5.1.1. The agent and its components

Air pollutants are ubiquitous – from anthropogenic activities and natural processes – and global, and cross international boundaries. Levels of individual contaminants can vary dramatically between locations, due to the contributing sources and atmospheric processing, which mixes and transforms pollutants and transports them across great distances. Several important species are formed in the atmosphere and are not directly emitted. Air pollution is a mixture of mixtures, which can be viewed from source-oriented or component-oriented perspectives. Any outdoor air pollution mixture includes gases and suspended particles that are constantly interacting. Gaseous pollutants include photochemical oxidants, numerous organic compounds, carbon monoxide (CO), volatile metals, and nitrogenous and sulfurous species. Suspended particles (a heterogeneous mixture of liquids and solids referred to as particulate matter [PM]) are also a very complex mixture, with variable and dynamic chemical composition and physical characteristics. Many of the individual species and mixtures present in air have been classified by IARC as Group 1 (e.g. benzene, benzo[a]pyrene, chromium (VI), and dioxin) and Group 2A carcinogens.

5.1.2. Sources

Although there are many sources of outdoor air pollution, the largest contributors of air pollutants in many locations are: mobile sources; stationary power generation; other industrial and agricultural emissions; residential heating and cooking; re-emission from terrestrial and aquatic surfaces; the manufacturing, distribution, and use of chemicals; and natural sources. The distribution of these sources, the implementation of emissions control technologies, and the resulting emissions vary considerably between and within regions. Many of these sources have diurnal, weekly, and seasonal patterns in emissions. Some important regional trends in source contributions to air pollution include: (1) substantial contributions from residential burning of solid fuels in developing countries in Asia, Africa, and parts of South America; (2) substantial contributions from coal-fired power plants in China; (3) substantial contributions from stationary sources in heavily industrialized cities where advanced emissions controls are not present; (4) large episodic dust storms in Asia, Africa, and the Middle East; and (5) mobile sources, which contribute to varying degrees in urban areas.

5.1.3. Measurement methods

A wide variety of measurement methods are applied to measure concentrations of air pollutants. Therefore, comparisons of measurements across space or time need to consider these differences. Most methods for regulated gaseous pollutants, such as CO, nitrogen dioxide (NO₂), sulfur dioxide (SO₂), and ozone, use in situ continuous monitors for hourly averaged (or shorter-duration) concentrations, whereas PM is most often measured using integrated sampling systems on filter substrates for air passed through size-selective inlets to determine mass concentration and major components such as multi-elements, ions, and carbon. Passive sampling is increasingly used to assess spatial variation, particularly for gases. With specified standard operating procedures and quality assurance/quality control, within- and between-network consistency may be achieved.

5.1.4. Environmental occurrence

In some countries outdoor air quality is monitored routinely in networks. Measurement methods and site selection differ between networks, partially limiting comparisons between countries. Satellite-based approaches provide global estimates, filling the gaps for a limited number of pollutants (e.g. PM with particles of aerodynamic diameter < 2.5 μm [PM_2.5], NO₂, SO₂, and formaldehyde). Network and satellite data have shown large variability across countries of concentrations of PM with particles of aerodynamic diameter < 10 μm (PM₁₀), PM_2.5, NO₂, and other pollutants. Annual average PM_2.5 concentrations range from below 10 µg/m³ to well above 100 µg/m³. In many areas, World Health Organization (WHO) air quality guidelines for PM_2.5 are substantially exceeded. High PM_2.5 concentrations are observed in South and East Asia and North Africa. High NO₂ concentrations are observed in areas with high population density and traffic density. Within countries, high concentrations are observed in urban areas and around traffic sites and industrial locations. Significant spatial variability is present, related to urban–rural differences and proximity to sources or source areas.

Trends differ by pollutant and region of the world. In North America and Europe, concentrations of major pollutants such as PM, SO₂, and NO₂ have decreased substantially in the past 30 years. In many developing countries, concentrations have increased with rapid economic development.

5.1.5. Exposure assessment in epidemiological studies

Epidemiological studies of relationships between air pollution exposure and cancer require long periods of observation and large populations. Therefore, it is almost impossible with currently available approaches to assess exposure via personal monitoring. Rather, epidemiological studies use measured or modelled concentrations of outdoor air pollution as the primary basis for exposure estimation. Air quality monitoring is usually limited to measurements of a relatively small number of indicator pollutants collected at discrete locations. Epidemiological studies have typically used centrally located outdoor monitors or geostatistical averaging of multiple measurements within a single study area in analyses of between-area variation in exposure, and various modelling techniques (e.g. atmospheric transport and land-use regression models), sometimes in combination with detailed spatial and temporal measurements, to assign individual estimates of exposure. Evaluation of these models indicates that they can accurately estimate outdoor concentrations at residential locations. More recently, satellite-based estimates, sometimes in combination with land-use information, have been used to produce relatively high resolution and compatible estimates of concentrations at national (and even global) scales, including rural areas, where in situ measurements are generally not available. Since it is important to estimate exposures over long time periods, assessments can be improved by using both estimates of air pollution concentrations for extended time periods and residential histories for the study population of interest.

5.1.6. Personal exposure

Personal exposure is typically not used to assess exposure in epidemiological studies of air pollution and cancer but can be used to assess the agreement with outdoor exposure estimates. Human biomarkers of exposure provide information about individual exposures that may be used in evaluation studies.

There is strong evidence that temporal variation of outdoor concentration is correlated with temporal variation of personal exposure to fine particles. The few studies that have evaluated the agreement between average outdoor concentration estimated at a fine scale and personal exposure have generally found a moderate agreement, which differed between pollutants and studies. A few studies comparing personal exposure of subjects in cities with different outdoor air pollution concentrations have shown a strong correlation between average personal exposure and city-average outdoor concentration.

Personal exposure to air pollution is determined by the pollutant concentration in the microenvironment and by the time–activity patterns and location of individuals, including outdoors, indoors, and in transit. Personal exposures differ from those estimated based solely on outdoor concentrations because of time–activity patterns, variable infiltration of outdoor air pollution into indoor environments, and indoor sources. Thus, for studies of health effects of outdoor air pollution, the contributions from indoor and outdoor sources to total personal exposure should be considered separately.

People generally spend a large fraction of their time indoors (typically about 80–90%). A substantial fraction of the time spent indoors is spent in the home, supporting the assessment of exposure based upon the residential address. Because of the high fraction of time spent indoors, infiltration of outdoor pollution indoors is an important factor that modifies exposure. Infiltration varies substantially between pollutants and homes/buildings and by season. Relatively high infiltration factors (≥ 0.5) have generally been found for fine particles (particularly sulfate and elemental carbon) and CO. Lower infiltration factors have been found for ultrafine and especially coarse particles, NO₂, and ozone. For pollutants with lower infiltration factors, the potential for misclassification of exposure based on outdoor concentrations is higher. Infiltration factors are affected strongly by air exchange rates, which differ between homes and by season. Despite the typically small fraction of their time (< 10%) that people spend in traffic, the contribution to average personal exposure may be substantial because of the high concentrations found in traffic areas. Outdoor workers such as street vendors, traffic police, and toll booth operators experience long exposure durations and elevated exposure levels.

5.1.7. Guidelines and regulations

Given the chemical complexity of outdoor air pollution and the large number of anthropogenic sources, air pollution is managed with a combination of air quality standards for selected pollutants, regulation of sources, emissions permitting, and indirect control of sources, such as land-use regulation. Although the regulatory framework for outdoor air pollution control differs considerably across countries and across local government agencies within countries, most regions of the world have air quality standards for key air pollutants (ozone, SO₂, NO₂, CO, and PM, although there is limited specification on the components of PM). The control of these pollutants has beneficial consequences for the control of other pollutants. Useful air quality regulations include an indicator, a specified averaging time, and a statistical form, which is effectively the number of exceedances that are allowed. In some locations where air quality standards have not been developed, WHO guidelines are used as a reference for air quality management. In many locations around the world, compliance with air quality standards and WHO guidelines is not achieved.