Drinking water which has been chlorinated to kill microbes may contain trace levels of chlorine. The World Health Organization recommends an upper limit of 5 ppm.
Although low, 5 ppm is enough to slowly attack certain types of plastic, particularly when the water is heated, as it is for washing.
Polyethylene, polybutylene   and acetal resin (polyoxymethylene) pipework and fittings are all susceptible. Attack leads to hardening of pipework, which can leave it brittle and more susceptible to mechanical failure.