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. Strange Matter cartoon included by kind permission of Nick Kim at the University of Waikato , New Zealand. Here is a brief summary of the isolation of chlorine. It is rarely necessary to make chlorine in the laboratory as it is readily available commercially in cylindes.
Chlorine is found largely in seawater where it exists as sodium chloride.
Electrolysis of molten salt, NaCl, also succeeds, in which case the other product is sodium metal rather than sodium hydroxide.