>Chlorine cannot 'break down' it is an element.

        Surely Dave Cushman, G8MZY can't be challenging me for the title of
'chemical heavy pedant' on this list?  :-}

        What Dave said is true, but only in a pedantic sense.  Elemental
chlorine Cl2 when dissolved in typical water supplies does react
chemically, breaking down some of those chlorine molecules and transferring
some of the chlorine atoms to create e.g. chlorinated phenols which are
reasonably suspected of causing cancer.

R