In June this year, the International Agency for Research on Cancer (IARC) reclassified diesel emissions from group 2A (probably carcinogenic to humans) to group 1 (carcinogenic to humans). This puts diesel emissions in the same category as other group 1 carcinogens such as arsenic, asbestos, benzene, ethanol, and formaldehyde. Diesel emissions are associated with an increased risk of lung cancer and possibly an increased risk of bladder cancer although the evidence for this is limited.
These findings may be of concern to some, particularly those who drive diesel vehicles or who are exposed to signficant amounts of diesel emissions in the workplace. Diesel based cars are increasingly in popularity around the world. In Europe for example, about half of new cars sold have diesel engines. In the USA, around 3% of new cars sold are diesel based however this figure is expected to double to 6% by 2015.
It is important however to put the IARC reclassification in perspective.
First, the health risks associated with diesel fumes have been known for some time. In 1988, the IARC classified diesel as probably carcinogenic to humans because at the time, only a relatively small number of studies existed that suggested a positive association between diesel fumes and lung cancer. The move to a group 1 carcinogen comes on the back of several recent studies, such as The Diesel Exhaust in Miners Study. These recent studies don’t necessarily indicate that diesel emissions are more dangerous than previously thought however the combined number of studies now indicating a link between diesel and lung cancer is sufficient for the IARC to classify diesel fumes as a group 1 carcinogen.
Interestingly, gasoline emissions are classified as possibly carcinogenic to humans (group 2B) however due to the difficulty of separating gasoline exposure from diesel exposure (those exposed to gasoline emissions have typically also been exposed to diesel), it is difficult to conclusively say whether gasoline exhaust emissions alone can cause lung cancer.
The increased risk of lung cancer as a result of diesel emissions also appears to be quite modest when compared to other risk factors for lung cancer.
A 1988 study of railroad workers found that among a cohort of 40-44 year olds who had worked in the railroad industry for at least 10 years, the risk of lung cancer was 45% higher than expected.
A Harvard study of more than 10,000 USA long haul truck drivers , published in 2008, found that drivers with at least 1 year of work history were 15% more likely to develop lung cancer than the general population.
An excess lung cancer risk of between 15% and 45% would suggest that a non-smoker occupationally exposed to diesel emissions has approximately a 1 in 200 to 1 in 600 chance of developing lung cancer during their life as a direct result of their diesel exposure.
The level of exposure to those who own diesel cars, or who experience occasional low-level diesel exposure (for example travelling to work at peak traffic times) would likely be a fraction of that experienced by the above groups and therefore excess lung cancer risk due to diesel fumes would be negligible in these groups.
Studies have estimated that railway workers in the 1990s were exposed to approximately 40-150 µg/m3 of elemental carbon (a measure of exposure to diesel combustion products) while truck drivers average exposure is around 3-5 µg/m3 of elemental carbon. In contrast background levels of elemental carbon range from 1 µg/m3 in urban centers to 0.1 µg/m3 in rural areas.
The graph to the right shows the relative risks of lung cancer in various situations. The risks of lung cancer due to occupational diesel exposure are similar to those for workers occupationally exposed to passive smoke or who live in a home with high radon levels (>4 pCi/L). In comparison, current smokers have a 10-fold increased risk of lung cancer while those occupationally exposed to asbestos have a 5-fold increased risk.
In addition to this, studies on the risk of lung cancer in those occupationally exposed to diesel fumes typically have long follow-up periods and therefore even recent studies rely on diesel exposures that occurred several decades ago. The level of occupational exposure to diesel has fallen dramatically over the past half century due to stringent diesel emissions standards imposed in many developed countries.
A 2011 study, published in the journal Environmental Health Perspectives, found that air concentrations of elemental carbon have declined significantly in the United States across 5 job categories where diesel exposure is common. For example, elemental carbon levels for long haul truck drivers declined from a median of 9 μg/m3 in 1971 to around 2 μg/m3 in 2000.
As a result, past studies probably overestimate the present day risks associated with modern diesel engines.
In summary, all forms of air pollution are harmful and while diesel emissions are now listed as carcinogenic, it is quite possible that gasoline emissions and other forms of air pollution carry similar risks. In any event, the risks to those not occupationally exposed to diesel emissions are quite small and are likely to become smaller as more stringent diesel emission standards continue to be phased in.
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