US Ozone Pollution by State: Why the Mountain West Tops the List
Ground-level ozone is the main ingredient in summer smog and one of the most widespread air pollutants in the country. But its geography is surprising: the highest statewide ozone levels aren't in the biggest cities — they're in the sunny, high-altitude Mountain West. This guide maps ground-level ozone by state, ranks them, and explains why sunshine and elevation, more than population, drive where ozone builds up.
What is ground-level ozone?
Ozone high in the atmosphere protects us from the sun's UV rays, but at ground level it's a harmful pollutant — the primary component of smog. It isn't emitted directly; it forms when sunlight cooks together pollutants from cars, power plants, and industry. Breathing it inflames the lungs, worsens asthma, and is especially dangerous for children, the elderly, and people with respiratory conditions. It's measured in parts per billion (ppb), and the EPA's health standard is 70 ppb.
Which states have the most ozone?
The highest statewide ozone levels are in the Mountain West — Colorado, Arizona, Wyoming, and Kansas — not, as you might guess, the most populous states. The lowest are in cool, coastal, and island states: Hawaii, Alaska, and Maine. This pattern is the opposite of what you'd expect if ozone simply tracked population and traffic. The ranking below lists every state.
Why ozone forms in sunny, high-altitude states
Three things drive the Mountain West's high ozone. Sunshine: ozone forms in sunlight, so sunny states cook up more of it. Altitude: high-elevation air starts with more natural background ozone and is closer to ozone drifting down from the upper atmosphere. And geography: mountain valleys can trap pollutants, and some Western ozone is even carried across the Pacific from Asia. Add energy development and traffic, and high, arid, sunny states end up with more ozone than cloudier, lower regions — even big cities — on a statewide-average basis.
Ozone vs. PM2.5
Ozone and PM2.5 (fine particles) are the two pollutants that drive most U.S. air-quality alerts, but they behave differently. PM2.5 is highest in the industrial Midwest and on wildfire days; ozone is highest in sunny, high-altitude areas and peaks in summer afternoons, when sunlight is strongest. That's why summer brings "ozone action days" while winter and wildfire season bring particle warnings. Both have fallen substantially since the Clean Air Act, but ozone has been more stubborn because rising temperatures help it form.
Frequently asked questions
What is ground-level ozone?
A harmful pollutant and the main ingredient in smog. It forms when sunlight reacts with emissions from cars, power plants, and industry, and it inflames the lungs and worsens asthma. The EPA standard is 70 ppb.
Which state has the highest ozone levels?
On a statewide-average basis, Mountain West states — Colorado, Arizona, and Wyoming — have the highest ground-level ozone, driven by sunshine and high altitude rather than population.
Which states have the lowest ozone?
Cool, coastal, and island states — Hawaii, Alaska, and Maine — have the lowest ground-level ozone.
Why is ozone high in the Mountain West?
Sunshine drives ozone formation, high altitude means more background ozone, and mountain geography can trap pollutants — some even drifting across the Pacific from Asia.
What's the difference between ozone and PM2.5?
PM2.5 is fine particles, highest in industrial areas and on wildfire days; ozone forms in sunlight and peaks on summer afternoons in sunny, high-altitude regions. Both drive air-quality alerts.