Ozone, NO2, PM2.5, PM10: The Pollutants Behind Your AQI

Your local air quality number is not one thing. The AQI is built from up to six pollutants, and the reading you see reflects whichever one is worst at that moment. Four of them do most of the work: ozone, nitrogen dioxide (NO2), and fine and coarse particles (PM2.5 and PM10). Each comes from a different place, peaks at a different time, and harms your body in a different way. Here is what all six are, and how to tell which is driving your air today.

What pollutants make up the AQI?

The US Air Quality Index combines up to six pollutants: ozone, PM2.5, PM10, nitrogen dioxide, sulfur dioxide, and carbon monoxide. Each gets its own score, and the AQI reports the single worst one, so on a smoggy summer afternoon your number might be all ozone, while on a wildfire day it is all fine particles. Our AQI scale guide explains what the number and colors mean; this guide is about what is actually in the air. The table sums up all six, the same six Skyvo shows in its pollutant grid, and the sections after it go one by one.

Pollutant	Where it comes from	When it peaks	Hits hardest
Ozone (O3)	Traffic and industry fumes cooked by sunlight	Hot, sunny afternoons	Asthma, children, older adults
Nitrogen dioxide (NO2)	Burning fuel: traffic, power plants, gas stoves	Rush hour, near busy roads	Asthma, children, older adults
PM10 (coarse)	Dust, pollen, construction, roads, fires	Dry, dusty, windy days	Nose, throat, airways
PM2.5 (fine)	Combustion: engines, industry, wood and coal fires	Cold, still days and wildfire smoke	Heart, lungs, bloodstream
Sulfur dioxide (SO2)	Coal and oil power plants, heavy industry, volcanoes	Near industrial sites, still winter days	Asthma, airways
Carbon monoxide (CO)	Incomplete burning: exhaust, faulty heaters and stoves, fires	Heavy traffic, enclosed spaces	Blood oxygen, heart

Ozone (O3): the sunshine pollutant

Ground-level ozone is unusual because nothing emits it directly. According to the EPA, it forms when nitrogen oxides and other fumes from cars, power plants, and industry react in the presence of sunlight. That is why it is a warm-weather problem: the EPA notes ozone is most likely to reach unhealthy levels on hot, sunny days in urban areas, building through the afternoon.

Breathing it irritates the airways. People with asthma, children, and older adults are most at risk, and in the AQI ozone typically becomes a concern for those sensitive groups once the index climbs past 100.

Nitrogen dioxide (NO2): the traffic and stove pollutant

Nitrogen dioxide comes from burning fuel. The EPA lists cars, trucks, buses, power plants, and off-road equipment as the main outdoor sources, which is why NO2 runs highest near busy roads and at rush hour. Short-term exposure irritates the airways and can aggravate asthma, and longer exposure may help bring it on.

The twist is indoors. A 2024 Stanford study found that a gas or propane stove can fill a home with as much NO2 as all outdoor sources combined, pushing millions of households past long-term safety limits and contributing to as many as 19,000 US deaths a year. Good ventilation helps, and it is a reminder that the outdoor AQI does not capture your indoor air.

PM10: coarse particles (dust and pollen)

PM10 means inhalable particles up to 10 micrometers across, which the EPA describes as coming from construction sites, unpaved roads, fields, smokestacks, and fires, along with wind-blown dust, pollen, and mold. For scale, a human hair is about 70 micrometers wide, so even these coarser particles are far too small to see.

They mostly irritate the nose, throat, and airways. PM10 tends to spike on dry, dusty, or windy days, and it is a bigger problem in arid regions and during dust storms.

PM2.5: fine particles (the most dangerous)

PM2.5 is the fine fraction, particles 2.5 micrometers and smaller, roughly thirty times narrower than a hair. It comes mostly from combustion: engines, industry, and wood or coal fires. Because it is so small, the EPA notes it can lodge deep in the lungs and pass into the bloodstream, which is why PM2.5 is tied to the largest share of pollution’s health toll and why it anchors the global rankings of the most polluted countries. It is the pollutant most worth watching, and it deserves its own detailed guide, so this is the short version.

Sulfur dioxide (SO2): the coal and industry pollutant

Sulfur dioxide comes from burning sulfur-rich fuels, mainly coal and oil power plants and heavy industry, with volcanoes as the natural source. It irritates the airways and can trigger asthma attacks, and in the atmosphere it turns into fine particles and acid rain. It drives the AQI less often than the big four, but near a plant or a port it can be the pollutant setting your number.

Carbon monoxide (CO): the enclosed-space pollutant

Carbon monoxide comes from incomplete burning of fuel: vehicle exhaust, faulty heaters and stoves, and fires. It binds to the blood and cuts the oxygen it can carry, which is why it is most dangerous indoors and in traffic tunnels rather than in open air. Notably the European index leaves carbon monoxide out entirely, one of several reasons the two scales can disagree, which we cover in our US vs. European index guide.

How to see which pollutant is driving your air

A single AQI number tells you how bad the air is, but not why, and the right response is not always the same. Ozone means rethinking the afternoon run, while fine particles from smoke mean keeping the windows shut. Same number, different action.

That is what a pollutant breakdown is for. Alongside your AQI on the US or European scale, Skyvo shows each pollutant and flags the dominant one driving the reading right now, so you know whether today is an ozone day or a particle day. If you are not sure where to get a reliable reading in the first place, our guide on how to check air quality covers the options.

Knowing the pollutants turns a vague number into a decision. Once you can see that today’s 95 is ozone and tomorrow’s is smoke, checking the air becomes as routine as checking the weather, which is exactly the habit Skyvo is built around.