Together with sulphur compounds, nitrogen compounds are the source of environmental acidification. Anthropogenic nitrogen is predominantly emitted as nitrogen oxides by transport sources, as well as by other energy uses and industrial processes. Airborne emissions of nitrogen oxides contribute to local pollution as well as to large scale pollution through long distance transport in the atmosphere. Another source of nitrogen is nitrogenous fertilisers when used in excessive quantities in agriculture. Nitrogen oxides are associated with both respiratory morbidity and mortality in humans.
Nitrogen dioxide can irritate the lungs and lower the resistance to respiratory infections.
The effects of short-term exposure are still unclear, but continued or frequent exposure to concentrations higher than those normally found in the ambient air may cause increased incidence of acute respiratory disease. Nitrogen oxides are also a precursor of nitric acid in rainwater, and they reinforce the deleterious effects of sulphur dioxide on artefacts, aquatic organisms, agriculture and habitat. Atmospheric deposition of nitrogen oxides can contribute to eutrophication. In some areas, nitrogen oxides are precursors to particulate matter concentrations. In addition to annual nitrogen oxide emissions and their percentage change, emission intensity expressed as quantities emitted per unit of GDP, per capita and per unit of gross consumption of energy should be presented in order to assess sustainability.