Health Effects of Engine Emissions

W. Addy Majewski

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• Introduction
• Particulate Emissions
• Hydrocarbons (HC)
• Carbon Monoxide (CO)
• Nitrogen Oxides (NOx)
• Sulfur Dioxide (SO₂)

Introduction

Engine Exhaust Pollutants

Motor vehicles are a significant source of air pollution and adverse health effects, particularly in large urban centers. Traffic-related air pollution (TRAP) is a complex mixture of gases and particles resulting from the use of engines and vehicles. Motor vehicles emit a variety of air pollutants including nitrogen oxides, elemental carbon (EC), particulate matter ≤2.5 μm in aerodynamic diameter (PM_2.5), ultrafine particles (UFP), heavy metals, polynuclear aromatic hydrocarbons, and volatile organic compounds (VOC). When emitted through vehicle exhaust, these pollutants are called exhaust or tailpipe emissions. When emitted by other means, such as evaporative emissions of fuel, the re-suspension of dust, the wear of brakes and tires, and the abrasion of road surfaces, they are called non-exhaust or non-tailpipe emissions.

The list of the main engine exhaust pollutants linked to adverse health effects includes:

• Particulate matter (PM)
• Hydrocarbons (HC)
• Carbon monoxide (CO)
• Nitrogen oxides (NOx)
• Sulfur dioxide (SO₂)

Among these compounds, PM, HC, CO and NOx (i.e., NO + NO₂) are regulated engine emissions. SO₂ emissions, while not directly regulated, are controlled by fuel quality standards that require increasingly lower sulfur content. In most jurisdictions, these pollutants are also subject to ambient air quality standards and occupational health exposure limits.

Engine and vehicle emissions undergo dilution and transformation in the ambient air. The rate at which vehicle emissions disperse depends on multiple factors that are highly variable, including wind speed, wind direction, atmospheric stability, and terrain and land use. In addition, air pollution from other sources—such as industry, oil, coal, and wood burning, and agricultural sources as well as atmospheric transport of pollutants from distant sources—contributes to the overall air quality. The results of these emissions are elevated concentrations of air pollutants through primary emissions and through the formation of secondary pollutants, such as secondary PM and ozone.

A meta-analysis by the Health Effects Institute (HEI) [5675] summarized research on emissions, exposure, and health effects from TRAP and drew conclusions about the associations between exposure and health outcomes. The study reviewed both toxicological and epidemiological evidence. The health outcomes associated with the most common air pollutants (NO₂, EC, and PM_2.5) with a confidence assessment rated ‘high’ or ‘moderate-to-high’ included: asthma onset in children and adults, acute lower respiratory infection in children, and mortality—from all-causes and circulatory, from lung cancer, and from ischemic heart disease.

Health Research Challenges

The task of quantifying the health effects of air pollution and correlating them with exposure to particular pollutants is challenging. TRAP emissions are complex and represented simplistically in models. As pollutants often have common sources, their ambient concentrations are highly correlated with each other [5741]. Therefore, it is difficult to ascertain which pollutant is chiefly responsible for the adverse effects of pollution, or whether they are attributable to the mixture as such.

There are several further challenges, including:

• Uncertainties in estimating exposures.
• Influence of other variables, for example cigarette smoking.
• Traffic noise effects. It has been suggested that transportation noise is associated with myocardial infarction (i.e., heart attack) mortality, independent from air pollution [5742]. Air pollution studies not adequately adjusting for traffic noise exposure may overestimate the cardiovascular disease burden of air pollution.
• Vehicle technology impacts. A key challenge is related to different emission levels, composition, and health effects from old and new technology vehicles [5743].
• Vehicle operational factors such as cold start and/or cold weather emissions, as well as high-emitting vehicles.
• Emission compliance issues such as tampering and emission cheating.
• Non-tailpipe emissions, which can be a major emission source in emission-controlled vehicles (such as diesels with particulate filters) and are the only emission source in zero-tailpipe-emission vehicles [5754].

As a result, conclusions of various health effects studies have not always been consistent. Notwithstanding the uncertainties and the lack of clear guidance on engine emission effects, the ultimate message from the health effects research is that in order to minimize adverse effects, all traffic-related pollutants must be controlled to low levels.

Traffic Emission Trends

In most high-income countries, tailpipe emissions from motor vehicles and ambient concentrations of most traffic-related pollutants have decreased steadily over the last several decades, Figure 1. This trend is a result of air quality regulations and improvements in engine and vehicle emission control technologies. However, under conditions of economic and population growth, decreases in emissions from individual motor vehicles do not fully compensate for the growth of the motor vehicle fleet or for the continued presence of older or malfunctioning vehicles on the roads [5675].

In contrast, traffic-related emissions and urban air quality in most middle- and low-income countries have shown modest decreases or increases during the past decades, due to more polluting vehicle fleets and weaker—and often poorly enforced—emission regulations. This trend underscores the fact that ambitious clean air policies and regulations do not come for free—clean vehicle technologies require considerable resources and are challenging to adopt in low-income countries.

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