The maximum acceptable concentration (MAC) for trihalomethanes1 (THMs) in drinking water is 0.100 mg/L (100 µg/L) based on a locational running annual average of a minimum of quarterly samples taken at the point in the distribution system with the highest potential THM levels.
The maximum acceptable concentration (MAC) for bromodichloromethane (BDCM) in drinking water is 0.016 mg/L (16 µg/L) monitored at the point in the distribution system with the highest potential THM levels.
Utilities should make every effort to maintain concentrations as low as reasonably achievable without compromising the effectiveness of disinfection.
Trihalomethanes are a group of compounds that can form when the chlorine used to disinfect drinking water reacts with naturally occurring organic matter (e.g., decaying leaves and vegetation). The use of chlorine in the treatment of drinking water has virtually eliminated waterborne diseases, because chlorine can kill or inactivate most microorganisms commonly found in water. The majority of drinking water treatment plants in Canada use some form of chlorine to disinfect drinking water: to treat the water directly in the treatment plant and/or to maintain a chlorine residual in the distribution system to prevent bacterial regrowth. The health risks from disinfection by-products, including trihalomethanes, are much less than the risks from consuming water that has not been disinfected. Utilities should make every effort to maintain concentrations of all disinfection by-products as low as reasonably achievable without compromising the effectiveness of disinfection.
The trihalomethanes most commonly found in drinking water are chloroform, bromodichloromethane (BDCM), dibromochloromethane (DBCM) and bromoform. Of these, chloroform has been most extensively studied, and there are some scientific data available on BDCM. However, insufficient data are available to develop a guideline for either DBCM or bromoform. Since chloroform is the trihalomethane most often found in drinking water, and generally at the highest concentrations, the trihalomethane guideline is based on health risks linked to chloroform. This guideline applies to the total concentration of chloroform, BDCM, DBCM and bromoform.
This Guideline Technical Document reviews all the health risks associated with trihalomethanes in drinking water, incorporating multiple routes of exposure to trihalomethanes via drinking water including ingestion, and both inhalation and skin absorption from showering and bathing. It assesses all identified health risks, taking into account new studies and approaches, and applies appropriate safety factors. Based on this review, the guideline for total trihalomethanes in drinking water is established at a maximum acceptable concentration of 0.1 mg/L.
Although the concentration of BDCM is included in the concentration of trihalomethanes in the guideline, a separate guideline for BDCM is also deemed necessary. The guideline for bromodichloromethane in drinking water is established at a maximum acceptable concentration of 0.016 mg/L.
Chloroform is considered to be a possible carcinogen in humans, based on limited evidence in experimental animals, and inadequate evidence in humans. Animal studies have shown links between exposure to specific trihalomethanes and liver tumours in mice and kidney tumours in both mice and rats; some studies in humans show data that are consistent with these findings. Human studies are suggesting a link between exposure to trihalomethanes and colorectal cancers.
Human studies also suggest a link between reproductive effects and exposure to high levels of trihalomethanes. However, an increase in the concentration of trihalomethanes could not be linked to an increase in risk, suggesting the need for more studies.
Preliminary animal studies indicate that BDCM and other trihalomethanes that contain bromine may be more toxic than chlorinated trihalomethanes such as chloroform. For this reason, and based on the availability of scientific data for BDCM, a separate guideline was also developed for BDCM. BDCM is considered to be a probable carcinogen in humans, with sufficient evidence in animals and inadequate evidence in humans. Animal studies have shown tumours in the large intestine in rats. Among the four trihalomethanes commonly found in drinking water, BDCM appears to be the most potent rodent carcinogen, causing tumours at lower doses and at more target sites than the other three compounds.
Exposure to BDCM at levels higher than the guideline value has also been linked to a possible increase in reproductive effects (increased risk for spontaneous abortion or stillbirth) above what can normally be expected. Further studies are required to confirm these effects.
Levels of trihalomethanes, including BDCM, are generally higher in treated surface water than in treated groundwater, because of the high organic content in lakes and rivers, and will be higher in warmer months, because of the higher concentrations of precursor organic materials in the raw water and especially because the rate of formation of disinfection by-products increases at higher temperatures. Trihalomethanes levels are also affected by the choice and design of treatment processes. Recent data indicate that, in general, average trihalomethanes levels in Canadian drinking water supplies are well below the guideline. However, some systems show average levels well above the guidelines; these systems serve only a small proportion of Canadians (less than 4%) and are generally smaller treatment systems with limited ability to remove organic matter before adding the chlorine disinfectant. It should be noted that the presence of brominated by-products such as BDCM will also depend on the presence of bromine in the source water.
Trihalomethanes and haloacetic acids are the two major groups of disinfection by-products found in drinking water and generally at the highest levels. Together, these two groups can be used as indicators for the presence of all disinfection by-products in drinking water supplies, and their control is expected to reduce the levels of all disinfection by-products and the corresponding risks to health. A guideline for haloacetic acids is expected to be available in 2006-2007.
The approach to reduce exposure to trihalomethanes is generally focussed on reducing the formation of chlorinated disinfection by-products. The concentrations of trihalomethanes and other chlorinated disinfection by-products in drinking water can be reduced at the treatment plant by removing the organic matter from the water before chlorine is added, by optimizing the disinfection process or using alternative disinfection strategies, or by using a different water source. It is critical that any method used to control trihalomethanes levels must not compromise the effectiveness of disinfection. The Federal-Provincial-Territorial Committee on Drinking Water also recommends that every effort be made not only to meet the guideline, but to maintain concentrations of trihalomethanes as low as reasonably achievable.