, although pollution episodes such as that of December 1991 in London will still be apparent. The relationships between daily and annual averages are shown in Figure 3 for central London over the twelve months from July 1991. It can be seen that the December episode raised the annual running average from around 3 ppb to 6 ppb, with a slow decline thereafter to about 4 ppb. The annual average for the period shown was 4.2 ppb.
16. In discussing the setting of an Air Quality Standard, the Panel have had in mind the importance of practical measures to ensure compliance. As stated above, the most important of these relate to control of traffic exhaust emissions. Whilst the Government has already implemented a number of important measures in this direction, there also remains much that individuals can do. Since traffic levels are predictable and since tomorrow's weather conditions can be forecast increasingly accurately, it is possible to estimate the likely atmospheric concentration of benzene and other pollutants the day before they occur. This means that episodes, such as that in December 1991, may be forecast and the public informed of action that could be taken to reduce such levels, and thus reduce the population's lifetime exposure. For example, by restricting the use of motor vehicles at these times it is possible to contribute to reducing the severity of such episodes.