Chlorine is a greenish yellow gas which combines directly with nearly all elements. Chlorine is a respiratory irritant. The gas irritates the mucous membranes and the liquid burns the skin. As little as 3.5 ppm can be detected as an odour, and 1000 ppm is likely to be fatal after a few deep breaths. It was used as a war gas in 1915. It is not found in a free state in nature, but is found commonly as NaCl (solid or seawater). Table: basic information about and classifications of chlorine. • Name: