Nitrogen Dioxide (NO2) Air Quality Trends
Nitrogen dioxide is the dominant pollutant in 4 of 1,020 monitored US cities. NO2 is a reddish-brown gas produced primarily by vehicle exhaust and power plant emissions. It is both a direct respiratory irritant and a key precursor to ozone smog and fine particulate matter formation.
How NO2 Affects Air Quality
Nitrogen dioxide plays a dual role in air pollution. It directly irritates airways at elevated concentrations, and it also reacts with other chemicals in sunlight to form ground-level ozone (smog) and contributes to fine particulate matter (PM2.5) formation. The EPA regulates NO2 with both an annual standard (53 ppb) and a 1-hour standard (100 ppb) to protect against short-term spikes.
NO2 and Traffic
Vehicle emissions are the largest source of NO2 in most US cities. Studies show NO2 concentrations can be 2-3 times higher within 50 meters of major roadways compared to background levels. This makes NO2 a particularly significant environmental justice concern, as low-income communities and communities of color are disproportionately located near major highways.
Cities Where NO2 Is Getting Worse
| City | State | 5yr Avg AQI | Grade | Trend |
|---|---|---|---|---|
| Guaynabo, Puerto Rico | PR | 29 | C | +1.4/yr |
| Belmont, Ohio | OH | 18 | B | +0.7/yr |
All Cities Where NO2 Is the Dominant Pollutant
| City | State | 5yr Avg AQI | Grade | Trend |
|---|---|---|---|---|
| Belmont, Ohio | OH | 18 | B | Worsening |
| Caguas, Puerto Rico | PR | 10 | A | Improving |
| Guaynabo, Puerto Rico | PR | 29 | C | Worsening |
| Norfolk City, Virginia | VA | 18 | A | Improving |
Frequently Asked Questions
Nitrogen dioxide is a reddish-brown gas with a sharp, biting odor. It forms when fuel is burned at high temperatures in vehicles, power plants, and industrial facilities. NO2 is both a direct health hazard and a precursor to ground-level ozone and fine particulate matter.
NO2 is the dominant pollutant in 4 of 1,020 monitored US cities. It is most concentrated near major highways, in dense urban centers with heavy traffic, and downwind of power plants and industrial facilities.
Short-term exposure to elevated NO2 can aggravate asthma and other respiratory diseases. Long-term exposure is linked to development of asthma in children and increased susceptibility to respiratory infections. The EPA annual standard for NO2 is 53 ppb.
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