Nitrogen Oxides (NOx)
Learn what nitrogen dioxide (NO2) and nitric oxide (NO) are, why cars and diesel engines create NOx, how NOx drives smog, and what cuts emissions fastest.
NOx usually refers to nitric oxide (NO) and nitrogen dioxide (NO2). These gases form during high-temperature combustion — especially in vehicle engines — and they contribute to smog (ozone) and fine particle pollution that harms lungs and hearts.
If you’re here for vehicle NOx emissions: diesel-powered vehicles and equipment are a major NOx source, and the fastest way to cut tailpipe NOx is to reduce gasoline/diesel burning — including switching high-gasoline drivers to EVs. You can learn more aboutpollution from vehicles here.
Vehicle NOx quick facts
- Transportation is about 45% of the U.S. NOx emissions inventory (EPA, 2020 NEI reference).
- Diesel-powered vehicles, vessels, locomotives, and equipment account for over 60% of NOx from U.S. transportation sources.
- In California, transportation is responsible for nearly 80% of nitrogen oxide pollution.
Sources: EPA transportation NOx inventory, UCS diesel + NOx, California Energy Commission.
What Is Nitrogen Dioxide (NO2)?
Nitrogen dioxide (NO2) is a nitrogen oxide and a common air pollutant. In air-quality reporting, NO2 is often used as an indicator for the broader NOx group (NO + NO2). Source: EPA basic information about NO2.
Note: It's important to distinguish NOx from nitrous oxide (N2O). In air pollution, “NOx” typically means NO and NO2 — while N2O is a different pollutant that’s discussed mainly as a greenhouse gas.
What Is The Formula For Nitrogen Dioxide?
The chemical formula of nitrogen dioxide is NO2: one nitrogen atom and two oxygen atoms.
What Is Nitric Oxide (NO)?
Nitric oxide (NO) is a nitrogen oxide that forms during combustion. In the atmosphere, NO can react and convert into NO2 — which is one reason NO and NO2 are often discussed together as NOx.
What Is The Formula For Nitric Oxide?
The chemical formula of nitric oxide is NO: one nitrogen atom and one oxygen atom.
Nitrogen Oxide (NOx) Sources
NOx is produced mainly by high-temperature combustion — including cars and trucks, off-road diesel equipment, ships, locomotives, power plants, and industrial boilers.
Where does nitrogen dioxide (NO2) come from?
In the U.S., the transportation sector is responsible for about 45% of the NOx emissions inventory (EPA, 2020 NEI reference). Source: EPA transportation air pollution overview.
Within transportation, diesel-powered vehicles and equipment are a major NOx contributor. Source: UCS diesel engines and public health.
Vehicle NOx Emissions: Why Diesel Is Often Higher
Vehicle engines create NOx because combustion can reach temperatures where nitrogen (N2) and oxygen (O2) in the air react and form nitrogen oxides. Diesel engines are often associated with higher NOx because they typically operate with excess oxygen (“lean” combustion), which makes NOx control more complex than in gasoline engines.
How NOx Affects Human Health
NOx pollution contributes to other harmful pollutants — especially ground-level ozone (smog) and fine particle pollution (including nitrate particles) that can worsen asthma and increase cardiovascular and respiratory risks.
- A widely cited global estimate linked long-term ozone exposure to about 1 million premature respiratory deaths (global, 2010 baseline). Source: Malley et al. (ozone mortality estimate).
- WHO estimates about 4.2 million premature deaths (2019) from ambient (outdoor) air pollution (PM2.5 is the main driver). Source: WHO ambient air pollution fact sheet.
Environmental Effects Of NOx
NOx contributes to smog (ozone), acid rain, and haze (including nitrate particles that reduce visibility in parks and scenic areas). Source: EPA NO2 basics.
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Nitrogen Oxide (NOx) Emissions
Nitrogen oxide (NOx) emissions, which include nitric oxide (NO) and nitrogen dioxide (NO2), are major contributors to air pollution. Originating primarily from vehicle exhaust, the nitrogen and oxygen in the air combine to form harmful compounds.
Exposure to these pollutants can lead to respiratory problems, cardiovascular diseases, and even premature death. The environmental impact of NOx emissions extends beyond human health and includes the formation of acid rain. Acid rain can severely damage ecosystems, including forests, lakes, and streams.
Reducing Nitrogen Oxide Pollution
To reduce NOx, we need to reduce how much gasoline and diesel we burn — especially in the vehicles and engines that generate the most pollution.
Drivers who use a lot of gasoline can have an outsized impact by switching to an EV. In Coltura’s research, the top 10% of U.S. drivers by gasoline use would save thousands per year on fuel costs alone by switching to an EV.
Imagine never having to visit a gas station again, or needing an oil change again or worrying about unpredictable gas prices. With an electric vehicle, this could be your reality—while potentially saving thousands of dollars a year.
Curious how much you could save switching to an EV? Get a quick estimate of your savings below!
*Disclaimer: This tool is only intended to provide an estimate of potential savings. Actual results will vary. Learn more about the calculation and the Electric vs Gas Calculator here.
Nitrogen Oxide Policy
Mitigating NOx is a major public-health priority, and policies include tighter vehicle emission standards, cleaner fuels, and enforcement to ensure real-world emissions controls work as intended.
Federal policy note: EPA proposed updated standards in 2023 and finalized a multi-pollutant rule on March 20, 2024 for model year 2027 and later light-duty and medium-duty vehicles (phasing in through MY 2032). Source: EPA final rule page.
We Can Cut Vehicle Emissions — Including NOx — Faster By Helping The Biggest Gasoline Users Switch To EVs
Prioritizing the switch to EVs for “Gasoline Superusers” (the top 10% of gasoline consumers) offers a faster path to cutting vehicle pollution — and it can deliver major cost savings for households that spend the most on gasoline.
Learn from our free webinar about how the gasoline superuser approach can help reduce car pollution faster and more equitably.
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