The evidence that has accumulated over the last half century linking acute and chronic exposures of these major air pollutants to morbidity and mortality is significant, and ongoing studies continue to strengthen such associations [9]. Particulate matter, for example, has been estimated to contribute to 800,000 premature deaths in the world annually [5]. Early studies of mortality from long-term outdoor exposures such as the “Harvard Six Cities” study and related follow-up studies observed that inhabitants of the most polluted cities had increased rates of all-cause and cardiopulmonary mortality compared to less polluted cities [5]. Subsequent studies that focused specifically on respiratory outcomes demonstrated associations between incremental increases in ambient PM and increased respiratory symptoms, lower lung function, and increased healthcare encounters for pulmonary disorders such as asthma and COPD, as well as more frequent lung infections in children and adults. The mechanisms behind such relationships have been proposed to involve the ability of particulates to induce inflammatory responses in human airway epithelial cells, which in turn cause oxidative stress and further airway damage [5].

Similarly, pollutants such as ozone are cytotoxic to cells in the airway and are also associated with local inflammatory responses and injury that may underlie the observed decrements in lung function and increase in bronchial hyper-reactivity. Some studies support the relationship of high-ozone days with increased hospital admissions for respiratory illness such as asthma [4], though it is difficult to separate the effects of ozone from the combined effects of other commonly occurring co-pollutants, thereby warranting further single-pollutant studies.

Ongoing efforts to fully characterize the health effects of air pollution have yielded consistent findings that link airborne exposures to respiratory morbidity but in ways not previously well understood. A recent multi-cohort study in Europe found ambient air pollution, particularly PM, to be linked to lung cancer [10], and new evidence on the relationship of increases in indoor NO₂ to more severe asthma and COPD symptoms has led to work aimed at developing interventions to reduce indoor exposures [6, 7, 11]. This and future work will help to elucidate the true breadth of air pollution’s impact on human health .

As described above, air pollution can originate from both natural and anthropogenic sources , and patterns of dispersion have greatly been affected by global climate change and the spread of industrialization over the last century. Regardless of the source, human exposure to air pollution is unfortunately ubiquitous, and the type and extent of these exposures depend not only on where they are being generated but also on lifestyle factors that shape where individuals spend their time. Meteorological factors also play a major role, since ambient temperatures and weather conditions motivate varying degrees of insulation and ventilation in constructed spaces, which in turn affects the communication of outdoor and indoor environments.

In the USA, Americans spend over 90 % of their time inside [12], and therefore indoor sources of pollution can have a significant impact on health. Moreover, despite the impact of the Clean Air Ac t on ambient pollution, indoor levels of air pollution are not centrally monitored or regulated and therefore may pose unrecognized hazards for those who spend extended periods of time indoors. Indoor environments where people spend most of their time are commonly divided into three realms: the household, the school, and the workplace.

The level of exposure in each of these places relies on such characteristics as the type and frequency of activities being performed, the degree of enclosure and ventilation patterns of the building, and any efforts to filter or shield from the inhalation of airborne gases and particles. For example, common household activities have been known to generate high indoor levels of particulate matter. In one study of urban homes in Baltimore, three major sources of particulate matter in the homes were identified: smoking, sweeping, and stove use, with increasing frequency of these behaviors significantly associated with increasing levels of PM₁₀ measured in the indoor air [13]. Cleaning activities contribute to indoor pollution, not only from the emissions from toxic cleaning agents used themselves, but also due to the re-suspension of particles in the settled dust. Similarly, NO₂ concentrations within the home are correlated to both the presence of a gas stove and gas heater, as well as use of a space heater or stove/oven for heat [6], demonstrating the effects of cooking activities upon the generation of indoor NO₂. Besides cleaning and cooking, other activities such as the burning of candles or incense, use of fireplaces/wood stoves and certain appliances, and smoking are common indoor sources of air pollution in the developed world.

In contrast, the burden of household air pollution in the developing regions of the world is largely the result of indoor burning of biomass fuels . Almost half of the world, mostly in the poorest of settings, depends on the burning of cheaper, but energy-inefficient, organic materials such as animal dung, charcoal, and crop waste, as well as kerosene and mixed fuels, in open fires in order to cook and heat homes. The indoor levels of smoke and particulate matter generated from this burning are orders of magnitude above those seen in developed regions and have been linked to significant mortality and morbidity from acute respiratory infections/pneumonia, COPD, and airway cancers [14].

Many of the same indoor activities common to personal residences pertain to school environments. Cleaning and heating activities within school buildings may contribute more to indoor levels of pollution. One specific source of indoor pollution relevant to schools that has gained recent interest is school bus exhaust-related pollutant exposure. Due to the common phenomenon of bus “idling” while waiting to pick up and drop off children at schools, as well as the general transport of children within buses, exposure to diesel fumes inside and around buses has been found to be a measurable source of particulate matter, elemental carbon, carbon monoxide, and other gaseous and particulate pollutants [15, 16]. This exposure has led to considerable concern regarding the effects of noxious diesel fumes on young children, and further research on this is underway.

A third portion of indoor time for a large number of the world’s adult population is spent at work. Occupational hazards remain a concern as the cause for a variety of acute and chronic respiratory conditions, including exposures that span from airborne industrial toxins to aerosolized agricultural pollutants to the common indoor exposures from routine human activity discussed above. Given its large scope, work-related exposures and their health impact is specifically discussed in another chapter of this book.

In contrast to the variety of indoor pollutant exposures thus far described, outdoor pollution often affects large populations at once and is not readily amenable to lifestyle modifications by the exposed individual. Pollution sources can be described as “stationary” sources, such as factories with smoke stacks, and “mobile” sources such as motor vehicle tail pipes [4]. Despite the lessons learned during the 1952 London Great Smog and other similar environmental disasters, industrial emissions continue to be a major source of ambient pollution, particularly in urban areas. Combustion of fuels by power plants, factories, and waste incinerators that directly release organic, acidic, and metallic compounds into the air, continue to dominate as sources of ambient pollution. Historically, in an effort to improve local air quality, industrial smoke stacks have been redesigned to be taller, removing their emissions from the mixing layers nearer to the ground and from the breathable air of the people who live around them [4]. However, the emissions from these tall stacks, which are often at higher temperature and linger longer in the suspended air, become more widely dispersed and are capable of blanketing a larger geographical area [17]. In contrast, domestic chimneys and other low-lying emissions disperse less and accordingly, contribute greater to local airborne pollution [17]. Vehicular traffic is another example of low-lying emissions that can rapidly elevate concentrations of particulate and gaseous pollution in the direct breathing space of an individual. Not only do vehicles contribute through by-products of combustion released from tail pipes, but operation of these vehicles over often long pathways of transport generate particulate emissions into the air that are released from tire wear and road abrasion [17].

Lastly, natural sources of outdoor pollution such as soil, dust, volcanic eruptions, and organic material such as pollen and fungi, can contribute significantly to ambient levels of particulate matter and gases and can cause meaningful health effects. For example, volcanic activity is known to release a unique diversity of particles and volatiles such as SO₂, CO, CO₂, HCL, HF, H₂S, and radon in ash [18]. In the case of the Eyjafjallajökull eruption in Iceland, up to 25 % of the ash was found to be less than 10 μm, well within the respirable range. Health effects of such eruptions can extend for great distances, as in the case of Icelandic events that led to fallouts across the European continent, and can manifest with acute respiratory effects including symptoms of airway irritation and exacerbations in people with asthma and chronic bronchitis [18].

Disparities in the exposure to, and health effects from, air pollution are a uniquely relevant dilemma due to the intimate association between the environment and respiratory system. Thus, the inequalities in the environment that characteristically divide groups often coexist with differences in air quality, which in turn lead to disparities in respiratory health. Experts describe three mechanisms that may translate variations in the air quality of microenvironments into further differences in the health of their inhabitants [19]. The first is “differential exposure” where certain groups are found to be more exposed to air pollution sources, thereby violating the principle of environmental justice, which upholds that no one population should bear the brunt of toxic exposure over another [20]. The second mechanism is that of “differential susceptibility,” such that a given exposure to air pollution may have different effects on an individual due to specific personal and social vulnerabilities of people in a group. This is especially relevant to those individuals with pre-existing lung disease such as asthma and COPD. Lastly, the “social coping” mechanism includes factors that relate to socioeconomic status that may modify the ability of an individual to manage risk imposed by the threats faced, such as adequate access to healthcare [21]. These mechanisms are useful to consider when assessing the true burden of poor air quality for specific groups (Table 3.2).

In the following sections, we will discuss the evidence for disparities in air pollution across various dividing lines—socioeconomic status, race, gender and age, and geography. While many of these divisions overlap, they highlight the ways in which various groups of people around the world disproportionately bear the burden of global air pollution.