Concern about air pollution from diesels has been stirred by health studies in the USA and elsewhere which suggested that elevated levels of particulates are correlated with long-term increases in mortality.
Diesel emissions are rich in the particulates which are of most concern, the PM10s - particles less than 10µm in diameter.
Medical experts believe that the link between levels of PM10 and health effects is now consistent enough to indicate a causal relationship. In a recent article in New Scientist magazine, an epidemiologist from the US Environment Protection Agency calculated that airborne PM10 levels in England and Wales may account for 10,000 deaths annually.
In January, the DoE's Quality of Urban Air Review Group (QUARG) warned that the rising sales of diesel cars pose a serious threat to urban air quality (ENDS Report 228, pp 9-10 ). Its Chairman, Roy Harrison (Professor of Environmental Health, University of Birmingham), suggested that a price differential against diesel fuel would be a suitable measure to counteract their increasing popularity.
However, members of QUARG appeared less than unanimous at a National Society for Clean Air conference in London on 28 March.
One, Dr Clare Holman, concluded that there is "no clear answer" to whether diesel or petrol cars are preferable from an air quality standpoint. Although particulate emissions from petrol cars are low, diesels emit lower levels of most hydrocarbons than petrol vehicles. The lower hydrocarbon emissions of diesels are particularly apparent when the poor performance of petrol engines with catalysts in cold start conditions is considered.
Dr Duncan Laxen told the conference that although the proportion of diesel cars is increasing, particulate emissions will decline in coming years through improvements in fuel and engine specifications. The sources of PM10 and the proportion emitted by diesels and other vehicles are also not clear. Although present estimates indicate that 90% of PM10 in London arises from vehicles (see table ), Dr Laxen noted that monitoring had shown that airborne concentrations do not follow those of primary transport pollutants such as nitrogen dioxide. The role of traffic in generating PM10 remains "very uncertain", he said, and further research is needed.
Professor Harrison was also at pains to correct the widespread misinterpretation of the 10,000 mortalities figure. He emphasised that the deaths attributable to diesels would be only around 3,000 because, on present evidence, only one-third of PM10 emissions are attributable to diesels.
A Government response to the QUARG report is likely later this year. Two further advisory bodies are contributing to the debate.
One, the Department of Health's Committee on the Medical Effects of Air Pollution, has set up a sub-group to investigate the relationship between particulates and health and advise on whether an estimate of deaths due to PM10 can be made.
But the full Committee has already warned that a 1992 report by the Government's Advisory Group on the Medical Aspects of Air Pollution Episodes may have been wrong to conclude that particulates do not pose a significant health threat. According to a parliamentary answer by junior Health Minister Tom Sackville, the Committee has advised that "there appears to be sufficient evidence from recent studies in a number of countries to give cause for concern about the possible effects of current levels of fine particles upon health."1The Committee is expected to report later this year. Meanwhile, the DoE's Expert Panel on Air Quality Standards is considering a standard for PM10.