Air pollution results from a complex mixture of thousands of pollutants. This mixture may include solid and liquid particles suspended in the air (particulate matter (PM)), and various gases such as ozone (O3), nitrogen oxides (NO2 or NOx), volatile organic compounds (VOCs), and carbon monoxide (CO). The mixture varies with geographical location and the sources of the emissions. Particles vary in number, size, shape, surface area and chemical composition, while both particles and gases may vary in solubility and toxicity. The most important processes causing air pollution relate to the combustion of fossil fuels used in cars and trucks, aeroplanes, vessels or other engines, as well as in industries, power plants or household heating systems. Due to the close proximity of people to emissions, transport-related activities, particularly the use of cars and trucks, are an important source of air pollutants.
Traditionally, studies of the health effects of air pollution have measured some marker of air pollution, e.g. size-specific PM fractions, such as particles with an aerodynamic diameter of <10 μm (PM10) or <2.5 μm (PM2.5), respectively, or NO2. Commonly used indirect markers of traffic-related pollutants are traffic density at the nearest road or residential distance from busy roads.