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June 2nd: Cold water just as good as hot for handwashing

Don Shaffner and colleagues report a useful new study on the effectiveness of handwashing with soap - but yet again – media reporting shows we simply do not understand how hand washing works.  Reports in the Sun, Mail and Express say: “Antibacterial handwash is NO better than soap – and cold water kills as many germs as hot, experts claim”. 

The scientific basis of handwashing is that soap helps to detach microbes (and dirt) from the hands which are then rinsed off the hands when we rinse them under running water.  To kill bacteria in any numbers, microbiologists know that we need to use temperatures of 60C or more - which the hands cannot tolerate.  There is very little data, but what there is suggests that soap has some bactericidal action, but relatively little and only against some types of organisms. The basis of US CDC recommendations to wash hands in warm water is that it works better to remove dirt, food, etc, but this new study shows that – although this may be true – it does not increase the numbers of bacteria removed from the hands by handwashing with soap

In the new study, the researchers recruited 20 volunteers. Before washing, their hands they were contaminated with E. coli. The experiment evaluated four variables:

  • lather time (5, 10, 20 and 40 seconds)
  • soap volume (0.5, 1.0 and 2.0ml)
  • water temperature (15, 26 and 38C)
  • product formulation (plain soap versus antimicrobial soap)

 Afterwards, the hands were sampled to determine the reduction in contamination.

The study found that using colder water (15C) was just as effective at getting rid of bacteria as using hot water (38C), and antibacterial soap was not significantly more effective at removing bacteria than plain soap. It also found washing your hands for slightly longer – 30 seconds as opposed to 15 seconds – is more effective at getting rid of the bacteria – so friction is important

If you look at the Daily Mail responses from readers, you can see that many people assume that handwashing with soap works by killing the bacteria – despite the lack of evidence to support this.  Readers comments also showed another source of confusion – people also don’t know how alcohol hand gels work. One reader said “ As long as you use plenty of soap you&;ll have clean hands, it always makes me smile when I see people using the antibacterial gel instead of washing their hands , they might be germ free but they are still filthy dirty....

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