Diesel Exhaust and Cancer

Diesel exhaust is not one substance. It is the whole set of combustion products from burning diesel fuel, and its exact makeup shifts with the engine, the load, the lubricating oil, the additives, the emission controls, and the fuel itself.

Diesel engines burn with excess air, roughly 25 to 30 parts air to 1 part fuel. Most of the gas coming out is ordinary: nitrogen, oxygen, carbon dioxide, and water vapor. Incomplete combustion adds the rest. The gas phase also carries carbon monoxide, sulfur oxides, and nitrogen oxides. It carries volatile hydrocarbons too, plus light polycyclic aromatic hydrocarbons.

Then there is the soot.

Most diesel exhaust particles measure 0.1 to 0.25 micrometers across. About 92% are smaller than 1 micrometer. The size spread has two humps: a nuclei mode from 0.0075 to 0.042 micrometers, made of freshly condensed material, and an accumulation mode from 0.042 to 1.0 micrometers, made of those nuclei clumping together.

Organic and elemental carbon make up roughly 80% of the particle mass. Most of the remaining 20% is sulfate, largely sulfuric acid. Because the particles have so much surface area for their size, they soak up organic material from unburned fuel, lubricating oil, and combustion itself. The extractable organic fraction is usually 20% to 30% of particle mass, and can reach 90%.

Two facts explain why the particles, not the gases, carry the cancer concern. First, nearly all of them are small enough to reach the alveolar region, the deepest gas-exchanging part of the lung. Second, mutagens and carcinogens can be pulled off those particles. PAHs and nitroarenes come off with organic solvents. They also come off with a lipid component of the surfactant that lines mammalian lungs. In rat studies, only unfiltered diesel exhaust caused lung tumors. Filter the particles out and the tumors did not appear.

Diesel engines also emit far more particulate mass than gasoline engines with catalytic converters. Depending on conditions, light-duty diesel engines emit 50 to 80 times as much, and heavy-duty engines 100 to 200 times as much.

IARC is the International Agency for Research on Cancer, part of the World Health Organization. It first looked at diesel exhaust in 1988. Back then it called the exhaust probably carcinogenic to humans, Group 2A. An IARC advisory group had flagged it for re-review as a high priority since 1998.

On 12 June 2012, after a week-long meeting, IARC moved diesel engine exhaust to Group 1, carcinogenic to humans. The basis was sufficient evidence that exposure raises lung cancer risk. The working group also noted a positive association with bladder cancer, which it graded as limited evidence rather than sufficient. Gasoline exhaust was evaluated at the same meeting and stayed in Group 2B, possibly carcinogenic, unchanged since 1989. The full review is IARC Monographs Volume 105.

That distinction between sufficient and limited matters. Lung cancer is the established outcome. Bladder cancer is a real signal that has not reached the same standard of proof.

The U.S. National Toxicology Program reaches a softer conclusion. Its Report on Carcinogens lists diesel exhaust particulates as reasonably anticipated to be a human carcinogen, not as known. The stated basis is limited evidence in humans plus supporting evidence from animal and mechanistic studies.

The dates explain the gap. NTP first listed diesel exhaust particulates in the Ninth Report on Carcinogens in 2000, a dozen years before IARC's re-evaluation. IARC's Group 1 classification is the more recent judgment. NTP also frames its listing around the particulates specifically, while IARC evaluated the whole engine exhaust.

Diesel exhaust particulates carry no CAS registry number, because they are a mixture and not a single chemical.

IARC's press release named one study as re-emphasizing the concern: a large joint NCI and NIOSH investigation of underground miners, published in March 2012. It is known as the Diesel Exhaust in Miners Study, or DEMS.

DEMS followed 12,315 workers at eight U.S. non-metal mining facilities. Historical measurements were reconstructed into estimates of respirable elemental carbon, job by job and year by year, for each worker. Respirable elemental carbon is the standard stand-in for how much diesel soot someone actually breathed.

The cohort analysis found a standardized mortality ratio for lung cancer of 1.26. The 95% confidence interval ran from 1.09 to 1.44. A nested case-control analysis then compared 198 lung cancer deaths with 562 matched controls. It adjusted for cigarette smoking, prior high-risk jobs, and respiratory disease history. Lung cancer risk rose steadily as cumulative exposure rose, with the trend significant at P = .001 once the most recent 15 years of exposure were excluded. Workers above the median of the top exposure quartile — at or above 1,005 micrograms per cubic meter-years — had about three times the risk of those in the lowest quartile, an odds ratio of 3.20 with a 95% confidence interval of 1.33 to 7.69.

The result among people who had never smoked is the one worth pausing on. Across three exposure tertiles, odds ratios were 1.0, then 1.47, then 7.30. The confidence interval on that last figure is wide, 1.46 to 36.57, because never-smokers with heavy exposure are a small group. But the gradient is there without tobacco in the picture at all.

Earlier work pointed the same way without pinning down dose. NTP notes that a meta-analysis found diesel exhaust raised the relative risk for lung cancer, and that later cohort and case-control studies reported relative risks in the range of 1.2 to 2.21. DEMS added what those studies lacked: quantitative historical exposure estimates.

Numbers make the gaps between jobs concrete. The National Toxicology Program compiled these measured figures.

• Truck drivers: 1 to 10 µg/m³ elemental carbon. One study found no real gap between truckers at 3.8 µg/m³ and highway background at 2.5 µg/m³, meaning the road, not the cab, was the source.
• Mechanics in bus garages and truck terminals: 20 to 40 µg/m³.
• Train crews: 4 to 20 µg/m³. Maintenance crews: 3 to 39 µg/m³.
• Locomotive workers, respirable particulate matter: 39 to 191 µg/m³, per EPA.
• Firefighters inside fire stations: 10 to 40 µg/m³. A diesel fire truck idling indoors can spread exhaust through a whole station.
• Underground mining, respirable elemental carbon from 1997 to 2004: 148 to 637 µg/m³. Surface mining: 13 to 23 µg/m³.
• Enclosed spaces inside mines: diesel particulate up to 1,280 µg/m³.

Underground mining sits at the top by a wide margin — roughly a hundred times a truck driver's typical level. Diesel machinery has been used in U.S. mines since the early 1950s.

Mines are where the regulation bites. The rule is 30 CFR 57.5060. It covers underground metal and nonmetal mines. A miner's personal exposure to diesel particulate matter must not exceed 160 micrograms of total carbon per cubic meter of air. That figure is an average eight-hour equivalent full shift. It took effect on 20 May 2008, and it arrived in steps. First came an interim limit of 308 µg/m³ of elemental carbon. Then 350 µg/m³ of total carbon from January 2007. Then 160. A mine that cannot meet it for technological or economic reasons can apply to a district manager for a special extension, which lasts no more than one year at a time.

Diesel exhaust also has three broad source groups. Mobile sources are the obvious ones. Stationary area sources include oil and gas production, repair yards, and shipyards. Stationary point sources include chemical plants and electric utilities. For the broader picture of what is in outdoor air, see air pollution and cancer. For how workplace exposures are handled generally, see occupational carcinogens, and for the lung disease itself, lung cancer.

• IARC — Diesel Engine Exhaust Carcinogenic, 12 June 2012
• IARC Press Release 213, PDF
• IARC Monographs Volume 105 — Diesel and Gasoline Engine Exhausts and Some Nitroarenes
• NTP Report on Carcinogens, 15th edition — Diesel Exhaust Particulates profile
• PubMed — Silverman DT, et al. DEMS nested case-control study, PMID 22393209
• PubMed — Attfield MD, et al. DEMS cohort mortality study, PMID 22393207
• govinfo — 30 CFR 57.5060, limit on exposure to diesel particulate matter