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Frequently Asked Questions - Trihalomethanes (THMs) facts sheet

  1. What are THMs?
  2. How do the various levels of government in Newfoundland and Labrador ensure the safety of drinking water?
  3. How can I obtain a copy of the current Canadian drinking water quality guidelines?
  4. What are chlorination disinfection by-products and how are they formed?
  5. Why is drinking water chlorinated?
  6. What is the current Canadian drinking water guideline for THMs?
  7. What are the health effects associated with THMs?
  8. Which public water supplies have the highest/lowest levels of THMs?
  9. What are the alternate disinfectants?
  10. What is being done to reduce the levels of THMs in municipal drinking water in Newfoundland and Labrador?
  11. Should I stop drinking my tap water?
  12. Are there risks from CDBPs through showering, bathing or swimming?
  13. How can I reduce exposure to THMs?
  14. How can I obtain information about my drinking water quality?

1. What are THMs?

THMs are disinfection by-products that form when chlorine is added to water that contains elevated levels of natural organic matter such as decaying leaves and vegetation. High THM levels are common for surface-based public water supplies in Newfoundland and Labrador because many of them contain high levels of natural organic matter. Formation of disinfection by-products continue to be an issue in the province and are being addressed through chlorine demand management and exploring various corrective measures. Disinfection is an essential component of public drinking water treatment. The health risks from disinfection by-products, including THMs, are much less than the risks from consuming water that has not been appropriately disinfected.

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2. How do the various levels of government in Newfoundland and Labrador ensure the safety of drinking water?

Both provincial and municipal governments have some level of responsibility in ensuring the safety of Newfoundland and Labrador's drinking water. The provincial government, in cooperation with municipal governments protects source water quality through the watershed protection program. Under a partnership program with municipal governments, the provincial government monitors drinking water quality on a regular basis in order to ensure compliance with the Guidelines for Canadian Drinking Water Quality and to deal with emerging issues on a pro-active basis. The Department of Environment provide the drinking water quality data along with a brief interpretation to municipal governments on a regular basis. The Department is an active member of the Federal-Provincial Subcommittee on Drinking Water (DWS) which is responsible for the development of the Guidelines for Canadian Drinking Water Quality.

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3. How can I obtain a copy of the current Canadian drinking water quality guidelines?

Contact the Department of Environment or check Health Canada's web site Health Canada for a summary table of the current guidelines. Information on the development of these Guidelines can also be found on Health Canada's web site.

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4. What are chlorination disinfection by-products and how are they formed?

Chlorination disinfection by-products (CDBPs) are chemical compounds that form when water containing natural organic matter (the decay products of living things such as leaves, human and animal wastes, etc.) is chlorinated. Chlorine disinfection of water can lead to the formation of a number of chlorination by-products of which trihalomethanes (THMs) are only one subgroup. Among the many chlorination by-products, THMs are most often present and in the greatest concentration in drinking water and as such are used as indicators of total disinfection by-product formation.

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5. Why is drinking water chlorinated?

Chlorination is necessary for two reasons. First, almost all sources of surface water contain microbiological organisms, which have to be removed in order to prevent the outbreak of waterborne diseases such as typhoid fever and cholera. Second, once the treated water leaves the treatment plant, it may travel through water mains and pipes sometimes at significant distances, before it reaches it's destination. During this time, it is necessary to maintain a residual level of disinfectant in the water to ensure no possible regrowth of microorganisms. Without adequate disinfection, the health risks from microorganisms far outweigh the risks from THMs.

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6. What is the current Canadian drinking water guideline for THMs?

The current Canadian drinking water quality guideline for THMs is 100 parts per billion (ppb) or micrograms per litre (µg/l). The guideline is based on an annual running average of quarterly samples to account for seasonal variations. THM levels are generally highest in the summer and lowest in the winter.

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7. What are the health effects associated with THMs?

The Federal-Provincial Subcommittee on Drinking Water established the current guideline for THMs in 1993. The guideline is based on the risk of cancer reported in animal studies of chloroform, the THM most often present and in greatest concentration in drinking water. Since then, new epidemiological (human) studies had been published which reported associations between THMs and bladder and colon cancer, and adverse pregnancy outcomes including miscarriage, birth defects and low birth weight. In response to these new findings Health Canada, in its role as Secretariat to the DWS, established a multi-stakeholder task group in 1998 to oversee a comprehensive update of health risk information on THMs and to develop recommendations for controlling the risks.

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8. Which public water supplies have the highest/lowest levels of THMs?

Levels of THMs are generally highest in treated water from sources with high organic matter content, such as rivers and lakes. Lower levels of THMs are usually found when the source water is groundwater.

THM levels can vary within single water supply depending on the season, water temperatures, amount of natural organic matter in water, pH, amount of chlorine added, point of chlorination, time in distribution system, and other factors such as treatment processes used.

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9. What are the alternate disinfectants?

Alternate disinfectants include chloramine, chlorine dioxide and ozone. Each of these alternate disinfectants have their own advantages and disadvantages regarding handling and storage, disinfection by-product formation and cost. The use of chlorine is, however, essential to maintain the required residual in the water distribution system in order to ensure microbiologically safe water.

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10. What is being done to reduce the levels of THMs in municipal drinking water in Newfoundland and Labrador?

The government of Newfoundland and Labrador in consultation with municipal governments has developed a three-phase approach to deal with this issue. The first phase is data collection through THM surveys, the second phase deals with data assessment and identification of remediation methods and the third phase will be the implementation of mitigation measures where necessary. It must be emphasized that any changes made to water treatment practices must not compromise the effectiveness of disinfection.

The government of Newfoundland and Labrador is also actively participating, with its provincial colleagues of the DWS in the Chlorinated Disinfection By-Products Task Group that is overseeing a coordinated effort to estimate the health risks from THMs and to develop risk management recommendations.

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11. Should I stop drinking my tap water?

Tap water provided by municipal governments is generally safe and regularly monitored by the provincial government for physical, chemical and bacteriological quality. You do not need to stop drinking tap water unless you are have been advised to do so by the provincial or municipal governments.

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12. Are there risks from CDBPs through showering, bathing or swimming?

While showering, bathing or swimming in chlorinated water may result in significant exposure to CDBPs through breathing in vapors and absorption through the skin, the health risks of prolonged exposure to CDBPs from these sources are currently unknown. Research is in progress to better understand the contribution of inhalation and skin absorption from showering in overall exposure to CDBPs.

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13. How can I reduce exposure to THMs?

Consumers wishing to reduce their exposure to chlorination disinfection by-products can use a filter containing activated carbon certified to the NSF Standard 53 for THM removal. If a filter device is used it should be properly maintained because such devices can become sources of bacterial contamination in water. Although blending and boiling water will remove volatile (meaning easily evaporated) CDBPs such as THMs, they do not eliminate or necessarily reduce the health risks of other CDBPs that may not evaporate easily. As such, blending and boiling of water are not recommended by Health Canada as methods for reducing chlorination disinfection by-products.

Health Canada laboratories are currently testing a range of carbon filters and other treatment methods to see if they are able to remove most CDBPs. The results will be made public within a year.

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14. How can I obtain information about my drinking water quality?

Contact your Town Council office or call the Department of Environment at:

  • (709) 729-2563 (Eastern)
  • (709) 292-4220 (Central)
  • (709) 637-2542 (Western and Labrador)

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