The hazardous compounds are: two forms of hydrazine, nitrogen tetroxide and a pure form of ammonia.

CNN: Four compounds from shuttle can harm on contact

Nitrogen tetroxide is a reddish-brown gas at room temperature and has a "pungent, sweetish" smell, according to experts.

CNN: Four compounds from shuttle can harm on contact

Nitrogen tetroxide and monomethyl hydrazine are "hypergolic" propellants, meaning they ignite spontaneously when they come in contact with each other, a useful property for maneuvering systems.

CNN: Four compounds from shuttle can harm on contact