Nitrogen dioxide / NO₂

Nitrogen dioxide is a precursor in the industrial production of nitric acid (HNO₃) and a major pollutant of Earth's atmosphere. It is produced by internal combustion engines and thermal power stations. As such, it is responsible for the acidifying and eutrophying nature of sulphur-free "acid rain" (NO₂ combined with the anthropogenic tropospheric ozone forms nitrates which are highly soluble in water).
In its concentrated form, it is a suffocating and toxic reddish-brown gas with a characteristically acrid and pungent odour. Due to being highly volatile (a boiling point of 21°C, i.e. very close to room temperature), this product is generally delivered in cylinders as a liquefied compressed gas.

Flammability and explosiveness: Nitrogen dioxide is non-flammable. However, it is a combustion agent which can react aggressively with several compounds, which may cause fires and explosions.