Report No FR0288

G O'Neill, J K Fawell

March 1992



To review the toxicology of the disinfectants chlorine, ozone, chlorine dioxide and chloramine and their principal by-products formed during water treatment. To determine whether any of the by-products discussed may, in the future, limit the use of any of the disinfectants.


Increasing regulation of chlorination by-products and the continuing concern, particularly in the USA, over the possibility of a low level of risk to consumers has prompted more research on various alternative disinfectants to chlorine such as ozone, chlorine dioxide and chloramine. Therefore there is a need for data on by-products formed by chlorine and alternative disinfectants and, in particular, their health effects.


  1. All disinfectants which rely on chemical oxidation produce by-products or breakdown products which could place constraints on their use.

  2. At present chlorate and bromate appear to be of greatest concern in this regard.

  3. There are many gaps in the available data on occurrence, concentrations and toxicology.

  4. The risk assessment process is based on imperfect knowledge and corresponding risk/benefit and cost/benefit assessments are essential.

  5. Regulations in Europe for a range of disinfection by-products are possible in view of the activity in North America and by the World Health Organization in this area.


  1. Basic toxicity studies of chlorate should be carried out with some urgency, although there will be great difficulties in doing this.

  2. The mechanism of carcinogenicity of bromate requires urgent investigation.

  3. Data on the concentrations of bromate in treated waters and the conditions under which it is formed are urgently needed .

  4. Studies on the mechanism of carcinogenicity of some other disinfection by-products such as trichloroacetic acid (TCA), dichloroacetic acid (DCA) and chloral hydrate should be considered.

  5. Studies on the toxicity of MX and other by-products giving strong positive results in bacterial mutation assays should continue with regard to carcinogenicity and foetotoxicity.


This report reviews the toxicology of the disinfectants chlorine, ozone, chlorine dioxide and chloramine and their principal by-products and considers how this may affect the use of these disinfectants. It would appear that all of these disinfectants, which are oxidising chemicals, will produce by-products or breakdown products which may limit their use. Bromate and chlorate would appear to be the by-products of most concern, although there are many gaps in the toxicological database of both these and other by-products.

The implications of these findings are discussed and recommendations are made. A table of the by-products considered and the likelihood of their future regulation is included.

Copies of the Report are available from FWR, price 15.00 less 20% to FWR Members