We’ve said for a decade hot water is not a factor in reducing microbial loads during handwashing. Friend of the blog Don Schaffner at Rutgers agrees.

And now, so do researchers at Tennessee’s Vanderbilt University.

We admit, warm water is often a preference, but scientifically, it does not lower microbial loads.

And we all want to be evidence-based.

As reported in the European Cleaning Journal, using hot water for hand washing is unnecessary while potentially being harmful for the environment, but nearly 70 per cent of Americans believe hot water to be more effective than cold or warm water – despite having no evidence to back this up.

According to research assistant professor Amanda Carrico: “It is certainly true that heat kills bacteria, but if you were to use hot water to kill them it would have to be way too hot for you to tolerate.”

She explains that pathogens can be killed by water at temperatures of 99.98°C – but hot water for hand washing is generally between 40°C to 55°C, and even at these temperatures the sustained heat required to kill some pathogens would scald the skin.

Carrico’s team found water as cold as 4.4°C to be just as effective at reducing bacteria as hot water if the hands were scrubbed, rinsed and dried properly. And they noted that hot water could even have an adverse effect on hygiene. “Warmer water can irritate the skin and affect the protective layer on the outside, which can cause it to be less resistant to bacteria,” said Carrico.

And she adds that no water temperature is specified in official guidelines from the Centre for Disease Control and Prevention nor the World Health Organization, which simply recommend using soap and water and scrubbing vigorously for at least 20 seconds followed by a thorough dry.

Now, about that 20 seconds …