Residential Water Heater Temperature: 49 or 60 Degrees Celsius?

Abstract

here are two opposing risks when it comes to water tem-perature inside domestic water heaters; exposure toLegionella, the bacteria responsible for Legionnaires’ disease(pulmonary legionellosis), and the risk of scalding. In 1986,this dilemma was the subject of an editorial in the CanadianMedical Association Journal(1). A few months ago, Safe KidsCanada launched a media campaign aimed at preventingscalding by lowering domestic hot water temperature to 49°Cat the tap (2). Among the means considered to reach thisobjective, Safe Kids Canada, with the support of some publichealth organizations, suggests and seems to favour lowering thetemperature setting of domestic hot water heaters to 49°C.Like other authors (3,4), including the World HealthOrganization (WHO) who published a recent monograph onthe Legionella problem in drinking water (3), we believe thatthere is evidence for the transmission of legionellosis throughthe drinking water distribution systems in private homes. Thisis a serious illness associated with high death rates (up to 12%).Primary groups at risk (the elderly, smokers, the immunocom-promised and patients suffering from chronic respiratory ill-nesses), are groups who include a large proportion of thepopulation at home. Although we support prevention againsttap water scalds, we are against setting water heater thermo-stats at 49°C because we believe this could facilitate prolifera-tion of Legionella inside the tank and increase the risk oflegionellosis.Domestic water heaters, particularly electric devices, can cer-tainly be contaminated by Legionella. In Quebec, a study of 211homes (178 electric water heaters, 33 oil or gas water heaters)found Legionella contamination in 40% of electric water heaters.No water heaters using fossil fuels were contaminated (5). Theauthors concluded that, because of design variables, use of anelectric water heater was the most significant factor leading toLegionella contamination in hot water (5) in the home.The clinical and epidemiological significance of this find-ing is much debated. However, in a case-control study of spo-radic cases of community-acquired legionellosis, Straus et al(6) concluded that the residential drinking water supply wasresponsible for a substantial proportion of sporadic cases ofLegionnaires’ disease. These findings are supported by Stout et al(7) in a study of 20 Pittsburgh patients with culture-confirmed Legionnaires’ disease. A link with residential drink-ing water contamination was established for eight (40%)patients. This included three private homes (one singledwelling, two multidwellings), two senior-citizen homes, twoout-patient hospital clinics, and one industrial plant. Theauthors concluded that drinking water distribution systemswere a significant source of transmission of Legionnaires’ dis-ease (7).The importance of Legionnaire’s disease is underestimatedbecause it is difficult to diagnose and because it is reportedthrough a passive surveillance system. In an active surveillancestudy of pneumonia requiring hospitalization in Ohio, the inci-dence of Legionnaire’s pneumonia was estimated to be approx-imately seven cases per 100,000 people (8). With theobservations from the Stout et al (7) study, if active surveil-lance was performed, an estimated two cases of Legionnaire’sdisease per 100,000 people per year could be attributable topotable water in private homes and senior-citizen residences.This is at least of the same order of magnitude as the annualrates of 0.45 per 100,000 for hospitalization and 0.043 fordeath due to scalding by tap water in Quebec (9).The optimal temperature for Legionella proliferation inwater varies between 32°C and 35°C, but it can easily prolifer-ate at temperatures of up to 45°C. Usually, there is no growthabove 55°C, and a temperature of over 60°C has a bactericidaleffect. Thus, the WHO recommends that water be heated andstored at 60°C (3). However, studies in Quebec have shown,even when the thermostat is set at 60°C, a high percentage(approximately 40%) of electric water heaters remain contam-inated because of the lower temperature, about 30°C to 40°Cat the bottom of the tank. The probability of contaminationwill increase considerably if the temperature setting is loweredto 49°C. The risk of contamination is much lower for waterheaters operating with fossil fuels, and is practically nonexist-ent for these heaters set at 60°C.In our opinion, it is important to reduce both the risk ofscalds and the risk of legionellosis associated with domesticwater supplies (9). For water heaters servicing a single housingunit, electric water heater manufacturers need to market, asquickly as possible, water heaters resistant to proliferation ofLegionella. At the least, all new water heaters must be preset at60°C and equipped with antiscald devices to deliver water at49°C to the entire household. Electric water heaters alreadyinstalled should be set at 60°C to limit the risk of Legionellacontamination. Gas or oil water heaters already installed