Air Quality and Health and Welfare
2.7 Toxic Materials
2.7.3 Diesel Exhaust or Diesel Particulate Matter
Diesel exhaust include gaseous and particulate components. Gaseous components of diesel exhaust include organic compounds, nitrogen-containing compounds, sulfur compounds, carbon monoxide, carbon dioxide, water vapor, and excess air (nitrogen and oxygen). Among these gaseous organic compounds are benzene (a known human carcinogen), formaldehyde, acetaldehyde, and 1,3-butadiene (possible or probable human carcinogens). Particulate components include many organic compounds that are mutagenic as well as several trace metals (including chromium, manganese, mercury and nickel) that may have general toxicological significance (depending on the specific species). In addition, small amounts of dioxins have been measured in diesel exhaust, some of which may partition to the particle phase.
Because diesel exhaust is a mixture of particles and gases, the choice of a measure of exposure (i.e., dosimeter) is important. EPA believes that exposure to whole diesel exhaust is best described, as many researchers have done over the years, by diesel particulate concentrations expressed in units of mass concentration (e.g., ?g/m3). The choice of this dosimeter implies that the contribution of the gaseous components and diesel particulate constituents to toxicity are related by diesel particulate mass. This assumption is consistent with historic practice, but can only be validated when there is a better understanding of the toxicological mode of action for diesel exhaust.
While some of the cancer and noncancer hazard may be associated with exposure to the gaseous component of diesel exhaust, studies suggest that the particulate component plays a substantial role in carcinogenicity and noncancer effects. Investigations show that diesel particles (the elemental carbon core plus the adsorbed organics) induce lung cancer at high doses and that the particles, independent of the gaseous compounds, elicit an animal lung cancer response. The presence of non-diesel elemental carbon particles, as well as the organic-laden diesel particles, correlate with an adverse inflammatory effect in the respiratory system of animals. Additional evidence suggesting the importance of the role of particulate matter in diesel exhaust includes the observation that the extractable particle organics collectively produce cancer and adverse mutagenic toxicity in laboratory experiments.
Given the available information, EPA has proposed to list diesel exhaust as a mobile source air toxic pollutant.
Diesel particulate matter is mainly attributable to the incomplete combustion of fuel hydrocarbons as well as engine oil and other fuel components such as sulfur. Diesel exhaust particulates are part of ambient PM2.5, since diesel engines are used to power numerous types of equipment in many places. Some geographic areas may have higher diesel particulate loading because of the number of engines that exhaust into the ambient air. While diesel particulate matter contributes to ambient levels of PM2.5, the high content of elemental carbon with the adsorbed organic compounds and the high number of ultrafine particles (organic carbon and sulfate) in diesel exhaust distinguish it from other noncombustion sources of PM2.5. In addition, diesel particulate matter from mobile source diesel engines is emitted into the breathing zone of humans and thus has a greater potential for human exposure (per kg of emissions) compared to other combustion particulates emitted out of stacks.
EPA believes that exposure to whole diesel exhaust is best described, as many researchers have done over the years, by diesel exhaust concentrations expressed in units of mass concentration, i.e., micrograms/m3. This does not directly account for the gaseous component of diesel exhaust. Another important aspect is to recognize that diesel exhaust particulate matter is part of ambient PM2.5. A qualitative comparison of adverse effects of exposure to PM2.5 and diesel exhaust particulates shows that the respiratory system is adversely affected in both cases, though PM2.5 has a wider spectrum of adverse effects for humans. Considerably more PM2.5 research also exists than is the case for diesel exhaust. A carcinogenicity hazard for PM2.5 has not yet been clearly shown, however.
Overall, information suggests that the diesel particle may be playing a key role(s) in contributing to the chronic noncancer and carcinogenicity hazards associated with exposure to diesel exhaust: both as a mechanism of delivery for many of the organics and trace metals into the respiratory system, and as a physical irritant in and of itself. Given the available information, it is a reasonable and prudent step to protect public health by proposing regulations on the particulate phase of diesel exhaust.
Because diesel exhaust is a mixture of particles and gases, the choice of a measure of exposure (i.e., dosimeter) is important. EPA believes that exposure to whole diesel exhaust is best described, as many researchers have done over the years, by diesel particulate concentrations expressed in units of mass concentration (e.g., ?g/m3). The choice of this dosimeter implies that the contribution of the gaseous components and diesel particulate constituents to toxicity are related by diesel particulate mass. This assumption is consistent with historic practice, but can only be validated when there is a better understanding of the toxicological mode of action for diesel exhaust.
While some of the cancer and noncancer hazard may be associated with exposure to the gaseous component of diesel exhaust, studies suggest that the particulate component plays a substantial role in carcinogenicity and noncancer effects. Investigations show that diesel particles (the elemental carbon core plus the adsorbed organics) induce lung cancer at high doses and that the particles, independent of the gaseous compounds, elicit an animal lung cancer response. The presence of non-diesel elemental carbon particles, as well as the organic-laden diesel particles, correlate with an adverse inflammatory effect in the respiratory system of animals. Additional evidence suggesting the importance of the role of particulate matter in diesel exhaust includes the observation that the extractable particle organics collectively produce cancer and adverse mutagenic toxicity in laboratory experiments.
Given the available information, EPA has proposed to list diesel exhaust as a mobile source air toxic pollutant.
Diesel particulate matter is mainly attributable to the incomplete combustion of fuel hydrocarbons as well as engine oil and other fuel components such as sulfur. Diesel exhaust particulates are part of ambient PM2.5, since diesel engines are used to power numerous types of equipment in many places. Some geographic areas may have higher diesel particulate loading because of the number of engines that exhaust into the ambient air. While diesel particulate matter contributes to ambient levels of PM2.5, the high content of elemental carbon with the adsorbed organic compounds and the high number of ultrafine particles (organic carbon and sulfate) in diesel exhaust distinguish it from other noncombustion sources of PM2.5. In addition, diesel particulate matter from mobile source diesel engines is emitted into the breathing zone of humans and thus has a greater potential for human exposure (per kg of emissions) compared to other combustion particulates emitted out of stacks.
EPA believes that exposure to whole diesel exhaust is best described, as many researchers have done over the years, by diesel exhaust concentrations expressed in units of mass concentration, i.e., micrograms/m3. This does not directly account for the gaseous component of diesel exhaust. Another important aspect is to recognize that diesel exhaust particulate matter is part of ambient PM2.5. A qualitative comparison of adverse effects of exposure to PM2.5 and diesel exhaust particulates shows that the respiratory system is adversely affected in both cases, though PM2.5 has a wider spectrum of adverse effects for humans. Considerably more PM2.5 research also exists than is the case for diesel exhaust. A carcinogenicity hazard for PM2.5 has not yet been clearly shown, however.
Overall, information suggests that the diesel particle may be playing a key role(s) in contributing to the chronic noncancer and carcinogenicity hazards associated with exposure to diesel exhaust: both as a mechanism of delivery for many of the organics and trace metals into the respiratory system, and as a physical irritant in and of itself. Given the available information, it is a reasonable and prudent step to protect public health by proposing regulations on the particulate phase of diesel exhaust.
