Water Disinfectants & disinfectant by-products

5. What are the risks posed by disinfectants and their by-products?

• 5.1 What tolerable daily intakes and guideline values has the WHO set?
• 5.2 How much disinfectants and by-products are we exposed to?
• 5.3 Are there uncertainties in assessing exposure?
• 5.4 Have epidemiological studies been able to evaluate the risks?

5.1 What tolerable daily intakes and guideline values has the WHO set?

For disinfectants and disinfectant by-products (DBPs) with sufficient data, the World Health Organisation (WHO) has set safe acceptable intake levels. These levels are called Tolerable Daily Intakes (TDIs). TDIs are determined by taking the level at which no harmful health effect has been observed (No Observed Adverse Effect Level or NOAEL) in a particular animal species (e.g. mice, rats, hamsters) and applying an uncertainty factor (in this case, between 100 and 1000 depending on studies available and confidence in the results).

These TDIs are used as a basis for developing the WHO drinking water guidelines that are, in turn, used by many countries, including the European Union (EU), as a basis for their drinking water standards. The guideline values are typically derived from the TDI assuming the average adult weighs 60 kg and drinks 2 litres of water per day.

The following tables present TDIs for disinfectants and disinfectant by-products and the NOAELs from which they were derived. In addition to the data from the IPCS report, presented in the first two columns, a third column is added, which is based on the "Third edition of the WHO Guidelines for Drinking-Water Quality" 2004, providing the most up-to-date guideline values in terms of concentrations in drinking water.

5.1.1 Disinfectants More...

Source & ©: * Source: "Third edition of the WHO Guidelines for Drinking-Water Quality" 2004 

5.1.2 Chlorine by-products More...

Table 2. Intake reference values for chlorine by-products

Source & ©: * : "Third edition of the WHO Guidelines for Drinking-Water Quality" 2004 

5.1.3 Chlorine dioxide by-products

Source & ©: * : "Third edition of the WHO Guidelines for Drinking-Water Quality" 2004 

Notes referring to the above table

1: The Tolerable Daily Intake (TDI) for chlorite and for chlorate has translated into a WHO provisional drinking water guideline value. The TDI reflects current lack of a specific long-term study which is in progress. More...

5.1.4 Ozonation by-products More...

Source & ©: * : "Third edition of the WHO Guidelines for Drinking-Water Quality" 2004

Notes referring to the above table

1: At present there are conflicting data regarding the exact mechanism by which bromate causes cancer in laboratory animals and how that would impact on risk assessment. The WHO has developed a drinking water guideline value of 10 µg/litre based on two approaches and taking into account the practicality of using ozone. Where bromide is present in the raw water, the guideline value can only be achieved by appropriate control of disinfection conditions.

5.2 How much disinfectants and by-products are we exposed to?

Typically, several milligrams of disinfectant per litre (mg/l) of water are employed in the system, corresponding to the dose necessary to kill the microorganisms in the treatment plant (primary disinfection) and the dose necessary to maintain a residual disinfection in the distribution system (secondary disinfection). The concentration at the tap is considerably less than initially injected.

The concentrations of disinfection by-products will vary according to the raw water quality and its physical and chemical properties such as pH and temperature, and the level of drink-water treatment.

Chlorine by-products:

• Trihalomethanes (THMs) are normally dominated by chloroform but the levels of brominated THMs will depend on the bromide present in the water.
• The concentration of total haloacetic acids (HAA) is generally about half of the total THMs but it is quite difficult to generalize. Dichloroacetic acid (DCA) and trichloroacetic acid (TCA) are the first and second most dominant species.
• If a hypochlorite solution is used instead of chlorine gas, chlorate may also occur during chlorination.

When the disinfectant is chloramine, the by-products depend on the mode of chloramination, application of ammonia followed by chlorine leading to lower levels of chlorine by-products.

Bromate is primarily formed by ozonation of water containing bromide: about 50% of the bromide is converted to bromate. In European water treatment plants, the concentration of bromate was found to be a maximum of 16 mg per litre.

When the disinfectant is chlorine dioxide, about 50 to 70% of the applied dose forms chlorite. More...

5.3 Are there uncertainties in assessing exposure?

Toxicological studies attempt to extrapolate the results of laboratory animal studies to humans. This may lead to an estimation of risk factors for some health effects. Epidemiological studies attempt to link human health effects (e.g. cancer) to a cause (e.g. exposure to a disinfectant by-product, DBP) and require exposure assessments.

Humans can be exposed to chemical risks from disinfected drinking-water through several routes:

• ingestion of disinfectant by-products (DBPs) in drinking-water;
• ingestion of drinking-water that has a residual of free chlorine at the tap and the resulting formation of DBPs in the mouth and stomach (ozone cannot be present at the tap and chloramines and chlorine dioxide do not react to form by-products);
• inhalation of volatile DBPs during showering.

However, it is generally assumed that the most significant route is the ingestion of DBPs.

Human health effects depend on both DBP concentration and duration of exposure. They are difficult to assess because DBP mixtures:

• are complex, e.g. trihalomethanes (THMs) versus haloacetic acids (HAAs), chlorinated versus brominated species ;
• change with time, as a function of temperature, nature and concentration of natural organic matters (NOM) in the source water ;
• change with location, i.e. throughout a distribution system.

For epidemiological studies, some historical databases exist for disinfectant doses (e.g. chlorine) and possibly for DBP precursors (e.g. total organic carbon, TOC) and THM concentrations. In contrast, data for HAAs, bromate and chlorite are much more recent and hence sparse. When DBP data is not available, exposure can be estimated indirectly from the chlorine dose and other data. More...

5.4 Have epidemiological studies been able to evaluate the risks?

5.4.1 Epidemiological studies are a very valuable tool in determining the possible risks from exposure to environmental chemicals. However, there are limitations to the use of Epidemiological studies because it is generally difficult to measure individual exposure to the chemical accurately and because there are usually other causes of the effect of interest, such as smoking or diet. This is also true for epidemiological studies of disinfectant by-products (DBPs). There is often no information on individual water consumption and generally very little information on the concentrations of DBPs other than trihalomethanes (THMs).

Another important point for epidemiology is the consistency of the results between different studies. There have been more studies looking at bladder cancer than any other cancer but the evidence is not consistent. Some studies show an increased risk while others do not. In those that do show an increased risk, there are differences in the risk reported and between the sexes and smokers and non-smokers. More...

5.4.2 The epidemiological studies available provide insufficient evidence to establish that water disinfectants and their by-products cause any of the observed effects, mainly because of incomplete information about exposures to specific water contaminants.

Risks may be due to other water contaminants or to other factors not taken into consideration. More...