Air Quality and Health and Welfare
2.7 Toxic Materials
2.7.2 Diesel Health Assessment
Diesel emissions deserve a special discussion because they tend to be a dominant source of mobile source cancer risk. The US EPA determined a reference concentration in 1993 to minimize noncancer health effects resulting from exposure to diesel exhaust. EPA has summarized available information to characterize the cancer and noncancer health effects from exposure to diesel exhaust emissions in the draft “Health Assessment Document for Diesel Emissions” (1994) (the Assessment). The key components of the current draft Assessment are: (1) information about the chemical components of diesel exhaust and how they can influence toxicity, (2) the cancer and noncancer health effects of concern for humans, and (3) the possible impact or risk to an exposed human population.
Based on information provided in the draft Health Assessment Document for Diesel Emissions (EPA is revising this draft document in response to comments by the CASAC) and other sources of information, EPA has concluded that diesel particulate is a probable human carcinogen. The most compelling information to suggest a carcinogenic hazard is the consistent association that has been observed between increased lung cancer and diesel exhaust exposure in certain occupationally exposed workers working in the presence of diesel engines. Approximately 30 individual epidemiological studies show increased lung cancer risks of 20 to 89 percent within the study populations depending on the study. Analytical results of pooling the positive study results show that on average the lung cancer risks were increased by 33 to 47 percent. The magnitude of the pooled risk increases is not precise owing to uncertainties in the individual studies, the most important of which is a continuing concern about whether smoking effects have been accounted for adequately. While not all studies have demonstrated an increased risk (six of 34 epidemiological studies summarized by the Health Effects Institute reported relative risks less than 1.0), the fact that an increased risk has been consistently noted in the majority of epidemiological studies strongly supports the determination that exposure to diesel exhaust is likely to pose a carcinogenic hazard to humans (“Diesel Exhaust,” 1995, pp. 253-292).
Additional evidence for treating diesel exhaust as a carcinogen at ambient levels of exposure is provided by the observation of the presence of small quantities of many mutagenic and some carcinogenic compounds in the diesel exhaust. A carcinogenic response believed to be caused by such agents is assumed not to have a threshold unless there is direct evidence to the contrary. In addition, there is evidence that at least some of the organic compounds associated with diesel particulate matter are extracted by lung fluids (i.e., are bioavailable) and, therefore, are available in some quantity to the lungs as well as entering the bloodstream and being transported to other sites in the body.
The concern for the carcinogenic health hazard resulting from diesel exhaust exposures is widespread and several national and international agencies have designated diesel exhaust or diesel particulate matter as a ‘potential’ or ‘probable’ human carcinogen (“Carcinogenic Effects of Exposure to Diesel Exhaust,” 1988 and “Diesel Fuel and Exhaust Emissions,” 1996). The International Agency for Research on Cancer (IARC) in the late 1980s concluded that diesel exhaust is a ‘probable’ human carcinogen (“Diesel and Gasoline Exhausts and Some Nitroarenes,” 1989). Based on IARC findings, the State of California identified diesel exhaust in 1990 as a chemical known to the State to cause cancer and after an extensive review in 1998 has listed diesel exhaust as a toxic air contaminant (“Proposed Identification of Diesel Exhaust as a Toxic Air Contaminant,” 1998). The National Institutes for Occupational Safety and Health has classified diesel exhaust a “potential occupational carcinogen.” The World Health Organization recommends that “urgent efforts should be made to reduce [diesel engine] emissions, specifically of particulates, by changing exhaust train techniques, engine design and fuel composition.” The Department of Health and Human Services (DHHS) will decide in 2000 whether to list diesel particulate matter in its Report on Carcinogens (ROC) in terms of its lung cancer hazard.
Just recently, the National Institute For Environmental Health Sciences (NIEHS) added diesel particulate to its list of substances which are reasonably anticipated to be human carcinogens in its 9th National Toxicology Report on Carcinogens.
Another aspect of diesel particulate which is a cause for concern is its size. Approximately 80-95 percent of diesel particle mass is in the size range from 0.05-1.0 micron with a mean particle diameter of about 0.2 microns. These fine particles have a very large surface area per gram of mass, which make them excellent carriers for adsorbed inorganic and organic compounds that can effectively reach the lowest airways of the lung. Approximately 50-90 percent of the number of particles in diesel exhaust are in the ultrafine size range from 0.005-0.05 microns, averaging about 0.02 microns. While accounting for the majority of the number of particles, ultrafine diesel particulate matter accounts for 1-20 percent of the mass of diesel particulate matter.
Based on information provided in the draft Health Assessment Document for Diesel Emissions (EPA is revising this draft document in response to comments by the CASAC) and other sources of information, EPA has concluded that diesel particulate is a probable human carcinogen. The most compelling information to suggest a carcinogenic hazard is the consistent association that has been observed between increased lung cancer and diesel exhaust exposure in certain occupationally exposed workers working in the presence of diesel engines. Approximately 30 individual epidemiological studies show increased lung cancer risks of 20 to 89 percent within the study populations depending on the study. Analytical results of pooling the positive study results show that on average the lung cancer risks were increased by 33 to 47 percent. The magnitude of the pooled risk increases is not precise owing to uncertainties in the individual studies, the most important of which is a continuing concern about whether smoking effects have been accounted for adequately. While not all studies have demonstrated an increased risk (six of 34 epidemiological studies summarized by the Health Effects Institute reported relative risks less than 1.0), the fact that an increased risk has been consistently noted in the majority of epidemiological studies strongly supports the determination that exposure to diesel exhaust is likely to pose a carcinogenic hazard to humans (“Diesel Exhaust,” 1995, pp. 253-292).
Additional evidence for treating diesel exhaust as a carcinogen at ambient levels of exposure is provided by the observation of the presence of small quantities of many mutagenic and some carcinogenic compounds in the diesel exhaust. A carcinogenic response believed to be caused by such agents is assumed not to have a threshold unless there is direct evidence to the contrary. In addition, there is evidence that at least some of the organic compounds associated with diesel particulate matter are extracted by lung fluids (i.e., are bioavailable) and, therefore, are available in some quantity to the lungs as well as entering the bloodstream and being transported to other sites in the body.
The concern for the carcinogenic health hazard resulting from diesel exhaust exposures is widespread and several national and international agencies have designated diesel exhaust or diesel particulate matter as a ‘potential’ or ‘probable’ human carcinogen (“Carcinogenic Effects of Exposure to Diesel Exhaust,” 1988 and “Diesel Fuel and Exhaust Emissions,” 1996). The International Agency for Research on Cancer (IARC) in the late 1980s concluded that diesel exhaust is a ‘probable’ human carcinogen (“Diesel and Gasoline Exhausts and Some Nitroarenes,” 1989). Based on IARC findings, the State of California identified diesel exhaust in 1990 as a chemical known to the State to cause cancer and after an extensive review in 1998 has listed diesel exhaust as a toxic air contaminant (“Proposed Identification of Diesel Exhaust as a Toxic Air Contaminant,” 1998). The National Institutes for Occupational Safety and Health has classified diesel exhaust a “potential occupational carcinogen.” The World Health Organization recommends that “urgent efforts should be made to reduce [diesel engine] emissions, specifically of particulates, by changing exhaust train techniques, engine design and fuel composition.” The Department of Health and Human Services (DHHS) will decide in 2000 whether to list diesel particulate matter in its Report on Carcinogens (ROC) in terms of its lung cancer hazard.
Just recently, the National Institute For Environmental Health Sciences (NIEHS) added diesel particulate to its list of substances which are reasonably anticipated to be human carcinogens in its 9th National Toxicology Report on Carcinogens.
Another aspect of diesel particulate which is a cause for concern is its size. Approximately 80-95 percent of diesel particle mass is in the size range from 0.05-1.0 micron with a mean particle diameter of about 0.2 microns. These fine particles have a very large surface area per gram of mass, which make them excellent carriers for adsorbed inorganic and organic compounds that can effectively reach the lowest airways of the lung. Approximately 50-90 percent of the number of particles in diesel exhaust are in the ultrafine size range from 0.005-0.05 microns, averaging about 0.02 microns. While accounting for the majority of the number of particles, ultrafine diesel particulate matter accounts for 1-20 percent of the mass of diesel particulate matter.
