This article is general information, not medical advice. For health concerns, speak with a qualified health professional.
What Counts as Air Pollution
Air pollution is the presence in the air of substances at levels that can harm people, other living things or materials. Some pollutants come from natural sources, such as wildfires, dust storms and volcanoes, but a large share comes from human activity, especially burning fuels. Air quality varies from place to place and hour to hour, depending on what is being emitted and how the weather disperses it.
Scientists and regulators usually focus on a short list of widespread pollutants. These are often described as “criteria pollutants” in some national systems, though the exact list and limits differ among countries and organizations.
The Main Pollutants
Particulate matter. Tiny solid or liquid particles suspended in the air. They are commonly sorted by size: PM10 refers to particles with a diameter of 10 micrometers or less, and PM2.5 to those 2.5 micrometers or less, much finer than a human hair. Sources include vehicle exhaust, power plants, industry, wood and charcoal burning, construction, road dust and wildfires. Some particles are emitted directly, while others form in the air from gases. The smallest particles can travel deep into the lungs, which is why PM2.5 receives so much attention.
Ground-level ozone. Ozone high in the atmosphere shields us from ultraviolet radiation, but at ground level it is a pollutant. It is not usually emitted directly. Instead, it forms when sunlight drives reactions between nitrogen oxides and volatile organic compounds, so it tends to peak on sunny, warm afternoons and in summer.
Nitrogen dioxide. A reddish-brown gas produced when fuel burns at high temperatures, especially in vehicle engines and power plants. It is often highest near busy roads and contributes to the formation of ozone and fine particles.
Sulfur dioxide. Released mainly by burning coal and oil containing sulfur and by some industrial processes and volcanoes. It can form fine particles and is a main ingredient of acid rain. Controls on fuel and smokestacks have reduced it in many countries.
Carbon monoxide. A colorless, odorless gas produced by incomplete burning of fuel. Outdoors, levels have fallen in many places with cleaner engines. Indoors, faulty heaters, stoves and generators can cause dangerous buildup, which is why many homes have detectors.
Volatile organic compounds. A broad group of carbon-containing chemicals that evaporate easily. They come from solvents, paints, fuels, and some plants and trees. Some are harmful directly and many help form ozone.
Lead and other toxic metals. Leaded gasoline was phased out in most countries, which sharply reduced airborne lead. Industrial sources such as smelters can still release metals in some places.
Methane and carbon dioxide. These are greenhouse gases rather than pollutants that harm lungs at typical outdoor levels, but they are closely connected to the same sources, and they affect climate.
Where They Come From
Common sources include:
- Transport. Cars, trucks, buses, ships and aircraft burn fuel and release a mixture of pollutants. Brake and tire wear also release particles.
- Power and industry. Burning coal, oil and gas for electricity and heat, and processes such as smelting and cement production.
- Homes. Cooking and heating with wood, coal, dung or kerosene, especially on open fires or inefficient stoves, which is a major source of both indoor and outdoor pollution in many parts of the world.
- Agriculture. Ammonia from fertilizer and animal waste can react in the air to form fine particles. Burning crop residue releases smoke.
- Wildfires and open burning. Smoke can carry fine particles over long distances.
- Natural sources. Dust, sea salt, pollen and volcanic emissions.
How the Weather Matters
The same emissions can produce very different air quality depending on conditions. Wind carries pollutants away or brings them in from elsewhere. Rain washes particles out of the air. Sunlight fuels ozone formation. During a temperature inversion, a layer of warm air sits above cooler air near the ground and acts like a lid, trapping pollution below. Cities in valleys or basins are especially prone to this. The structure of the lower atmosphere explains why bad days often cluster in calm, stable weather. For readers curious about the broader structure of the sky, the atmospheric layers set the stage for all of this.
Pollution can also influence weather and climate. Particles scatter or absorb sunlight, can act as seeds for cloud droplets and can reduce visibility, producing haze. Some, like soot, absorb sunlight and warm the air, while others, like many sulfate particles, reflect sunlight and have a cooling effect. Acid rain, caused by sulfur and nitrogen compounds, can damage forests, lakes and buildings.
General Health Effects
Researchers have studied the health effects of air pollution extensively, and the broad conclusion is that exposure is associated with respiratory and cardiovascular problems. Short-term exposure to high pollution can aggravate asthma and other lung conditions and cause symptoms such as coughing and irritated eyes and throat. Long-term exposure is linked to increased risk of heart and lung disease and other health problems. Children, older adults, pregnant women and people with existing heart or lung conditions are generally considered more sensitive.
The size of any individual’s risk depends on levels of exposure, duration, age, health and other factors, and it is difficult to link a particular illness to air pollution in a single person. Population-level studies are what show the patterns. Health agencies generally publish guideline levels and advice, which are the best sources for specific guidance.
Measuring and Reporting
Many places publish an air quality index, a scale that converts measurements of key pollutants into a single number and color-coded category, along with suggested precautions. Because indexes differ between countries, the same number may mean different things in different places. Monitoring networks use ground sensors, and satellites and low-cost sensors are increasingly used to fill gaps. Some people check forecasts and adjust outdoor activity on days with poor air quality, as local health authorities advise.
What Has Worked
Air quality has improved markedly in many regions through targeted measures: cleaner fuels, filters on smokestacks, vehicle emission standards, phasing out leaded gasoline, and switching away from coal in cities. Cleaner cooking solutions can reduce pollution in homes. In some regions, however, pollution is still serious and rising with industrialization and growing cities. Many of the steps that clean the air, like reducing fossil fuel combustion, also reduce greenhouse gas emissions, which is one reason policymakers often consider them together.