Consumer Concerns
Disinfection By-products
What are they?
Disinfection By-Products (DBP) are potentially toxic chemical compounds that are formed in extremely low concentrations during the disinfection of water supplies. They include chloroform and chloroacetic acids formed during chlorination, and bromate which is a by-product of ozone treatment. They are the products of the reaction between disinfectants and natural, or sometimes man-made, organic and inorganic substances present in water sources.
Why the concern?
Studies on laboratory animals using very high doses of DBP have shown that some, such as chloroform and bromate, are implicated in the development of cancers. Concentrations of DBP in drinking water are extremely low but it has been suggested that prolonged exposure to very low doses might have the same effect as short-term exposure to high doses.
Concerns have also been raised about possible links between exposure to chlorinated water (containing chlorinated DBPs) and adverse pregnancy outcomes. These concerns have been investigated and a large national study found no links, the conclusions of the study are outlined below.
What is being done about DBP in drinking water?
It is not possible to simply stop using disinfectants. The introduction in the UK of general disinfection of water supplies after the Croydon typhoid outbreak in 1936, reduced dramatically the incidence of drinking water related illnesses and eliminated deaths from cholera and typhoid. The theoretical risk of DBP toxicity must be weighed against the certainty that water that has not been disinfected can cause illness and even death.
Exposure to DBPs is controlled by means of the European standards for trihalomethanes (which include chloroform) and bromate that are enforced by the Inspectorate. The standards are set in the European Commission's Drinking Water Directive and the Inspectorate has advised water companies to optimise disinfection processes in order to minimise DBP concentrations in drinking water. The Inspectorate is responsible for checking whether water companies are complying with the standards and will take action if water companies fail to comply.
DWI research on DBP
A number of epidemiological studies have reported a relationship between consumption of chlorinated drinking water and small increases in the incidence of certain types of cancer. In these studies the rates of illness in the community was compared statistically with consumption of water and exposure to DBP. DWI maintains a small programme of research to investigate the conclusions of these studies and receives expert advice from the Health Protection Agency and the independent scientific committees that advise government.
There are significant difficulties with the design and interpretation of drinking water epidemiological studies. Everyone drinks water and it is very difficult to isolate the drinking water contribution of DBP from other sources of exposure to carcinogenic chemicals e.g. smoking or workplace exposure. It is also very difficult to estimate either the drinking water consumption or the concentration of DBP in the water consumed over a particular period.
Studies which seek to investigate a particular relationship also tend to suffer from sources of bias which favour a particular conclusion e.g. the global assumption that everyone drank 2 litres of water per day is often made when retrospectively assessing mortality statistics. In reality, there is no certainty as to what amount of water was consumed and no allowance is made for the high proportion of water consumed in hot beverages, where the heating causes significant loss of volatile DBP such as chloroform. Such studies also fail to take account of other significant water related exposures to DBP e.g. inhalation in swimming pools.
The Inspectorate has received advice from the Department of Health's expert committees. This advice confirms that disinfection is vital to the prevention of waterborne disease and that, providing steps are taken to minimise exposure to DBP, there are no grounds for changing current disinfection practices. This advice is contained in DWI Information Letter 12/99 that is posted on the Inspectorate's website under Regulation of Water Companies - Information Letters. All the new epidemiological studies on chlorinated drinking water and cancer published since 1999 were reviewed by the the Committee on Carcinogenicity of Chemicals in Food, Consumer Products and the Environment, and they produced their 2nd Statement on Chlorinated Drinking Water and Cancer (COC/08/S1 – May 2008). This reached broadly the same conclusions as in 1999.
Recently DWI part funded a study conducted by the Small Area Health and Statistics Unit (SAHSU) which examine possible links between THM concentrations and adverse pregnancy outcomes. This large national study found no statistically significant trends across exposure categories for either broadly defined or more restricted sets of anomalies and concluded that there is little evidence for a relationship between THM concentrations in drinking water and risk of congenital anomalies. The full study can be found on the DWI website http://www.dwi.gov.uk/research/reportlist.shtm under the heading “Drinking Water Quality and Health”.
This and other studies have been considered by the Committee on Toxicology and they concluded that in human studies there is no consistent relationship between chlorinated drinking-water and adverse pregnancy outcomes, including low birth weight, pregnancy loss, pre-term delivery and congenital malformations. In animal studies, effects have largely been seen at high doses associated with maternal toxicity and these are not considered to be predictive of effects in humans exposed to far lower levels of DBPs full detail of the CoT assessment can be viewed on the CoT website at: http://cot.food.gov.uk/pdfs/cotstatementchlorwater0408.pdf
