Halogens bind so well to other atoms that great energy has to be applied if they are to be separated from their compounds. This may be just as well, because all of the halogen elements are very dangerous. As compounds, though, the halogens are harmless, even beneficial. Think of the halogens as health-givers. Iodine salts are essential to prevent thyroid gland problems, fluoride strengthens your teeth, and chlorine salts can be used to preserve food and disinfect water supplies.
Halogens also make up many of the commonly used anaesthetics used in surgery.
It forms an amber-coloured liquid at its boiling point. Nearly one-fiftieth of sea water is made of chlorine, where it occurs combined with sodium as sodium chloride. People become sensitive to the presence of chlorine in concentrations as little as three to five parts per million because it irritates the membranes of the nose. At concentrations of 30 parts per million its effects on the eyes and nose become severe. At only slightly higher concentrations, it causes difficulty in breathing.