Nitrogen dioxide (NO2) is a gas with poignant odor released during the burning of oil, wood, and coal-based fuels. The gas can be toxic to most living organisms, and it is a key agent in the development of acid rain and associated ecological damage. In moderate and high concentrations NO2 can render the horizon with a reddish-brown atmospheric haze. Information source: U.S. EPA.
Short-term exposure to NO2 of less than one day can cause bronchioconstriction and increased asthma symptoms. Long-term health effects are unknown. Note that NO2 (and other gaseous nitrogen oxides) can react with ammonia and water molecules in the atmosphere forming small particulates, which have additional adverse health effects. Information source: U.S. EPA.