Nitrogen Dioxide (NO2)
NO2 is a reddish-brown gas produced by combustion at high temperatures. It is the strongest indicator of traffic-related air pollution.
Reviewed by Hayden Williams. Last reviewed 2026-05-01. Unit: ppb.
NO2 forms inside hot combustion chambers (engines, boilers, power plants) when atmospheric nitrogen and oxygen react. It is a precursor for both ozone and secondary PM2.5, and it has direct respiratory effects on its own. Concentrations track traffic density closely - kerbside readings can be 5-10x higher than background urban readings just 100 meters away.
Health effects
Inflames airways, increases asthma severity, reduces lung development in children, and is associated with increased respiratory infections.
Chronic NO2 exposure near busy roads is associated with new-onset asthma in children, reduced lung function growth, and worsened cardiovascular outcomes. Short-term exposure increases hospital admissions for asthma and COPD. Studies near schools and homes within 100 m of major roads show measurable health impact gradients.
Vulnerable groups
Children living or attending school near busy roads, people with asthma, COPD patients, elderly with cardiovascular disease.
Common sources
- Diesel and gasoline vehicles (largest urban source)
- Heavy-duty trucks and buses
- Coal and gas power plants
- Industrial boilers and furnaces
- Gas stoves and unvented gas heaters indoors
- Aircraft (around airports)
- Marine shipping (in coastal cities)
Regional context
Highest in dense urban centers with heavy diesel traffic - London, Paris, Madrid, Milan, Mexico City, Beijing, Delhi, Cairo. Cities that have implemented Low Emission Zones (London ULEZ, Paris ZFE) show measurable NO2 declines. Suburban and rural areas can still exceed WHO guidelines along major highways.
Regulatory thresholds
How NO2 is measured
Chemiluminescence is the reference method - NO2 is converted to NO and reacted with ozone, the resulting light is measured. Passive diffusion tubes provide cheap monthly averages widely used in citizen-science monitoring. Low-cost electrochemical sensors are emerging but currently noisy.
How to reduce your exposure
- Walk or run on side streets, not main roads (30-50% lower exposure)
- Avoid rush hour for outdoor exercise (NO2 peaks 7-9 AM and 5-7 PM)
- Schools located >100 m from major roads have measurably lower exposure
- Switch from gas cooking to induction or electric, or use a vented range hood
- Drive less, choose EVs, support Low Emission Zones
Trends
NO2 has dropped roughly 25-40% in many EU cities over the past decade due to Euro 6 emissions standards, ULEZ-style restrictions, and EV adoption. The US has seen similar declines under Tier 3 standards. Asia has flattened or risen as vehicle fleets grow. Diesel vehicles remain a stubborn legacy in Europe.
Cities where NO2 matters most
Live readings for cities historically affected by elevated NO2:
Frequently asked
Why is NO2 important?
NO2 is the strongest indicator of traffic pollution. Living near busy roads exposes you to chronic NO2, linked to childhood asthma onset and cardiovascular outcomes. It is also a precursor to ground-level ozone and secondary PM2.5.
Do gas stoves produce NO2?
Yes, significantly. Gas cooking can spike indoor NO2 above 200 ppb during use - well above outdoor air quality standards. Use a vented range hood or switch to induction.
How far does NO2 from a road reach?
Concentrations drop sharply with distance. Kerbside levels can be 5-10x higher than 100 m away. Schools, playgrounds, and homes set back from main roads have meaningfully lower exposure.
Has NO2 declined?
In Western Europe and the US, yes - due to vehicle emissions standards and electrification. Asia has flattened. Diesel passenger cars remain a slow-decaying problem in Europe.
Sources + further reading
Track NO2 live across 217 cities on atmos.today. See methodology for how readings are sourced and calculated.