Although reactive nitrogen (N) exists in the natural environment, relatively small amounts of reactive N circulate and balance. The increased load of reactive N associated with human activities causes various environmental impacts. The effects are diverse, including climate change, stratospheric ozone depletion, air pollution, water pollution, eutrophication, and acidification, and are collectively referred to as N pollution.

Reactive N has environmental effects depending on its chemical species. Nitrous oxide is a greenhouse gas that is approximately 300 times more potent than carbon dioxide, and is currently the most contributing substance to stratospheric ozone depletion. Nitrogen oxides are a typical air pollutant. Ammoniacal and nitrate N are the causative compounds of fine particulate matter (PM2.5) as an air pollutant, and of water pollution and eutrophication.