Page 3: Guidance on Chloral Hydrate in Drinking Water

Part A - Guidance on chloral hydrate in Canadian drinking water supplies

Chloral hydrate is a chlorinated disinfection by-product, and is also used in human and veterinary medicine as a sedative. Because exposure levels in Canada are far below any concentration that would cause health effects, the Federal-Provincial-Territorial Committee on Drinking Water has determined that there is no need to establish a guideline for chloral hydrate in drinking water and has chosen instead to make the results of the risk assessment available through this guidance document.

Chloral hydrate is a disinfection by-product of the drinking water treatment process. It is a compound that can form when the chlorine used to disinfect drinking water reacts with naturally occurring organic matter (e.g., decaying leaves and vegetation). However, it is formed at concentrations that are significantly lower than those that could pose health risks.

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.

Chloral hydrate is also used in human and veterinary medicine as a sedative and in the manufacturing of pharmaceuticals, pesticides, and other chemicals. Chloral hydrate can occur in drinking water if it is released into the environment from these processes or as a by-product from the drinking water treatment process. However, the amount typically found in drinking water is well below the level at which health effects may be observed.

There are no studies linking exposure to chloral hydrate with significant health effects in humans, including cancer, despite the fact that chloral hydrate has been used for many decades (and still is used) as a sedative and hypnotic drug in adults and children for dental procedures. Only one study has linked chloral hydrate with an increase in the number of tumours in mice.

A health-based value of 0.2 mg/L (200 µg/L) can be derived for chloral hydrate in drinking water, based on a lifetime study in mice, which showed liver cell abnormalities at lower doses and a slight increase in the number of tumours at higher doses. Uncertainty factors were included to account for the possibility that chloral hydrate may be a human carcinogen. There are no long-term studies of chloral hydrate exposure in humans.

Surveys conducted in 1995 and 1997 show levels of chloral hydrate in Canadian drinking water supplies ranging from 1.2 to 3.8 µg/L in winter and from 3.6 to 8.4 µg/L in summer, with a maximum level of 22.5 µg/L observed in winter. Although slightly higher levels may be associated with smaller-scale treatment plants with a limited ability to remove organic matter prior to the addition of the chlorine disinfectant, these levels are still expected to be much lower than any level of concern.

Chloral hydrate can be analysed by gas chromatography/electron capture detection with a detection limit of 0.005 µg/L. Levels of chloral hydrate in drinking water can be reduced by removing DBP precursors, through enhanced coagulation and softening, by moving the point of disinfection to reduce the reaction between chlorine and DBP precursors, and by using chloramines instead of chlorine for residual disinfection. There are currently no certified drinking water treatment devices for residential use that specifically remove chloral hydrate.

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