The Problem With Diesel

Abstract

Diesel engine exhaust (DEE) is a complex mixture of substances characterized by polycyclic aromatic hydrocarbons (PAH) surrounding an elemental carbon core. Diesel engines have a wide range of industrial applications including on-road equipment (most heavy and medium duty trucks and buses use diesel engines) and off-road applications in the mining, rail, construction, distribution, and farming industries and in the military, including the use of diesel-powered heavy equipment, locomotives, forklift trucks, ships, tractors, and generators. In a recent review of the literature , the highest levels of elemental carbon were reported for enclosed underground work sites in mining and construction with intermediate levels for above-ground semi-enclosed work areas for workshop mechanics, dock workers, and fire station workers, and the lowest levels being reported for enclosed areas separated from the source such as drivers, train crew, parking attendants, vehicle testers, and utility service workers ( 1 ). A large body of epidemiological work has shown consistent associations between particulate matter in ambient air and several health outcomes including chronic bronchitis, ischemic heart disease, stroke, and respiratory infections and exacerbation of asthma. The effect of DEE from traffi c on people with preexisting disease has also been shown, for example, in the reduction of lung function resulting from a 2- hour walk by people with mild or moderate asthma down London’s busiest shopping street (Oxford Street), where only diesel- powered buses and taxis are permitted access, compared with a similar walk round Hyde Park ( 2 ). The possibility that DEE might cause cancer in humans has been raised since 1955 when it was demonstrated that the particulate fraction of DEE contained PAHs such as benzo(a)pyrene known by then to cause tumors in experimental animals ( 3 ). The gas phase includes carbon monoxide and nitrogen oxides, but it is the particulate phase of the exhaust that appears to be implicated as a lung carcinogen. An effect of DEE on bladder cancer is also plausible because metabolites of PAH present in DEE are concentrated in the urine and may interact with the urothelium of the bladder ( 4 ). In this issue of the Journal, two related articles ( 5 , 6 ) report results from studies of DEE in miners based on quantitative estimates of respirable elemental carbon (REC) and focusing on lung cancer. In the cohort study ( 5 ) , inclusion of co-exposures such as silica did not change the fi ndings substantially. In the case – control study ( 6 ) , adjustment was carried out for several potential confounders including smoking. A dose – response relationship was found with increasing exposure to REC in both the cohort and case – control studies but is particularly clear in the latter, where an interaction between smoking and DEE was also found. These studies in miners make an important contribution to the body of evidence about DEE and are timely given the forthcoming International Agency for Research on Cancer (IARC) monograph meeting this year at which the current IARC categorization of DEE as a group 2A (probable human) carcinogen ( 7 ) will be reconsidered. The issue of causality is fundamental when estimating and ranking burden of disease attributable to various exposures. A recent study estimating the burden of occupational cancer in Great Britain chose to include all IARC group 1 (defi nite) and 2A carcinogens ( 8 ). DEE emerged as one of the most important problems. For bladder cancer related to exposure to DEE, the estimated total attributable fraction for bladder cancer was 1.00% (95% confi dence interval [ CI ] = 0.17 to 2.03), with an estimated 47 (95% CI = 8 to 94) deaths in 2005 and 106 (95% CI = 18 to 214) cancer registrations in 2004; for lung cancer, the attributable fraction was 1.84% (95% CI = 0.00% to 3.37%), with 605 (95% CI = 272 to 1107) deaths and 695 (95% CI = 313 to 1269) cancer registrations. DEE was the sixth most important occupational carcinogen, contributing 8.1% of the deaths and 5.9% of the cancer registrations. The majority occurred in land transport as expected but also in the construction industry where nearly 500 000 workers were estimated to have been exposed to DEE over the 40-year risk exposure period before the year of estimation. As the authors point out in the mining studies published in this issue, the levels of exposure they have found are high compared with other studies. The median elemental carbon values in the Oxford Street study were 7.5 μ g/m 3 (range 3.9 – 16) compared with