It is a brownish gas. It chokes when it is breathed in. When it is compressed, two of the molecules bond, making dinitrogen tetroxide. It also makes the dinitrogen tetroxide when it is cooled. It is an oxidizing agent. It turns back into nitric oxide and oxygen when it is heated. It reacts with iodides to make nitrites and iodine. It is what makes nitric acid yellow.
It is used to make nitric acid by dissolving it in water. Nitric oxide is also released.
Nitrogen dioxide is very toxic. It is also released by some cars, polluting the atmosphere. It has a bad odor. It makes mushroom clouds (clouds produced when a nuclear weapon explodes) reddish-brown. It can stop the nose from smelling, making the person unaware that they are breathing toxic fumes.