Chlorination of water has probably saved millions of lives (see biological hazards). Some concern has been raised about possible increased cancer risks in association with chlorinated water but there is as yet no proof that a causal association between the two exists. Fluoride added to water reduces the risks of caries but can also have unwanted effects such as mottling of the teeth.
The accidental tipping of a large quantity of aluminium sulphate in the Camelford incident a few years ago resulted in acute health effects although there is controversy about chronic effects. Nitrate in water usually arising from fertiliser leaching (natural or artificial) can increase the risk of methaemoglobinaemia ('blue babies') in bottle fed infants but this is extremely rare. Although pesticides can and do leach into water, there is no evidence that the current standards for water quality are inadequate in this respect, but most standards are based on evidence other than human epidemiology which in this context is extremely difficult to conduct.
Beyond the point of supply further problems in drinking water quality
may result. Thus for example water tanks containing lead may increase the
burden of this metal in the water, while water softeners may increase its
sodium content (This can be harmful for bottle fed infants).
Non-bacterial microbiological contamination may arise from organisms such as Cryptosporidium and Giardia.
There can be other opportunities for further bacteriological contamination. Thus Legionella can grow in sumps or dead legs in the plumbing system and may then be dispersed as aerosols from showers.
Recreational water which is heavily contaminated with pathogens, notably coliform bacteria has been shown to be associated with an increased risk of gastrointestinal and other infectious illness, usually self-limiting. There is evidence that the organisms causing many of these cases of ill-health are not necessarily the ones that are tested for, in order to establish the quality of the water. cryptosporidium