Among the gases that burning fuels produce are nitrogen oxides. Traffic fumes are a major source of these gaseous combinations of nitrogen and oxygen. Nitrogen oxides can have various damaging effects: acid rain, the greenhouse effect, ozone layer depletion, and direct harm to human health when they react with hydrocarbon vapours and sunlight to form photochemical smog.
In 2002, 81.6 million tonnes of nitrogen oxides were emitted worldwide. 27% of these were from North America; 0.1% were from Central Africa. North America also contributes the highest nitrogen oxides emissions per person living there, Central Africa pollutes the least