Health Effects of Engine Emissions

W. Addy Majewski

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Abstract: The common air pollutants emitted with engine exhaust include particulate matter, carbon monoxide, sulfur dioxide, and nitrogen oxides. Epidemiological studies have shown that exposure to traffic-related air pollution causes increased morbidity and mortality, such as from cardiovascular and pulmonary disease. Long-term exposure to diesel exhaust is also associated with a small increase in the relative risk of lung cancer.


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 (PM2.5), 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:

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 PM2.5) 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:

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].

Figure 1. Global air pollution trends, 1990-2015

(Source: Health Effects Institute)

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.