Diesel Exhaust

Exposure to diesel exhaust is widespread in the modern world. Trucks, buses, trains, construction and farm equipment, generators, ships, and some cars have diesel engines. Exhaust from these engines brings a complex mixture of soot and gases to roadways, cities, farms, and other workplaces. Health concerns about diesel exhaust relate not only to cancer, but also to lung (respiratory) and heart diseases.

What Is Diesel Exhaust?

The exhaust from diesel engines is made up of both gases and soot, each of which is made up of thousands of different substances. The gas portion of diesel exhaust is mostly carbon dioxide, carbon monoxide, nitric oxide, nitrogen dioxide, sulfur oxides, and hydrocarbons, including polycyclic aromatic hydrocarbons (PAHs). The particulate portion of diesel exhaust, also known as soot, is mainly made up of elemental carbon, organic material (including PAHs), and traces of metallic compounds. So, PAHs are found in both the gases and the soot of diesel exhaust.

How Are People Exposed to Diesel Exhaust?

People are exposed to diesel exhaust by breathing in the soot and gases, rather than by eating contaminated foods or through skin absorption. Measuring exposure is difficult because diesel exhaust is chemically complex and many parts of it are also found in a lot of other sources. This has been, and remains, a major challenge in scientific studies. People may be exposed to diesel exhaust at work, through daily living, or on the way to school.

At Work

The amount and length of time people are exposed to diesel exhaust varies greatly. Those with the highest exposures include truck drivers, bridge and tunnel workers, mine workers, forklift drivers, railroad and dock workers, and garage workers. Farm workers and car, truck, and bus maintenance garage workers may also spend a lot of time around diesel exhaust.

Every Day

The general public is also exposed to diesel exhaust, although less often and at much lower levels than in the workplace. Exposures are highest where diesel traffic is heaviest, such as along major highways and in cities.

On the Way to School

One particular area of concern that has been studied is children’s exposures to diesel exhaust and other vehicle pollutants while going to school on diesel school buses. The California Environmental Protection Agency published the results of The Children’s School Bus Exposure Study in 2003. The study found that diesel exhaust exposure is indeed increased during bus travel and needs to be reduced. Many schools now use the study's suggestions of minimizing travel times; using the newest, cleanest buses for the longest trips; retiring older buses as quickly as possible; and decreasing bus-to-bus lineups and idling time in an effort to reduce children's exposure to bus-related air pollutants.

Does Diesel Exhaust Cause Cancer?

Background Information

How Normal Cells Become Cancer Cells

Cancer cells develop because of damage to DNA. DNA is in every cell and directs all the cell's activities. When DNA becomes damaged it is often able to repair itself. In cancer cells, the damage is not repaired.

Carcinogens

A substance that causes cancer is called a carcinogen (car-SIN-oh-gin). Some carcinogens do not act on DNA directly, but lead to cancer in other ways. For example, the carcinogen may cause cells to divide at a faster rate, which could increase the chances that DNA changes will occur.

Carcinogens do not cause cancer in every case, all the time. Substances classified as carcinogens may have different levels of cancer-causing potential. Some may cause cancer only after prolonged, high levels of exposure. And for any one person, the risk of developing cancer depends on many factors, such as the length and intensity of exposure to the carcinogen, the person's overall health, and the person’s family history.

Evidence

Two kinds of evidence tell us about the cancer-causing (carcinogenic) potential of diesel exhaust:

1. studies of the parts of diesel exhaust, such as soot and PAHs
2. studies of diesel exhaust itself

A full discussion of what is known about the parts of diesel exhaust, such as soot and PAHs, is beyond the scope of this article, but soot and PAHs have been classified as carcinogens. Evidence about diesel exhaust as a whole is presented here.

What Do Epidemiologic Studies Suggest?

Epidemiologic studies provide information about the possible causes of disease, occurrence of disease in a population or its subgroups, and trends in the frequency of disease over time. Epidemiologic studies of lung cancer risk in diesel-exposed workers have been done. Lung cancer is the major cancer thought to be linked to diesel exhaust.

These studies face the usual challenges of such studies. For example, it is very hard to correctly define and measure work-place exposure. It is also difficult to take into account the impact of other risk factors, such as smoking.

Epidemiologic studies of workers exposed to diesel exhaust have shown small but significant increases in risk of lung cancer. A case-control study found that railroad workers with at least 20 years of service were more likely to die from lung cancer than were members of the general population. Another study of railroad workers by the same researchers found that lung cancer risk increased with length of exposure to diesel exhaust; the relative risk was highest among workers with the longest exposure.

Several studies of Teamsters also linked diesel exhaust exposure with lung cancer. Still another study of almost a half-million American men looked at the effect of work-place exposure to diesel exhaust on their risk of developing lung cancer. Men with the heaviest and most prolonged exposures, such as railroad workers, heavy equipment operators, miners, and truck drivers, had higher lung cancer death rates than unexposed workers. Although most studies have found a link between diesel exhaust exposure and lung cancer, many have not. Still, with more than 1 million exposed workers, diesel exhaust may pose a substantial health risk.

The relationship between lung cancer and diesel exhaust in the general environment has been studied less than occupational exposures.

What Do Animal and Laboratory Studies Suggest?

In laboratory studies, diesel exhaust (as soot or chemical extracts) can cause changes in DNA. Several studies also provide proof that diesel exhaust can cause cancer in animals. For example, a long-term inhalation study in mice, rats, and hamsters exposed to diesel exhaust showed a higher rate of lung cancer in the exposed animals. However, the carcinogenic effects of diesel exhaust in animals may relate to soot overload in their lungs. If true, this would suggest that normal lung defenses must be overcome in order for diesel exhaust to cause cancer.

What Do the Experts Say?

Based on animal and human evidence, expert agencies have evaluated the potential of diesel exhaust to cause cancer.

The National Toxicology Program

The National Toxicology Program evaluates exposures that may cause cancer. Exposures that are thought to cause cancer are included in the "Reports on Carcinogens," which is published every 2 years. Each exposure is assigned to 1 of 2 categories: "known to be human carcinogens," and "reasonably anticipated to be human carcinogens." The first category includes substances for which human studies (epidemiologic and/or experimental studies) provide "sufficient evidence" that they cause cancer in humans. The second category includes substances for which there is limited evidence that they cause cancer in humans and/or sufficient evidence of causing cancer in experimental animals. In 2005, the National Toxicology Program classified diesel exhaust particulates as "reasonably anticipated to be human carcinogens."

The International Agency for Research on Cancer

The International Agency for Research on Cancer (IARC) also evaluates exposures that may cause cancer. IARC classifies exposures into 1 of 4 categories.

* Group-1 exposures are those "known to be carcinogenic to humans," usually based on "sufficient" human evidence, but sometimes based on "sufficient" evidence in experimental animals and "strong" human evidence.
* Group-2 exposures are divided into 2 categories. Group-2A ("probably carcinogenic to humans") has stronger evidence, and Group-2B ("possibly carcinogenic to humans") has weaker evidence.
* Group-3 exposures are not considered classifiable because the available evidence is limited or inadequate.
* Group-4 exposures are "probably not carcinogenic to humans" based on evidence suggesting these substances do not cause cancer in humans or in experimental animals.

IARC puts diesel exhaust in Group-2A, as "probably carcinogenic to humans."

The Environmental Protection Agency

The Environmental Protection Agency (EPA) uses a classification system very similar to that of the IARC. EPA considers diesel exhaust "likely to be carcinogenic to humans." The EPA also reports that the fine particles that diesel exhaust contains "are a serious risk to public health. They pose a significant health risk because they can pass through the nose and throat and become lodged in the lungs. Fine particles cause lung damage and premature death. They can also aggravate respiratory conditions such as asthma and bronchitis."

The National Institute for Occupational Safety and Health

Yet another expert agency, the National Institute for Occupational Safety and Health (NIOSH) "has determined that diesel exhaust is a potential human carcinogen, based on a combination of chemical, genotoxicity, and carcinogenicity data. In addition, acute exposures to diesel exhaust have been linked to health problems such as eye and nose irritation, headaches, nausea, and asthma." They go on to state that though the cancer-causing potential of diesel exhaust has not been measured, exposure should be minimized to reduce any possible cancer risk.

Does Diesel Exhaust Cause Any Other Health Problems?

Lung cancer is the main cancer to have been linked to diesel exhaust, but there is also suspicion that other cancers, especially those of the larynx, pancreas, bladder, and kidney, may be associated with diesel exhaust.

Diesel exhaust is a major part of outdoor air pollution. As mentioned above, diesel exhaust is believed to play a role in other health problems, such as eye irritation, headache, lung damage, asthma and other lung diseases, heart diseases, and possibly immune system problems.

What Should I Do If I Have Been Exposed to Diesel Exhaust?

If you are or have been heavily exposed to diesel fumes, your risk of lung cancer may be increased. If you have been exposed to diesel fumes in the general environment, the increase in your risk is likely to be very small. If you have had exposure to high levels over a long period of time, your risk is higher. However, using study results to predict individual problems is not easy, so measuring the exact amount of an exposed person’s risk is difficult.

At Work

If your workplace exposes you to diesel exhaust, there are many ways to reduce or prevent exposures. Some of these measures will also protect you from other chemical exposures that are likely to happen in the workplace. Engineering changes, such as ventilating the exhaust away from workers’ breathing zones, are important. Good work practices, such as changing clothes after work, washing hands regularly, and keeping food out of the work area, are also important. Finally, personal protective equipment, such as respirators, may be a key part of a workplace protective program. You should work with your employer to be sure that enough protection is in place.

For more information on preventing or reducing occupational exposures in your current job, consult your company’s safety and health manager. Additional technical assistance is available from the National Institute for Occupational Safety and Health (NIOSH).

Every Day

If you are exposed to diesel exhaust fumes in the general environment, you can take some of the same precautions. For example, you can avoid spending time near large sources of diesel exhaust, such as behind trucks and buses. Over the last 7 years the EPA has enforced the Clean Air Act, which included reducing diesel emissions from cars and trucks by mandated changes in both engines and fuel. This has decreased public exposure to diesel exhaust.

Cigarette smoking is a much more powerful risk factor for lung cancer than are diesel fumes. The two exposures combined, however, may increase risk to a greater extent than the sum of their individual risks. People with a history of diesel exhaust exposure should avoid any contact with tobacco smoke, either through smoking or through secondhand exposure.

What's the Bottom Line?

Prolonged exposure to diesel exhaust probably increases the risk of lung cancer and maybe other cancers, too. Parts of diesel exhaust, such as soot and PAHs, have been shown to cause cancer. As a result, several government and research organizations have stated that diesel exhaust probably causes cancer in humans. Another reason for limiting exposure is that diesel exhaust, like many other air pollutants, also causes lung, heart, and other health problems.

Short-term exposure to diesel exhaust is very unlikely to affect anyone's risk of developing lung cancer. However, US regulatory agencies, such as the EPA and OSHA, continue to look for ways to limit diesel exhaust exposures of the general public and of workers, because this exposure is common and the combined risk for the population overall may be considerable.