Cold Water And Regular Soap Kills Germs Just As Well As Hot Water

It’s conventional wisdom that to really kill germs and disinfect your hands, you should be using water as hot as you can stand it and antibacterial soap, right? Not so, according to researchers at Vanderbilt University. In fact, cold water and regular soap can be just as effective. Here’s why.

Photo by Katherine Johnson.

It may be more comfortable to wash your hands with hot water, and it may be better at getting debris and dirty off of your hands than cold water, but when it comes to making sure your hands are germ free, the hot water from your tap just isn’t hot enough to make a meaningful difference. Amanda R. Carrico, a research assistant professor at the Vanderbilt Institute for Energy and Environment, explained to National Geographic:

Carrico said, “It’s certainly true that heat kills bacteria, but if you were going to use hot water to kill them it would have to be way too hot for you to tolerate.”

She explained that boiling water is sometimes used to kill germs-for example, to disinfect drinking water that might be contaminated with pathogens. But “hot” water for hand washing is generally half that temperature or less. At the high end of that range, heat could kill some pathogens, but the sustained contact that would be required would scald the skin.

Carrico said that after a review of the scientific literature, her team found “no evidence that using hot water that a person could stand would have any benefit in killing bacteria”. Even water as cold as 4.4°C appeared to reduce bacteria as well as hotter water, if hands were scrubbed, rinsed and dried properly.

In fact, she noted that hot water can often 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.

It’s for this reason that the World Health Organisation (WHO) doesn’t actually specify water temperature in its handwashing guidelines. It does suggest scrubbing with soap for at least 20 seconds (as in singing the “Happy Birthday” song in your head to mark the time), but the key is to wash well and wash often, not necessarily wash hot.

Carrico also notes in her interview with National Geographic that organisations and households can save money on energy by washing with lukewarm water instead of trying to get their tap as hot as they can stand it. Hit the link below to read the full story.

The environmental cost of misinformation: why the recommendation to use elevated temperatures for handwashing is problematic [The International Journal of Consumer Studies (May 2013, Vol. 34, Issue 4) via National Geographic and Tested]