Nitrogen is not harmful by itself, in fact, it makes up about 78% of the Earth's atmosphere and is essential for life. Nitrogen becomes harmful when it reacts with other elements, such as oxygen, to form compounds that can cause health problems. For example, nitrogen dioxide (NO2) and nitric oxide (NO) are both produced by combustion engines and other industrial processes and can cause respiratory problems, such as asthma, bronchitis, and emphysema. Nitrogen also contributes to the formation of smog and acid rain, both of which can damage the environment and human health.