The most common air pollutants are six substances known as “criteria air pollutants,” regulated under the Clean Air Act: particulate matter, ground-level ozone, carbon monoxide, nitrogen dioxide, sulfur dioxide, and lead. Together with volatile organic compounds and indoor pollutants like formaldehyde and radon, these substances accounted for 8.1 million deaths globally in 2021, making air pollution the second leading risk factor for death worldwide.
Particulate Matter
Particulate matter is a catch-all term for tiny particles and liquid droplets suspended in the air. It comes in two sizes that matter for health: PM10 (particles under 10 micrometers) and PM2.5 (particles under 2.5 micrometers, roughly 30 times smaller than a human hair). Sources include vehicle exhaust, construction sites, wildfires, power plants, and industrial facilities.
The size distinction matters because it determines where particles end up in your lungs. PM10 tends to deposit in the upper airways, worsening conditions like asthma and COPD. PM2.5 travels deeper, settling on the surfaces of the smallest lung tissue, where it triggers inflammation and tissue damage. Short-term exposure to PM2.5 over even 24 hours has been linked to asthma attacks, bronchitis flare-ups, and emergency room visits. Long-term exposure over months or years can reduce lung function growth in children.
The World Health Organization’s 2021 guidelines recommend annual PM2.5 levels stay below 5 micrograms per cubic meter. Many cities around the world routinely exceed this by several times over.
Ground-Level Ozone
Ozone in the upper atmosphere protects us from ultraviolet radiation. At ground level, it’s a lung irritant and the primary ingredient in smog. Unlike most pollutants, ground-level ozone isn’t released directly from a smokestack or tailpipe. It forms when nitrogen oxides and volatile organic compounds from cars, power plants, refineries, and chemical plants react with each other in sunlight.
This is why ozone levels peak on hot, sunny days in urban areas, though it can reach unhealthy concentrations in colder months too. The WHO recommends keeping 8-hour ozone exposure below 100 micrograms per cubic meter. Because ozone requires sunlight and heat to form, it’s one pollutant that climate change is expected to make worse in many regions.
Nitrogen Dioxide
Nitrogen dioxide (NO2) is a reddish-brown gas released by any combustion source: car engines, trucks, buses, coal plants, oil and gas facilities. Anywhere fossil fuels burn, NO2 enters the air. It irritates the airways, worsens asthma, and increases susceptibility to respiratory infections. It also plays a direct role in forming both ground-level ozone and particulate matter, amplifying the effects of other pollutants.
The WHO’s recommended annual limit for NO2 is 10 micrograms per cubic meter, with a 24-hour limit of 25. Satellite monitoring from NASA shows that NO2 concentrations closely track traffic density and industrial activity, making it one of the most reliable indicators of local air quality.
Sulfur Dioxide
Sulfur dioxide (SO2) comes primarily from burning coal to generate electricity. It’s the main driver of acid rain, which damages forests, lakes, and buildings. In the air, SO2 irritates the nose, throat, and airways, and it reacts with other compounds to form fine particulate matter. People with asthma are especially sensitive to even brief exposures.
SO2 levels have dropped significantly in countries that have shifted away from coal power, but it remains a serious problem in regions that still rely heavily on coal for electricity and industrial processes.
Carbon Monoxide
Carbon monoxide (CO) is colorless and odorless, which makes it especially dangerous. It binds to the same spot on red blood cells where oxygen normally attaches, effectively starving your organs of oxygen. The most common symptoms of CO exposure, including headache, dizziness, weakness, nausea, and confusion, are often mistaken for the flu. At high concentrations, CO can cause unconsciousness and death. People who are sleeping or intoxicated can die from CO poisoning before ever noticing symptoms.
Outdoors, CO comes mainly from vehicle exhaust. Indoors, it’s produced by fuel-burning appliances like gas stoves, furnaces, and water heaters. Infants, older adults, and people with chronic heart disease or anemia are at higher risk from exposure.
Lead
Lead in the air has declined dramatically since leaded gasoline was phased out, but it hasn’t disappeared. Metal processing facilities, waste incinerators, and small aircraft that still use leaded aviation fuel continue to release lead particles. Once inhaled or ingested, lead accumulates in the body, particularly in bones. In children, even low levels of lead exposure can impair brain development and reduce IQ. In adults, chronic exposure raises the risk of cardiovascular and kidney disease.
Volatile Organic Compounds
Volatile organic compounds (VOCs) are gases released from certain solids and liquids, both indoors and out. Common sources include paints, cleaning products, stored fuels, building materials, and vehicle emissions. Benzene, one of the most well-studied VOCs, is a known carcinogen found in tobacco smoke, gasoline fumes, and paint supplies.
Outdoors, VOCs react with nitrogen oxides in sunlight to form ground-level ozone. Indoors, concentrations of many VOCs are consistently higher than outdoors, sometimes by several times, because they off-gas from furniture, adhesives, and household products in enclosed spaces with limited ventilation.
Common Indoor Pollutants
Several pollutants are primarily indoor problems. Formaldehyde off-gases from pressed wood products, insulation, and some textiles, irritating the eyes, nose, and throat. Radon, a radioactive gas, seeps into buildings through cracks in foundations and is the second leading cause of lung cancer after smoking. Outdoor pollutants also migrate indoors through open windows, doors, and on clothing and shoes, meaning your indoor air quality is never fully separate from what’s happening outside.
Fuel-burning appliances like gas stoves deserve particular attention. Cooking with gas produces both nitrogen dioxide and carbon monoxide inside the home, sometimes raising indoor NO2 to levels that would violate outdoor air quality standards.
How Air Quality Is Measured
In the United States, air quality is communicated through the Air Quality Index (AQI), a scale from 0 to 500 divided into six color-coded categories:
- Green (0 to 50): Good. Air pollution poses little or no risk.
- Yellow (51 to 100): Moderate. Acceptable for most people, though unusually sensitive individuals may notice effects.
- Orange (101 to 150): Unhealthy for sensitive groups, including people with asthma, older adults, and children.
- Red (151 to 200): Unhealthy. The general public may begin to experience health effects.
- Purple (201 to 300): Very unhealthy. Health risk increases for everyone.
- Maroon (301 and above): Hazardous. Emergency conditions where everyone is likely affected.
You can check real-time AQI readings for your area through AirNow.gov or most weather apps. The AQI is calculated separately for each major pollutant, and the highest individual reading becomes the overall number reported. On days when the AQI exceeds 100, limiting outdoor exertion, especially for children and anyone with respiratory conditions, makes a meaningful difference in exposure.