Guidance for Issuing and Rescinding Boil Water Advisories
2001
Help on accessing alternative formats, such as PDF, MP3 and WAV files, can be obtained in the alternate format help section.
 (85K)
Table of Contents
Decisions concerning boil water advisories can only be made at the local level based upon site-specific knowledge and conditions. The purpose of this guidance is to provide water purveyors and health and environment authorities with instructions on how to boil or disinfect tap water during a boil water advisory and to summarize the important factors that should be considered before boil water advisories are issued or rescinded.
Communication Processes
An emergency team consisting of those responsible for source water protection, treatment plant operation and distribution, water quality monitoring and public health surveillance should be in place to quickly respond to any drinking water-related incident that has had or may have had an effect on water quality or public health. This arrangement will allow for the rapid exchange of information so that remedial measures that may be required in the watershed, at the treatment plant or in the distribution system can be instituted without delay. Depending on the seriousness of the incident, the public health representative or other designated member of the team may issue a boil water advisory. The team should also have criteria in place to determine when a boil water advisory can be rescinded. The arrangement should also allow for prompt communication of the boil water advisory and related health risks to elected officials, the news media and the general public. Detailed instructions should also be provided to the operators of health care institutions, day care centres, schools, hotels, restaurants, public pools and spas, supermarkets and other public facilities.
Instructions for Boiling and Disinfecting Tap Water
During a boil water advisory, it is essential that all water destined for drinking, preparing infant formulas, juices and ice cubes, washing fruits and vegetables, cooking or dental hygiene be boiled. Recent research indicates that holding water at a rolling boil for one minute will inactivate all waterborne pathogenic microorganisms.1 Water can be boiled in a pot or kettle on a stove, in an electric kettle without an automatic shut-off or in a microwave oven. If water is boiled in a microwave oven, it is advisable to include a glass rod or wooden or plastic stir stick in the container to provide nucleation sites for bubble formation and energy diffusion and prevent the formation of superheated water. Under most circumstances, it is not necessary to boil water used for other household purposes. Adults, adolescents and older children may shower, bathe or wash using tap water, but should avoid swallowing the water. Toddlers and infants should be sponge bathed. Some local health authorities recommend that water for bathing and showering not contain more than 200 E. coli / 100 mL as specified in the Guidelines for Canadian Recreational Water Quality.2 In non-outbreak situations, dishes and laundry may be washed in tap water, either by hand or by machine.
In the event of a waterborne outbreak, it may be necessary to advise the public to take additional precautions. In this situation, hands should be washed in a dilute solution of household bleach and water (1 mL or 20 drops of bleach per litre of water). This is particularly important before preparing or eating meals and after using the toilet, changing diapers and handling animals. The solution should be allowed to stand for 10 minutes before use. If dishes are hand washed, they should be washed and rinsed in hot tap water, soaked in a dilute solution of household bleach (20 mL of bleach in 10 L of water) for one minute and air dried. Alternatively, dishwashers with a hot water cycle will disinfect dishes. These precautions should reduce the possible spread of illness and at the same time minimize the need to boil tap water and the risk of serious burns and scalds.
Issuing a Boil Water Advisory
In most cases, boil water advisories are issued: a) on evidence of conditions such as: significant deterioration in source water quality; equipment malfunction during treatment or distribution; inadequate disinfection or disinfectant residuals; unacceptable microbiological quality; unacceptable turbidities or particle counts; situations where operation of the system would compromise public health; or b) where epidemiological evidence indicates that the drinking water is or may be responsible for an outbreak of illness.
Boil water advisories are most frequently based on unacceptable bacteriological quality. In some jurisdictions, advisories are issued exclusively in response to the presence of E. coli. In others, they are issued on the confirmed presence of total coliforms or thermotolerant coliforms. The confirmed presence of total coliforms or thermotolerant coliforms in the absence of E. coli in the distribution system, but not in water leaving the treatment plant, usually indicates that the distribution system is experiencing bacterial regrowth problems. As total coliform bacteria are ubiquitous in nature, their presence in the distribution system does not necessarily indicate a health risk. Nevertheless, if remedial measures, such as flushing water mains and increasing chlorine residuals, do not correct this problem, then the local authorities may wish to issue a boil water advisory after consultation with the water purveyor and appropriate municipal officials. Conversely, the presence of E. coli is a definite indication of human or animal faecal contamination and the possible presence of pathogenic microorganisms. If E. coli are found in drinking water, a boil water advisory should be issued immediately. Additional guidance in response to the presence of E. coli in drinking water can be found in the Bacteriological Quality supporting document.
Some authorities prefer to use the presence of thermotolerant coliforms (faecal coliforms) as a trigger for the issuance of boil water advisories. However, as some species in this group — for example, Klebsiella pneumoniae — occur naturally in vegetation and soils as well as in faeces, they are not absolute indicators of faecal contamination. When thermotolerant coliforms are detected, their identity should be determined. If E. coli are present, a boil water advisory should be issued immediately; if species other than E. coli are present, it is likely that the distribution system is experiencing bacterial regrowth. The water purveyor may wish to attempt to correct the problem before any decision on the issuance of a boil water advisory is made. The possible negative consequences of boil water advisories — for example, the risk of scalding, especially to young children and elderly people — should also be considered.
The mere presence of parasitic cysts or oocysts in treated drinking water is not usually sufficient justification for issuing a boil water advisory. Because the current methods for the routine detection of cysts or oocysts do not measure viability or human infectivity, their public health significance is unknown. Nevertheless, the presence of cysts or oocysts in drinking water receiving full conventional treatment may indicate inadequate filtration, a malfunction in treatment or penetration of sewage into the distribution system. In such cases, health officials may wish to monitor the general public for the associated gastrointestinal illnesses before considering issuance of a boil water advisory. Certain parasitic illnesses — for example, cryptosporidiosis — may pose a more serious threat to people who have weakened immune systems. Severely immunocompromised individuals should be advised to discuss these risks and remedial measures with their physicians.
Rescinding a Boil Water Advisory
Boil water advisories are usually rescinded:
a) as soon as the microbiological quality, turbidity, particle counts or disinfectant residual of the treated water in at least two consecutive sets of samples has returned to an acceptable level; or
b) when the treatment, distribution or operational malfunction has been corrected and sufficient water displacement has occurred in the distribution system to eliminate any remaining contaminated water.
In the case of an outbreak, advisories are usually rescinded after the above conditions have been met and when surveillance indicates that the incidence of the illness in the community has returned to background levels. Owing to lengthy incubation periods for some pathogens and their secondary spread, new cases of illness may occur after the period of contamination has passed. Conversely, a lack of new cases may indicate that the boil water advisory is being followed and not that the causative situation has been rectified.
Other Drinking Water Quality Advisories
There are certain water quality problems that cannot be rectified by boiling, yet for which boiling has been considered or advised as a solution. For example, boiling will not destroy the heat-stable cyanobacterial toxins. Boiling drinking water may remove some disinfection by-products (DBPs), but the exact conditions for their complete removal are unknown at this time. Many DBPs are not volatile and are not removed by boiling. For these reasons, the most appropriate position to take is that boiling has not been shown to reduce the health risks related to cyanobacterial toxins and DBPs and therefore cannot be recommended as a solution at the present time. It has not been demonstrated that boiling increases any negative health consequences from cyanobacterial toxins or DBPs.
References
1. Working Group on Waterborne Cryptosporidiosis.
Cryptosporidium and Water: A Public Health Handbook. American Water Works Association, Denver, CO (1997).
2. Health and Welfare Canada. Guidelines for Canadian
Recreational Water Quality. Minister of Supply and Services Canada (1992).
Guidelines for Canadian Drinking Water Quality: Supporting Documentation
|
