In September of 2002, the EPA released the final Health Assessment Document for Diesel Engine Exhaust, which stated that exposure to smog and particulate matter could potentially be responsible for respiratory ailments in as many as one million people, asthma attacks in as many as 400,000 people, and the cause of cancer in several thousand Americans each year. The particulate matter discussed in the report includes that generated from diesel-powered automobiles, and is described as a complex mixture of substances generated in the combustion of diesel fuel. The incidence rate of lung cancer for a variety of occupations subject to exposure to diesel exhaust is also mentioned, including railroad workers, miners, bus garage workers, and transportation workers. The fact of the matter is, however, that due to other factors such as smoking, there is no obvious proof of a causal association between exposure to particulate matter from diesel exhaust and excess risk of lung cancer.
The particulate matter from diesel exhaust amounts to approximately 23%, with this percentage increasing in urban areas up to a maximum of approximately 30% in California, Colorado, and Arizona. The remainder is from natural ambience. Although based on limited results, the report indicates that this particulate is from industrial-use diesel engines, which are less improved, rather than from automotive diesel engines, which have been subject to constant improvement on a yearly basis.
The present report was initiated at the insistence of public opinion to study the causal association between particulate matter from diesel exhaust and the incidence of cancer, and it reports that particulate matter from diesel exhaust accumulates in the deepest part of the lungs, where it poses a potential health hazard. Nevertheless, the content of this report comprises mainly previously published information, and does not contain the results of any new studies. The present EPA report states clearly the uncertainties inherent in the study, and does not conclude that there is a causal association between exposure to particulate matter from diesel exhaust and excess risk of lung cancer, but rather recommends issues for further research. Much of the research is pertinent only to diesel engines built in the early and mid-1990s, and specifically states that as new diesel engines with cleaner exhaust emissions replace existing engines, the applicability of the conclusions in the report will need to be reevaluated.
The EPA's current policy is to strengthen regulation of new diesel-powered vehicles as well as to strengthen exhaust emissions regulations for industrial-use diesels by bringing them in line with EU regulation. In Europe as well, the strengthening of regulations for industrial-use diesel engines predicated on the application of after-treatment equipment is under study, and it is expected that after-treatment equipment for industrial-use diesel engines will become indispensable in the US as well. There remain many issues that require clarification regarding industrial-use diesel engines, however, including those related to fuel, which still contains high levels of sulfur, and the feasibility of installing after-treatment equipment.