Nitrogen oxides help cause some kinds of air pollution. Nitrogen dioxide combines with other chemicals to make smog. It also combines with water droplets in the air to make nitric acid. Nitric acid is one of the kinds of acid in acid rain. Volcanoes and lightning make some of the nitrogen oxides in Earth's atmosphere. Humans make lots of nitrogen oxides, too. When fuel burns, nitrogen in the air combines with oxygen to make nitrogen oxides. The engines of cars, trucks, and buses make lots of nitrogen oxides. Nitrogen oxides are not all bad. Humans use nitric oxide to make nitric acid. Nitric acid is used to make fertilizer and some kinds of explosives.