What is Nitrogen Dioxide (NO2)?

When it comes to air pollution, Nitrogen Dioxide (NO2) is not only one of the biggest health hazards for our environment, but it’s also one of the main contributors to urban smog. But what is NO2 exactly and what can we do to protect ourselves from it?

What is NO2 & Where Does it Come From?

Nitrogen Dioxide is a gas, composed of one nitrogen atom and two oxygen atoms. Along with Nitric Oxide (NO), NO2 is a member of a family of chemicals composed of nitrogen and oxygen, collectively known as Nitrogen Oxides (NOX). Nitrogen Dioxide is a primary air pollutant, directly emitted from both natural and human sources.

During combustion processes, Nitrogen Oxides (including NO2 and NO) form when oxygen reacts with nitrogen at high temperatures. Human-caused NOx emissions typically get produced by the burning of fossil fuels (coal, oil, and gas) by vehicles such as cars, trucks, and buses, as well as from power plants and off-road equipment. Natural sources of NO2 include volcanoes, oceans, biological decay, and even lightning strikes.

Is Nitrogen Dioxide (NO2) also an Indoor Air Pollutant?

Yes, Nitrogen Dioxide can also be present as an indoor pollutant. Indoor sources of NO2 can include gas stoves and kerosene heaters as well as tobacco smoke. Of course, due to the dynamic nature of air pollution, NO2 from outside can travel indoors through open windows or air exchange systems that operate while outdoor nitrogen dioxide levels are high.

Learn more about air pollution migration between outdoors and indoors here

Where Do We See High NO2 Levels? 

Cities. High levels of NO2 pollution are generally found in urban environments where there is a high volume of traffic and use of vehicles – in particular prominently busy intersections, roads, and major highways. 
In its gaseous form, Nitrogen Dioxide possesses a deep red-orange color that is responsible for giving city haze its reddish-brown shade. NO2 absorbs ultraviolet and visible solar radiation, which decreases atmospheric visibility.

Nitrogen Dioxide is More Localized than Other Pollutants

Interestingly, in contrast to some types of pollutants, the impact of NO2 on air quality often remains fairly localized, highlighting the value of traffic pollution information for managing exposure here.

Example: Po Valley, Italy, a highly polluted area due to traffic and industry, effectively sees NO2 pollution trapped within the confines of the region due to the surrounding Alps mountains. During a 2020 COVID-19 lockdown period in which travel was more limited, NO2 levels dropped significantly, highlighting the local nature of NO2 emissions from vehicles and industry.

Northern Italy NO2 2020
NO2 Concentrations in Northern Italy (image originally featured on Copernicus.eu)

Nitrogen Dioxide + Ozone Formation

Ozone (O3) is produced by chemical reactions between NOX and Volatile Organic Compounds (VOCs) that get triggered by sunlight and heat. This then begs the question: is Nitrogen Dioxide a greenhouse gas? The answer is no. NO2 is not regarded as a greenhouse gas but rather as a precursor pollutant for the formation of ground-level ozone (O3), along with VOCs and other NOx pollutants from fossil-fuels vehicles, power plants, industrial boilers, chemical plants, refineries, and other sources. 

Learn more about how ozone forms and its impact on health here

It’s also important to note that a decrease in NO2 does not always lead to a decrease in ground-level ozone: in China, for example, researchers observed NO2 concentrations decrease by over 50% during COVID-19 lockdowns compared to the previous period. However, in Northern and Central China, O3 concentrations did not decrease but rather increased by over 100%

How Does Nitrogen Dioxide (NO2) Impact Health?

Short-term exposure to NO2 in the air has been linked to inflammation of the airways and aggravation of respiratory diseases, leading to respiratory symptoms such as coughing, wheezing, or difficulty breathing. Over the long term, the EPA warns that exposure to elevated concentrations of NO2 on a regular basis may contribute to the development of asthma and potentially increase susceptibility to respiratory infections and allergens. 


Scientists further warn that even short-term exposure to NO2 may increase the risk of death from respiratory diseases. In fact, in 2020, NO2 became involved in the tragic and untimely death of 9-year-old Ella Adoo-Kissi-Debrah, which marked the first time in the UK’s history where air pollution was listed as the ‘cause of death’.

According to the coroner’s testimony, Nitrogen dioxide levels near Ella’s home exceeded World Health Organization and European Union guidelines, and “There was a recognized failure to reduce the levels of nitrogen dioxide, which possibly contributed to her death”.

Who is Most Vulnerable to NO2?

Children and infants whose lungs are still developing are more vulnerable to Nitrogen Dioxide exposure, especially if they spend a lot more time outdoors in busy urban environments. The elderly and people with respiratory illnesses or chronic respiratory conditions may also be particularly vulnerable to NO2 exposure. 

How Much NO2 in the Air is Considered Unhealthy?

In September 2021, the World Health Organization updated its guidelines for long-term exposure to Nitrogen Dioxide (NO2), cutting it drastically from 40 μg/m3 to 10 μg/m3. For the first time in history, the WHO also introduced a daily recommended level for NO2 exposure at 25 μg/m3.

These new guidelines further emphasize the growing understanding of Nitrogen Dioxide as a dangerous air pollutant and the need to curb its emissions to protect public health.

How Can We Prevent Dangerous NO2 Exposure?

Some positive changes are taking place. US government regulations have proven effective in decreasing the national average of NO2 by 64% from 1980 to 2021, an improvement that speaks volumes when visualized:

annual mean NO2 Ohio River Valley 2005 & 2016
Annual mean NO2, Ohio River Valley (2005 & 2016); image courtesy of NASA

However, other parts of the world have experienced increasing NO2 levels as larger populations move into cities and vehicle use increases. 

Regardless, even in regions with annual average decreases, the dynamic nature of air pollution still makes NO2 a major environmental hazard at different times of day and in specific locations. For example, data from the UK government shows clear peaks during morning and evening rush hour.

Protecting People From NO2 On The Road with Cleaner Route Planning

In the past, exposure to NO2 and other harmful air pollutants during daily commutes in urban environments has been regarded as unavoidable, as vehicle emissions are a primary source. Neither motorists, cyclists, nor pedestrians had an easy way to ‘see’ the environmental hazards along their routes. Thanks to emerging climate technology, this is changing. 

Environmental intelligence now enables automotive, micro-mobility, and fitness companies to empower users with Cleanest Route Insights. These insights can be integrated into travel apps, vehicle dashboards, and other connected products to empower users with healthier travel options while still prioritizing safety and speed.

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Eli Ginsburg

MSc in environmental technology and water management from Imperial College London. Former environmental consultant, specialising in modelling of air pollution and environmental impact assessments.