Why are nitrogen oxides considered harmful to humans?

Nitrogen dioxide is an irritant gas, which at high concentration cause inflammation of the airways. ... Nox gases react to form smog and acid rain as well as being central to the formation of fine particles and ground level ozone, both of which are associated with adverse health effects.

Nitrogen dioxide levels above a certain limit can harm the human respiratory system, increasing a person's sensitivity to and severity of respiratory infections and asthma. Chronic lung disease can be caused by long-term exposure to high levels of nitrogen dioxide.

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the answer would be 3