Air pollution is the presence of gases, particles, or biological materials in the atmosphere at concentrations that can harm health, ecosystems, materials, or climate. Clean, dry air near sea level is composed mostly of nitrogen and oxygen, but the atmosphere also contains small yet important amounts of argon, carbon dioxide, water vapor, and trace gases. Water vapor is highly variable, ranging from nearly zero in cold, dry air to several percent in humid conditions, and it influences cloud formation and chemical reactions. Natural processes such as sea spray, windblown dust, wildfires, volcanic activity, and plant emissions continuously add substances to air. Human activity adds pollutants at rates or in locations that can overwhelm natural removal processes. Whether a substance becomes a pollution problem depends on its concentration, chemical form, duration of exposure, and the sensitivity of people and ecosystems. The lower atmosphere, called the troposphere, is especially important because it contains the air people breathe and most weather systems that mix, transport, and remove pollutants.

Primary pollutants are emitted directly from a source in a harmful or potentially harmful form. Examples include carbon monoxide from incomplete combustion, sulfur dioxide from burning sulfur-containing fuels, nitrogen oxides from high-temperature engines, and soot particles from diesel exhaust or fires. Secondary pollutants are not emitted directly in their final form; instead, they are produced through atmospheric reactions involving precursor chemicals. Ground-level ozone is a major example: sunlight drives reactions between nitrogen oxides and volatile organic compounds, producing ozone and other oxidants. Fine particulate matter can also form secondarily when sulfur dioxide, nitrogen oxides, ammonia, and organic vapors react and condense into particles. This distinction matters because controlling a secondary pollutant requires reducing the appropriate precursor emissions, not merely measuring pollution at the place where it appears. Atmospheric chemistry is influenced by sunlight, temperature, humidity, and the availability of reactants, so secondary pollution can be strongest far downwind of cities or industrial sources.

Pollutant emissions are commonly organized by sector because each sector has characteristic fuels, processes, and control options. Transportation emits nitrogen oxides, carbon monoxide, volatile organic compounds, and particles from cars, trucks, ships, aircraft, and off-road equipment. Electric power generation and industrial facilities may emit sulfur dioxide, nitrogen oxides, metals, carbon dioxide, and particles, especially when burning coal, oil, biomass, or waste. Residential and commercial buildings contribute through space heating, cooking, solvents, and wood burning. Agriculture is an important source of ammonia from fertilizers and livestock waste; ammonia can react in air to form fine particles. Construction, mining, unpaved roads, and wind erosion generate dust, while waste disposal can release methane and odorous gases. Emissions may also be classified as point sources, such as a refinery stack; mobile sources, such as vehicles; and area sources, such as many small homes or businesses. This classification helps agencies develop inventories, identify major contributors, and choose practical emission-control strategies.

Air pollution operates on several spatial and temporal scales. At the microscale, concentrations can change within meters of a busy road, warehouse loading area, or indoor combustion source. Neighborhood-scale patterns reflect local traffic, building layout, industrial activity, and terrain. Urban and regional pollution can extend across metropolitan areas or multiple states when winds transport gases and particles downwind. Some pollutants, including greenhouse gases, are mixed sufficiently to influence the global atmosphere. Time also matters: emissions and atmospheric conditions vary by hour, season, and year. Traffic-related pollutants often peak during commuting periods, while ozone commonly reaches its highest levels on warm, sunny afternoons after photochemical reactions have occurred. Temperature inversions, in which warmer air overlies cooler surface air, suppress vertical mixing and can trap pollution near the ground. Rain and snow can remove soluble gases and particles, whereas dry deposition transfers pollutants to soil, water, vegetation, and surfaces. Understanding scale is necessary for placing monitors, interpreting exposure, and designing controls that match the area affected.

Major health-relevant air pollutants include fine particulate matter, ozone, nitrogen dioxide, sulfur dioxide, carbon monoxide, lead, and certain toxic organic compounds. Fine particulate matter with aerodynamic diameter of 2.5 micrometers or less, known as PM2.5, is especially concerning because particles can penetrate deep into the lungs and some components may enter the bloodstream. Exposure is associated with aggravated asthma, cardiovascular disease, reduced lung function, and premature death, particularly among older adults, children, pregnant people, and those with heart or lung disease. Ozone irritates airways and can make breathing difficult during exercise. Nitrogen dioxide is associated with traffic and combustion and can worsen respiratory symptoms. Carbon monoxide reduces blood oxygen delivery by binding strongly to hemoglobin, creating particular risk in enclosed or poorly ventilated spaces. Health risk depends on pollutant concentration, exposure duration, particle composition, and individual susceptibility. U.S. air-quality regulations use standards and monitoring to reduce population exposure, but personal exposure can differ from regional averages because people spend time near sources and indoors.

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