Global anthropogenic nitrogen dioxide emissions, 1750-2022

In atmospheric chemistry, NOx (“knocks”) is shorthand for nitric oxide (NO) and nitrogen dioxide (NO₂)—the nitrogen oxides most relevant to air pollution. These gases contribute to the formation of several of the most harmful air pollutants. The European Union, the United States, and other countries regulate emissions of nitrogen dioxide as an indicator for the broader group of nitrogen oxides.

Exposure to high levels of NOx can cause a range of health impacts, including respiratory impairment, asthma exacerbation, decreased lung function, and cardiovascular effects.¹ Children and the elderly are particularly susceptible to lung function impairments and chronic disease. NOx also contributes to the formation of particulate matter (PM) and tropospheric (ground-level) ozone, both of which are damaging air pollutants.