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Safe hot tap water and the risk of scalds and legionella infection
  1. R Hockey
  1. Senior Data Analyst, Queensland Injury Surveillance Unit, Mater Hospital, South Brisbane QLD 4101, Australia; rhockey{at}mater.org.au

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    The recent paper in Injury Prevention by Jaye et al on the barriers to safe hot tap highlights the problem I raised last year regarding the perception of plumbers of the risk of legionella infection from hot water systems compared with that of scalds.1 The paper reports that half the respondents thought it more important to control for legionella than to prevent hot tap water burns. Much of the blame for this perception can be attributed to the Australian and New Zealand Standard (AS/NZS 3500.4.2:1997) which sets a minimum temperature of 60°C for hot water storage systems to protect against legionella.2

    In 1994 the standard was amended to require that the delivery temperature be a maximum of 50°C in personal hygiene areas in domestic installations through the installation of tempering devices. Several years ago the NSW Heath Department tried, as part of its “Burns like fire” campaign, to have this minimum revised down to 50°C and presented compelling evidence supporting the view that there would be no increased risk from legionella at this temperature.3

    This view is also reinforced by the situation in the US where the maximum storage temperature is set at 50°C, a measure that has resulted in a reduction in scalds without any discernible increase in legionella infections.4,5 Unfortunately the Joint Technical Committee for Plumbing Standards, which not unexpectedly is dominated by plumbers, dismissed this evidence and we are left with a confusing, contradictory, and expensive standard when a simple solution exists. Setting the storage temperature at 50°C, which is also easily implemented in most existing homes, would also protect children and more particularly elderly persons from hot water burns in the kitchen and laundry (a not insignificant problem).

    I would also contend that in a warm country like Australia, where during the summer months cold water is often delivered at temperatures exceeding 30°C, it is likely that cold water is a more significant source of legionella infection (the ideal temperature for the growth of legionella is 20–43°C).6

    The other issue that was touched on in the paper was the reliability of tempering valves, something which plumbers I have spoken to have alluded to and needs further investigation.

    Both scalds and legionella are important public health problems, however, very little is known about the contribution of a domestic hot water supply to legionella. No one would deny that hot water systems are a source of infection, with up to 30% of systems testing positive to the organism, but the one study published examining domestically acquired legionella failed to show a relationship between hot water heater temperature and the disease.7 On the other hand the evidence of the association between hot water temperature and scalds is compelling.

    I hope other countries can learn from Australia's experience.

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