The concentration of NO₂ in the air has significantly decreased while cities are on lockdown during the coronavirus pandemic. Air quality readings on website World Air Quality Index show that weekly average concentration of the gas waned in Rome, Madrid, New York and New Delhi between February and April.

NO₂ is a gas emitted by the burning of fossil fuels mostly (traffic, factories) and is therefore a good indicator of human activity outside the home. Air pollution, which is most commonly measured by the concentration of PM2.5 (particles with a diameter of 2.5 μm or less), is made up by a combination of many factors – for example traffic, factories, natural events like storms or fires and the burning of solid fuels – not all of them being influenced by the virus outbreak.