Health and Environmental Effects

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Abstract: Emissions from internal combustion engines include a number of biologically active substances. Among them, emissions of diesel particulates and nitrogen oxides became a major health concern. Many air pollutants, including ambient particulate matter, can cause a number of adverse health effects, including increased morbidity, increased hospital admissions, or increased risk of cardiovascular and respiratory illness. Engines fueled by fossil fuels also produce greenhouse gas emissions linked to climate change.

Introduction

Emissions from combustion engines can be a significant contributor to air pollution by a vast array of pollutants—including particulate matter, NOx, HCs, and a number of unregulated emissions. These emissions, together with their atmospheric reaction products such as ozone, have been a major public health concern. Combustion engines fueled by petroleum fuels are also a major source of GHG emissions—mainly CO₂ but also other gases such as CH₄ and N₂O—that have been linked to the ongoing climate change and warming of the planet.

From a public health perspective, the most important emissions that must be controlled to ensure healthy air, especially in urban settings, include particulate matter and nitrogen dioxide. Since both PM and NO₂ are always present together in traffic pollution, their relative significance has been a subject of debate. However, a number of analyses have found that particle pollution may present a higher health risk than NO₂ emissions [5673][5672]. The World Health Organization (WHO) has been ordinarily using the levels of PM_2.5 (ambient particulates of diameter below 2.5 μm) pollution as a ‘yardstick’ to compare air quality in cities across the world.

As diesel combustion can produce much higher quantities of particulate matter than gasoline or alternatively fueled engines, diesel emissions have become a focus in both public and occupational health research. High levels of diesel particulates can be produced by older generation diesel engines, without diesel particulate filters (DPF), as well as by newer technology engines with damaged or removed DPFs. More recently, direct injection gasoline engines became another important source of particulate emissions. Diesel particulate matter (PM or DPM)—defined by most engine emission regulations as a mixture composed of solids, organics, and sulfates—has often been regarded as a diesel emission of the greatest effect on health [265]. Diesel particulates have been identified as a toxic air contaminant in California [261].

Diesel emissions contain numerous compounds—in the particulate and/or the gas phase—that, even though present in smaller quantities, may still pose health risks. Examples include polynuclear aromatic hydrocarbons (PAH), nitro-PAHs, aldehydes, and other hydrocarbon derivatives. In the United States, the Environmental Protection Agency (EPA) included “polycyclic organic matter” (POM) in the list of urban hazardous air pollutants (HAP) [302]. The POM, defined as compounds with more than one benzene ring and a boiling point of 100°C and higher, includes practically all the diesel PAH material.

Some health studies investigate the effects of “whole diesel exhaust” that includes both gaseous pollutants and the particulate phase. Diesel particulates are frequently used in these studies as an indicator of the diesel exhaust exposure, but no effort is made to determine which emission components are responsible for particular health effects. Importantly, the International Agency for Research on Cancer (IARC) has classified diesel engine exhaust as carcinogenic to humans [3351]. 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.

The major diesel emission components, their possible biological impacts and atmospheric reaction products (secondary pollutants) are listed in Table 1 [260]. The compounds shown in the table also occur, at different concentration levels, in emissions from gasoline and alternatively fueled engines. 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 engine exhaust.

**Table 1**
Biological impact of diesel emission components
Emission Component	Atmospheric Reaction Products	Biological Impact
Gas Phase
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, fluoranthene)	Nitro-PAHs (<5 rings)	Some of these PAHs and nitro-PAHs are known mutagens and carcinogens.
Nitro-PAHs (2 and 3 rings) (e.g. nitronaphthalenes)	Quinones and hydroxylated-nitro derivatives	Some reaction products are mutagenic in bacteria (Ames assay).
Particulate Phase
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).

From the climate change perspective, diesel engine had traditionally produced lower GHG emissions than other types of engines. Compared to diesels, gasoline engines had a lower fuel economy, while natural gas engines could produce emissions of methane—a greenhouse gas with a higher global warming potential than CO₂. Due to this CO₂ superiority, EU climate change policies of the early 2000s favored and supported diesel engines as a means of CO₂ emission reduction. These policies were abandoned in the wake of the 2015 Volkswagen diesel emissions scandal, which damaged the public’s perception of diesel vehicles. In the following years, ‘net-zero’ emission targets have been adopted with the objective to eliminate or at least significantly reduce the use of fossil energy. Under these policies, future diesel engines would have to be fueled with low-carbon biofuels or synthetic fuels.