Common Air Pollutants Explained

Last updated April 29, 2026 · 11 min read

"Air pollution" is shorthand for at least six different things, and they don't behave the same way. They come from different sources, peak at different times, and harm the body in different ways. This guide takes the EPA's six "criteria" pollutants — the ones with National Ambient Air Quality Standards and AQI breakpoints — and explains each one in plain English.

1. Fine particulate matter (PM2.5)

What it is: particles 2.5 micrometers in diameter or smaller. For scale, a human hair is roughly 70 µm; a red blood cell is about 7 µm; PM2.5 is small enough to bypass the upper airway, deposit deep in the alveoli, and in some cases cross into the bloodstream.

Where it comes from: combustion. Burning anything produces particles, and PM2.5 is the small fraction. The biggest sources in the U.S. today are wildfires (now the dominant source on bad days for much of the country), residential wood burning, diesel engines, gas-powered vehicles, industrial processes, and secondary formation in the atmosphere from sulfate, nitrate, and ammonia precursors.

What it does: short-term, PM2.5 spikes trigger asthma exacerbations, cardiovascular events, and emergency-room visits within hours to days. Long-term, it's linked to heart disease, stroke, lung cancer, and reduced life expectancy. The WHO classifies particulate matter from outdoor air as a Group 1 carcinogen. The EPA tightened its annual PM2.5 standard from 12 to 9 µg/m³ in 2024 in response to accumulating health evidence.

When it peaks: winter, when residential wood burning combines with stagnant cold-air inversions; and summer/fall during fire season, sometimes thousands of miles from any active fire.

2. Coarse particulate matter (PM10)

What it is: particles 10 micrometers and smaller (which includes PM2.5 — the categories overlap). The PM10-minus-PM2.5 fraction is the "coarse" part: dust, pollen, mold spores, road grit, construction debris.

Where it comes from: mechanical sources rather than combustion. Wind-blown dust, agricultural operations, unpaved roads, construction sites, and brake/tire wear from vehicles all generate PM10. Saharan dust events visible from satellite can spike PM10 across the U.S. East Coast.

What it does: coarse particles deposit higher in the respiratory tract than PM2.5. They're a less potent driver of cardiovascular outcomes but still trigger asthma and exacerbate COPD. Workers with high occupational dust exposure (construction, agriculture, mining) face additional silicosis and lung-disease risk.

When it peaks: dry, windy days; agricultural burning seasons; dust storms.

3. Ground-level ozone (O₃)

What it is: ozone in the upper atmosphere is good (it blocks UV); ozone at ground level is bad. The same molecule, in the wrong place. Ozone is a secondary pollutant: it isn't emitted directly. It forms when sunlight hits a chemical soup of nitrogen oxides (NOₓ) and volatile organic compounds (VOCs).

Where it comes from: the precursor chemistry happens wherever NOₓ and VOCs meet sunlight. NOₓ comes from vehicle exhaust, power plants, and industrial combustion; VOCs come from gasoline vapors, paints, solvents, and biogenic sources (trees emit isoprene). On a hot, sunny, stagnant afternoon in a city, the cocktail is everywhere.

What it does: ozone is a strong oxidant. It irritates the airway, reduces lung function during exercise, exacerbates asthma, and provokes coughing and chest tightness. Long-term exposure is associated with reduced lung function in children and increased respiratory mortality.

When it peaks: hot summer afternoons. Unlike PM2.5, ozone is highly diurnal — it builds during the day, peaks in mid- to late afternoon, and falls overnight. Ozone alerts almost always mention "particularly during afternoon hours" for that reason. Outside of fire season, ozone is the dominant AQI driver in most U.S. cities.

4. Nitrogen dioxide (NO₂)

What it is: a reddish-brown gas, member of the broader NOₓ family.

Where it comes from: combustion at high temperatures. Vehicle exhaust (especially diesel) is the dominant source, followed by power plants and industrial boilers. Indoors, gas stoves are a meaningful NO₂ source — kitchens with gas cooking can briefly exceed outdoor regulatory standards while a burner is on.

What it does: NO₂ inflames the airway, increases susceptibility to respiratory infections, and worsens asthma — particularly in children. It's also a key precursor to both ozone and secondary PM2.5, so reducing NO₂ has knock-on effects on those.

When it peaks: rush hour in dense traffic corridors. NO₂ has steep gradients near roads — concentrations within 100 meters of a freeway can be several times the urban background.

5. Sulfur dioxide (SO₂)

What it is: a colorless gas with a pungent rotten-egg-meets-burning-match smell.

Where it comes from: historically, coal-fired power plants and oil refining. SO₂ emissions in the U.S. have fallen dramatically since the 1970s as power generation shifted away from high-sulfur coal and as flue-gas desulfurization ("scrubbers") became standard. Today the largest U.S. sources are remaining coal plants, marine vessels burning bunker fuel near ports, and certain metal-smelting operations. Volcanic eruptions are also a major natural source.

What it does: SO₂ irritates the airway and provokes bronchoconstriction, particularly in people with asthma. Even brief exposure (5–10 minutes) at moderate concentrations can trigger an asthma attack. It also reacts in the atmosphere to form sulfate, a major component of secondary PM2.5.

When it peaks: near specific industrial sources. Most U.S. ZIP codes rarely if ever see elevated SO₂; communities near refineries, smelters, or older coal plants are the exceptions.

6. Carbon monoxide (CO)

What it is: a colorless, odorless gas produced by incomplete combustion.

Where it comes from: vehicles (particularly cold-start gasoline engines), residential wood and gas combustion, generators, and industrial processes. Outdoor ambient CO is rarely a problem in the modern U.S. — it has fallen by more than 80% since 1980 as catalytic converters and fuel reformulation took hold. CO remains a serious indoor and occupational hazard, though.

What it does: CO binds to hemoglobin with about 200 times the affinity of oxygen, displacing oxygen and starving tissues. Acute CO poisoning causes headache, dizziness, confusion, and at high doses death. Chronic low-level exposure exacerbates cardiovascular disease.

When it peaks: ambient CO peaks in winter (cold-start emissions, residential heating). The biggest CO risk for most people is indoor — improperly vented gas appliances, generators run in garages, charcoal grills brought inside. Every home should have a working CO alarm.

The pollutants that aren't in the AQI

The AQI covers only the six criteria pollutants. Several health-relevant exposures don't appear in any AirNow reading:

• Volatile organic compounds (VOCs) — benzene, formaldehyde, trichloroethylene, and others. Some are carcinogens. Indoor concentrations from paints, cleaners, building materials, and personal-care products often dwarf outdoor levels.
• Polycyclic aromatic hydrocarbons (PAHs) — products of incomplete combustion that travel attached to particles. Some are potent carcinogens. Concentrations spike during wildfire smoke events.
• Ammonia (NH₃) — from agriculture and wastewater. Major precursor to secondary PM2.5 in some regions.
• Hazardous air pollutants (HAPs) — 188 chemicals the EPA tracks separately. Industrial monitoring, not part of the AQI.
• Radon — naturally occurring radioactive gas. Indoor problem, not in AQI; second-leading cause of lung cancer in the U.S. after smoking.

This is why a "Good" AQI doesn't mean "perfectly clean air" — it means the six criteria pollutants are at low levels.

Two other categories of airborne exposure also sit outside the AQI by design: biological aeroallergens (pollen, mold spores) and infectious respiratory aerosols (influenza, RSV, SARS-CoV-2). Allergy symptoms during high pollen counts can look like AQI-driven symptoms but the cause and the protective response are different — a respirator helps with both, but antihistamines only help with one. For tracking infectious respiratory exposure alongside your local AQI, Pandemic Watch publishes current surveillance data.

Quick reference: when to worry about which

Primary sources: EPA — Criteria Air Pollutants · EPA — PM Pollution · EPA — Ground-Level Ozone · WHO — Ambient Air Quality