Carbon Monoxide (CO) Air Quality Trends
Carbon monoxide is the dominant pollutant in 3 of 1,020 monitored US cities. CO is a colorless, odorless gas produced by incomplete combustion — primarily from vehicle exhaust, industrial processes, and residential heating. While outdoor CO has declined dramatically since the Clean Air Act, it remains the primary pollutant concern in some traffic-heavy urban areas.
Why Carbon Monoxide Matters
Carbon monoxide reduces the blood's ability to carry oxygen. At high concentrations, it causes headaches, dizziness, confusion, and can be fatal. Outdoors, CO is most dangerous near busy roadways and in enclosed urban areas with poor ventilation. The EPA monitors CO levels and sets national ambient air quality standards to protect public health.
CO Trends: A Success Story
Outdoor carbon monoxide is one of the biggest Clean Air Act success stories. National CO levels have declined over 75% since 1980, largely due to catalytic converters in vehicles and cleaner-burning fuels. However, localized CO hotspots persist near highways, industrial facilities, and in cities with temperature inversions that trap pollutants close to the ground.
Cities Where CO Is Getting Worse
| City | State | 5yr Avg AQI | Grade | Trend |
|---|---|---|---|---|
| Harrison, Ohio | OH | 10 | B | +6.3/yr |
All Cities Where CO Is the Dominant Pollutant
| City | State | 5yr Avg AQI | Grade | Trend |
|---|---|---|---|---|
| Bayamon, Puerto Rico | PR | 18 | A | Improving |
| Harrison, Ohio | OH | 10 | B | Worsening |
| San Juan, Puerto Rico | PR | 10 | A | Improving |
Frequently Asked Questions
Carbon monoxide (CO) is a colorless, odorless gas produced by incomplete combustion of fossil fuels. Major sources include vehicle exhaust, industrial processes, and heating systems. Outdoors, CO levels are highest near busy roads and in areas with heavy traffic congestion.
CO is the dominant pollutant in 3 of 1,020 monitored US cities. It is less common than PM2.5 or ozone as a dominant outdoor pollutant because catalytic converters and cleaner fuels have dramatically reduced vehicle CO emissions since the 1970s.
The EPA 8-hour CO standard is 9 ppm (parts per million). AQI values above 100 for CO indicate levels that may cause health effects for sensitive groups. CO reduces the oxygen-carrying capacity of blood, so people with heart disease are most vulnerable.
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