The six common air pollutants, officially called “criteria air pollutants,” are ground-level ozone, particulate matter, carbon monoxide, nitrogen dioxide, sulfur dioxide, and lead. The U.S. Environmental Protection Agency regulates all six under the Clean Air Act by setting National Ambient Air Quality Standards, or NAAQS, that define the maximum concentration allowed in outdoor air. Each pollutant comes from different sources, behaves differently in the atmosphere, and poses distinct risks to your health.
Ground-Level Ozone
Unlike the protective ozone layer high in the atmosphere, ground-level ozone is a harmful gas that forms when emissions from cars, power plants, refineries, and chemical plants react with sunlight. The key ingredients are nitrogen oxides and volatile organic compounds. Neither one is ozone on its own, but together, in the presence of heat and UV light, they produce it. That’s why ozone levels tend to spike on hot, sunny afternoons in urban areas, though elevated readings can occur in colder months too.
Breathing ground-level ozone irritates the airways, worsens asthma, and reduces lung function. The current federal standard limits the 8-hour average concentration to 0.070 parts per million. The World Health Organization is more conservative, recommending a peak-season 8-hour mean no higher than 60 micrograms per cubic meter.
Particulate Matter
Particulate matter is a catch-all term for tiny solid particles and liquid droplets suspended in the air. It includes dust, soot, smoke, and chemical droplets from industrial processes. The EPA splits it into two size categories: PM10 (particles 10 micrometers or smaller in diameter) and PM2.5 (particles 2.5 micrometers or smaller). For perspective, a single human hair is about 70 micrometers wide, so even the “coarse” particles in the PM10 range are invisible to the naked eye.
PM2.5 poses the greatest health risk. Particles that small can travel deep into the lungs and even cross into the bloodstream, contributing to heart disease, stroke, lung cancer, and chronic respiratory illness. Fine particulate matter is also the leading cause of haze that obscures visibility in national parks and wilderness areas. The EPA’s current primary annual standard for PM2.5 is 9.0 micrograms per cubic meter, while the WHO’s 2021 guideline recommends a much stricter limit of 5 micrograms per cubic meter.
Home air purifiers equipped with HEPA filters are effective at capturing particulate matter. A true HEPA filter removes at least 99.97% of airborne particles down to 0.3 microns, which covers virtually all PM2.5. However, HEPA filters do not remove gaseous pollutants like ozone or carbon monoxide.
Carbon Monoxide
Carbon monoxide is a colorless, odorless gas produced by incomplete combustion of fossil fuels. Cars, trucks, gas stoves, furnaces, and generators all release it. Outdoors, vehicle exhaust is the dominant source. Indoors, poorly ventilated appliances can push concentrations to dangerous levels quickly.
What makes carbon monoxide especially dangerous is how it interacts with your blood. Hemoglobin, the protein in red blood cells that carries oxygen, binds carbon monoxide 200 to 300 times more readily than it binds oxygen. Once carbon monoxide latches on, it also changes the shape of hemoglobin so that the remaining oxygen molecules cling tighter and don’t release as easily into your tissues. The result is that your organs can be starved of oxygen even while standard blood oxygen readings appear normal. Low-level exposure causes headaches, dizziness, and confusion. High-level exposure can be fatal.
The federal standard sets an 8-hour limit of 9 parts per million and a 1-hour limit of 35 parts per million.
Nitrogen Dioxide
Nitrogen dioxide is a reddish-brown gas with a sharp smell, produced mainly by burning fuel at high temperatures. Road traffic is the principal outdoor source. Indoors, gas stoves, kerosene heaters, and poorly ventilated garages can raise levels significantly.
Even relatively short exposures can trigger airway inflammation and heighten sensitivity to allergens and other irritants, particularly in people with asthma. Controlled studies have shown measurable increases in airway reactivity at concentrations as low as what you might encounter near a busy highway. Long-term exposure is linked to the development of asthma in children and worsening of chronic lung disease in adults.
The EPA sets a 1-hour standard of 100 parts per billion and an annual standard of 53 parts per billion. The WHO’s 2021 guidelines are far stricter, recommending annual mean concentrations no higher than 10 micrograms per cubic meter, a fourfold reduction from its own 2005 recommendation of 40 micrograms per cubic meter.
Sulfur Dioxide
Sulfur dioxide is a pungent gas released primarily by burning fossil fuels that contain sulfur, especially coal and oil. Electric power generation is the single largest source, accounting for roughly two-thirds of all sulfur dioxide emissions. Oil refineries, manufacturing facilities, and heavy diesel equipment contribute most of the rest. Volcanoes are a natural but comparatively minor source.
Short-term exposure tightens the airways and is particularly harmful for people with asthma. Longer-term, sulfur dioxide plays a central role in acid rain: it reacts with water and oxygen in the atmosphere to form sulfuric acid, which falls back to earth and damages forests, acidifies lakes and streams, and corrodes buildings. The current 1-hour federal standard is 75 parts per billion.
Lead
Lead is a heavy metal that enters the air as fine particles. After leaded gasoline was phased out in the 1990s, airborne lead levels dropped dramatically. Today, the major sources are ore and metals processing, piston-engine aircraft that still run on leaded aviation fuel, waste incinerators, and lead-acid battery manufacturing. The highest concentrations are typically found near lead smelters.
Once inhaled or ingested, lead distributes through the bloodstream and accumulates in bones, where it can remain for decades. It damages the nervous system, kidneys, immune system, and cardiovascular system. Children are especially vulnerable. Even low levels of lead exposure in infants and young children can contribute to behavioral problems, learning deficits, and measurably lower IQ. The EPA’s standard for lead in air is 0.15 micrograms per cubic meter, averaged over a rolling three-month period.
How the Air Quality Index Tracks These Pollutants
The Air Quality Index, or AQI, is the tool most people encounter when checking pollution levels in their area. It converts measured concentrations of individual pollutants into a single number on a 0 to 500 scale. An AQI reading of 100 for any pollutant corresponds roughly to the federal standard for that pollutant, so anything above 100 means the air quality has crossed the threshold the EPA considers safe for short-term exposure.
The scale uses six color-coded categories:
- Green (0 to 50): Air quality is good, posing little or no risk.
- Yellow (51 to 100): Acceptable for most people, though unusually sensitive individuals may notice effects.
- Orange (101 to 150): Unhealthy for sensitive groups, including people with asthma, children, and older adults.
- Red (151 to 200): Unhealthy for everyone, with sensitive groups at greater risk.
- Purple (201 to 300): Very unhealthy, with increased risk for the entire population.
- Maroon (301 to 500): Hazardous, representing emergency-level conditions.
You can check real-time AQI readings for your area on AirNow.gov, which reports values for ozone, particulate matter, carbon monoxide, nitrogen dioxide, and sulfur dioxide. Lead is monitored separately because its concentrations change more slowly and are tracked through longer-term averaging rather than daily readings.