Air pollution affects health when harmful gases, particles or biological and chemical contaminants in the air are inhaled or otherwise encountered at concentrations sufficient to damage biological systems. Important outdoor pollutants include particulate matter, nitrogen dioxide, ozone, sulphur dioxide and carbon monoxide, while indoor pollution can arise from tobacco smoke, solid-fuel combustion, cooking, building materials, mould and other sources.
Particulate matter is particularly important because very small particles can penetrate deeply into the respiratory tract, and some ultrafine particles can interact with biological tissues and potentially enter systemic circulation. Pollution exposure depends on concentration, duration, particle characteristics, source, weather, geography, indoor ventilation and individual activity. Health effects range from short-term irritation and exacerbation of asthma to long-term cardiovascular disease, chronic respiratory disease, adverse pregnancy outcomes and cancer. Vulnerability is not uniform: children, older adults and people with respiratory or cardiovascular disease can be more susceptible to certain pollutants.
Particulate matter is commonly classified according to aerodynamic diameter, with PM10 referring approximately to particles smaller than 10 micrometres and PM2.5 to particles smaller than 2.5 micrometres. Smaller particles can penetrate further into the lungs because they remain suspended in air for longer and can reach the bronchioles and alveolar regions. Deposited particles can activate macrophages and other immune cells, producing inflammatory signalling and oxidative stress. Some particles can carry metals, organic compounds and other reactive substances on their surfaces.
The biological effects therefore depend not only on particle mass but also on composition, size distribution and source. Chronic exposure to PM2.5 is strongly associated with increased cardiovascular and respiratory morbidity and mortality. Short-term increases in particulate pollution can also increase hospital admissions and exacerbate respiratory conditions. Indoor exposure can be substantial because people spend much of their time indoors, although infiltration from outdoor pollution and indoor sources vary considerably between buildings.
Air pollution affects the cardiovascular system through mechanisms extending beyond the lungs. Inhaled pollutants can trigger pulmonary inflammation and oxidative stress, with signalling molecules subsequently influencing systemic inflammation, vascular function and blood coagulation. Fine particles may also contribute to autonomic nervous-system changes and endothelial dysfunction. These pathways can promote atherosclerotic processes and increase susceptibility to acute cardiovascular events. Long-term exposure to particulate pollution is associated with increased risk of ischaemic heart disease, stroke and other cardiovascular outcomes.
People with established cardiovascular disease can be particularly vulnerable to pollution episodes. This is one reason why air pollution is not simply a respiratory issue. The cardiovascular effects also demonstrate why pollution exposure can matter even when an individual does not experience obvious coughing or breathlessness, because significant biological effects can occur without immediate symptoms.
Respiratory health is directly affected because the respiratory tract is the first major biological interface with inhaled pollutants. Ozone and nitrogen dioxide can irritate and inflame the airways, while particulate matter can impair airway and alveolar function. Air pollution can trigger asthma symptoms, increase the frequency of exacerbations and contribute to reduced lung development or function during childhood. Long-term exposure is also associated with chronic obstructive pulmonary disease and other chronic respiratory problems.
Ozone is a secondary pollutant formed through atmospheric chemical reactions involving nitrogen oxides and volatile organic compounds, particularly under sunlight. It can damage airway tissues through oxidative mechanisms. Nitrogen dioxide is strongly associated with combustion sources such as traffic and gas appliances, although concentrations and exposure patterns vary by location. Different pollutants frequently occur together, making it difficult to attribute every health outcome to one pollutant independently.
Cancer is another important long-term consequence of some forms of air pollution. Outdoor air pollution and particulate matter are recognised as carcinogenic to humans, with lung cancer being the clearest established association. Diesel exhaust contains particulate and gaseous components that can contribute to carcinogenic exposure, while indoor combustion of coal, wood or other fuels can produce substantial particulate and chemical exposures.
Radon, a naturally occurring radioactive gas that can accumulate indoors, is another important environmental lung-cancer risk and is distinct from conventional urban air pollution. The cancer risk from air pollution depends on pollutant type, concentration and cumulative exposure. Unlike smoking, which involves a highly concentrated direct source, ambient air pollution usually involves lower concentrations spread across large populations, meaning individual exposure can be less obvious while population-level effects remain substantial because millions of people are exposed.
Air pollution can affect development and pregnancy. Maternal exposure to certain pollutants has been associated with adverse pregnancy outcomes, including preterm birth and impaired fetal growth, although the magnitude of risk varies by pollutant and exposure context. Children can be particularly susceptible because their lungs and immune systems are still developing and because they breathe more air relative to body size during physical activity. Early-life exposure can therefore influence respiratory health over subsequent years.
Air pollution can also affect neurological and cognitive processes through inflammatory and vascular pathways, although specific causal relationships vary by pollutant and outcome and remain an active area of research. Vulnerability can be increased by poverty, occupational exposure, housing conditions and proximity to major roads or industrial sources, meaning air pollution is also an environmental and social determinant of health.
Reducing exposure can involve measures at individual, household, community and policy levels. Individuals can sometimes reduce exposure by avoiding heavy traffic during pollution peaks, improving indoor ventilation appropriately, using effective filtration where suitable and avoiding indoor combustion sources such as tobacco smoke. However, individual behaviour cannot eliminate pollution generated by traffic, industry, agriculture or regional atmospheric processes. Indoor air quality also requires balancing ventilation with outdoor pollution concentrations, because opening windows is not universally beneficial during high outdoor pollution episodes.
Well-maintained heating and cooking equipment, effective extraction and avoidance of indoor smoking can reduce important indoor sources. Respirators or high-efficiency masks can reduce inhalation of particulate matter in some circumstances, but their effectiveness depends on fit, filter characteristics and the pollutant involved, and they do not address all gaseous pollutants. Population-level emission controls generally have much broader effects than individual protective measures.
Air pollution is therefore a complex mixture of exposures with effects determined by pollutant properties, concentration, duration and individual susceptibility. The strongest established health consequences include respiratory irritation and disease, exacerbation of asthma and chronic respiratory conditions, cardiovascular disease, stroke and lung cancer, with additional evidence concerning pregnancy, development and neurological outcomes. The risks are cumulative and often invisible because polluted air does not necessarily have a noticeable smell or appearance.
Air-quality measurements and regulatory standards are therefore important because sensory detection is not a reliable indicator of safety. Improvements in air quality can produce measurable health benefits by reducing exposure across entire populations. The distinction between outdoor and indoor pollution is also important because exposure can originate from multiple sources, and reducing one source does not necessarily eliminate all risk.
