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The principal toxic gas compounds found in diesel exhaust include carbon monoxide (CO), nitric oxide (NO), nitrogen dioxide (NO2), and sulfur dioxide (SO2). Biological activity and toxic characteristics of these compounds have been studied for years and are relatively well understood.
In recent years, emission of diesel particulate matter (PM or DPM) has become one of the major health concerns among all diesel emissions. Medical research on health effects of PM is still in the initial phase of exploring this new area of human knowledge. There are many controversial opinions and many questions have not been answered, awaiting the results of ongoing and future studies. These uncertainties as to the effects of PM and its components are also reflected in the lack of a precise, universal definition of diesel particulates. While practically all of the public health/engine emission regulations define PM as a mix of solids, organics, and sulfates, such definitions as total carbon (i.e., excluding sulfates), or elemental carbon (i.e., excluding sulfates and organics) have been proposed and/or implemented by various occupational health regulations.
Diesel emissions contain numerous other compounds that are present in diesel emissions in smaller quantities, but still may be posing health threat to humans. The most important substances in this group include polynuclear aromatic hydrocarbons (PAH), nitro-PAHs, aldehydes, and selected other hydrocarbons and their derivatives. In their pure state, several of these species have been classified as human carcinogens (e.g., benzo[a]anthracene, benzo[a]pyrene, benzo[b]fluoranthene, benzo[k]fluoranthene, chrysene, dibenzo[a,h]anthracene, indeno[1,2,3-c,d]pyrene). Even though their concentrations in diesel exhaust are orders of magnitude lower in comparison to the principal diesel pollutants, they are still seen as a potential serious health hazard. In the USA, the Environmental Protection Agency (EPA) included “polycyclic organic matter” (POM) in the list of urban hazardous air pollutants (HAP) . The POM, defined as compounds with more than one benzene ring and a boiling point of 100°C and higher, includes practically all of the diesel PAH material.
Most of the heavy organic compounds, such as PAHs, are found in the particulate phase of diesel emissions. This association, in combination with their very low concentrations, makes it rather difficult to differentiate between the health effects of the solid DPM fraction and the particular organic species. A common approach is to study the effects of DPM as a whole. Diesel particulates, including both the solid and organic phase, have been identified as a toxic air contaminant in California .
Some health studies take an even more simplistic approach, investigating the effects of “whole diesel exhaust”, which includes both gaseous pollutants and particulates. Diesel particulates are frequently used in these studies as an indicator of the diesel exhaust exposure, but no effort is made to analyze which components of the exhaust gases are responsible for particular health effects. From the perspective of diesel emission control, this approach is not practical. Blaming the entire diesel exhaust for adverse health effects is not useful in setting emission control targets or selecting suitable control technologies. After all, the diesel exhaust gas is composed in over 99% of non-toxic materials, including nitrogen, oxygen, water vapor, and carbon dioxide. A consensus in newer publications is that the particulate phase in diesel exhaust, including solid inorganic carbon and the associated organic material, has the greatest effect on health .
Major diesel emission components that have adverse health or environmental effects are listed in Table 1 . A summary of the possible biological impacts and atmospheric reaction products (secondary pollutants) are also provided. This summary refers to biological activity of the chemical compounds in their pure state. The biological effects of particular compounds may or may not occur at the concentration levels that are found in diesel exhaust.
|Emission Component||Atmospheric Reaction Products||Biological Impact|
|Carbon monoxide||-||Highly toxic to humans; blocks oxygen uptake.|
|Nitrogen oxides||Nitric acid, ozone||Nitrogen dioxide is a respiratory tract irritant and major ozone precursor. Nitric acid contributes to acid rain.|
|Sulfur dioxide||Sulfuric acid||Respiratory tract irritation. Contributor to acid rain.|
|Carbon dioxide||-||Major contributor to global warming.|
|Saturated hydrocarbons (Alkanes, < C19)||Aldehydes, alkyl nitrates, ketones||Respiratory tract irritation. Reaction products are ozone precursors (in the presence of NOx).|
|Unsaturated hydrocarbons (Alkenes < C5)||Aldehydes, ketones||Respiratory tract irritation. Some alkenes are mutagenic and carcinogenic. Reaction products are ozone precursors (in the presence of NOx).|
|Formaldehyde||Carbon monoxide, hydroperoxyl radicals||Formaldehyde is a probable human carcinogen and an ozone precursor (in the presence of NOx).|
|Higher aldehydes (e.g., acrolein)||Peroxyacyl nitrates||Respiratory tract and eye irritation; causes plant damage.|
|Monocyclic aromatic compounds (e.g. benzene, toluene)||Hydroxylated and hydroxylated-nitro derivatives||Benzene is toxic and carcinogenic in humans. Some reaction products are mutagenic in bacteria (Ames assay).|
|PAHs (< 5 rings) (e.g. phenanthrene, fluoroanthene)||Nitro-PAHs (<5 rings)||Some of these PAHs and nitro-PAHs are known mutagens and carcinogens.|
|Nitro-PAHs (2 and 3 rings) (e.g. nitronaphtalenes)||Quinones and hydroxylated-nitro derivatives||Some reaction products are mutagenic in bacteria (Ames assay).|
|Elemental carbon||-||Nuclei adsorb organic compounds; size permits transport deep into the lungs (alveoli).|
|Inorganic sulfates||-||Respiratory tract irritation.|
|Aliphatic hydrocarbons (C14-C35)||Little information; possibly aldehydes, ketones, and alkyl nitrates||Unknown.|
|PAHs (4 rings and more) (e.g., pyrene, benzo(a)pyrene)||Nitro-PAHs (4 rings and more), nitro-PAH lactones||Larger PAHs are major contributors of carcinogens in combustion emissions. Many nitro-PAHs are potent mutagens and carcinogens.|
|Nitro-PAHs (3 rings and more) (e.g., nitropyrenes)||Hydroxylated-nitro derivatives||Many nitro-PAHs are potent mutagens and carcinogens. Some reaction products are mutagenic in bacteria (Ames assay).|