NO₂ — Nitrogen Dioxide Air Quality
Nitrogen dioxide (NO₂) is a reddish-brown gas produced mainly by burning fossil fuels. It is a major contributor to urban air pollution, smog formation, and acid rain — and is directly harmful to the respiratory system.
What is NO₂?
NO₂ belongs to the nitrogen oxides (NOₓ) family. In the lower atmosphere it reacts with volatile organic compounds (VOCs) under sunlight to form ground-level ozone (smog). It also dissolves in water to form nitric acid, contributing to acid rain and fine particle formation.
NO₂ is measured in parts per billion (ppb) or micrograms per cubic meter (μg/m³) — 1 ppb ≈ 1.88 μg/m³ at sea level. The US EPA measures 1-hour averages for its AQI; WHO guidelines use 24-hour and annual averages.
Sources of NO₂
- Road vehicles — especially diesel engines (cars, trucks, buses)
- Power plants — coal and gas combustion at high temperatures
- Industrial boilers and furnaces
- Aircraft engines
- Indoor gas appliances — gas stoves, unvented heaters (indoor NO₂ can exceed outdoor)
- Wildfires and agricultural burning
Health Effects of NO₂
Short-term exposure
- Airway inflammation
- Respiratory infections
- Worsening of asthma symptoms
- Coughing, wheezing, difficulty breathing
Long-term exposure
- Development of asthma in children
- Increased risk of respiratory disease
- Cardiovascular effects
- Contributes to PM2.5 formation (indirect harm)
NO₂ AQI Breakpoints (US EPA)
Based on 1-hour average concentration.
WHO and EPA Standards for NO₂
| Standard | Limit | Averaging Period |
|---|---|---|
| WHO (2021) | 25 μg/m³ (≈13 ppb) | 24-hour |
| WHO (2021) | 10 μg/m³ (≈5 ppb) | Annual mean |
| US EPA NAAQS | 100 ppb | 1-hour (98th percentile) |
| US EPA NAAQS | 53 ppb | Annual mean |
| EU Directive | 200 μg/m³ (≈106 ppb) | 1-hour (not to exceed 18×/yr) |
| EU Directive | 40 μg/m³ (≈21 ppb) | Annual mean |
| India NAAQS | 80 μg/m³ | Annual mean |
How to Protect Yourself from High NO₂
- Check daily AQI and NO₂ levels before outdoor exercise
- Avoid busy roads and high-traffic areas during rush hours
- Use N95 or KN95 masks during high-NO₂ events
- Ventilate your home but keep windows closed during high-traffic peak hours
- If you use a gas stove: use the range hood, crack a window, and consider an air purifier with activated carbon filter
- People with asthma should carry their rescue inhaler during high-pollution days
Frequently Asked Questions
What is a safe NO2 level?
The US EPA 1-hour standard for NO2 is 100 ppb. The WHO guideline is much stricter: 25 μg/m³ (about 13 ppb) for 24-hour average and 10 μg/m³ for annual mean.
Is NO2 worse indoors or outdoors?
Both. Outdoor NO2 primarily comes from traffic. Indoor NO2 from gas stoves and unvented heaters can actually exceed outdoor levels in small, poorly ventilated kitchens — sometimes reaching 400–900 ppb during cooking.
How does NO2 relate to smog?
NO2 is a precursor to ground-level ozone (O3) smog. In sunlight, NO2 reacts with volatile organic compounds (VOCs) to produce ozone. This photochemical smog is most severe on hot, sunny days in polluted cities.