Pungent, acrid odor detectable at 0.12 ppm

Highly toxic by inhalation; high concentrations of the gas and liquid NO₂-N₂O₄ are corrosive to the skin, eyes, and mucous membranes.

The acute toxicity of nitrogen dioxide by inhalation is high. Inhalation may cause shortness of breath and pulmonary edema progressing to respiratory illness, reduction in the blood's oxygen carrying capacity, chronic lung disorders and death; symptoms may be delayed for hours and may recur after several weeks. Toxic effects may occur after exposure to concentrations of 10 ppm for 10 min and include coughing, chest pain, frothy sputum, and difficulty in breathing. Brief exposure to 200 ppm can cause severe lung damage and delayed pulmonary edema, which may be fatal. Nitrogen dioxide at concentrations of 10 to 20 ppm is mildly irritating to the eyes; higher concentrations of the gas and liquid NO₂-N₂O₄ are highly corrosive to the skin, eyes, and mucous membranes. Nitrogen dioxide can be detected below the permissible exposure limit by its odor and irritant effects and is regarded as a substance with adequate warning properties.

Animal testing indicates that nitrogen dioxide does not have carcinogenic or reproductive effects. It does produce genetic damage in bacterial and mammalian cell cultures; however, most studies in animals indicate that it does not produce heritable genetic damage.