June 1, 2017


Hot water and antimicrobial soaps don't get your hands any cleaner : You're probably washing your hands wrong. (Kendra Pierre-Louis, 6/01/17, Popular Science)

[H]istorical surveys have found that most people are told to wash their hands in the warmest water they can tolerate. In general, people associate warmer water temperatures with cleaner hands. The study, however, didn't find this to be true. Water temperature had no effect. That's probably because the temperatures that we know kill bacteria (think boiling) are also likely to burn our flesh (ouch).

This isn't the first study to find that temperatures don't affect hand cleanliness. A 2002 study in the journal Food Service Technology garnered a similar result.

The authors also found that anti-bacterial soaps didn't really work either. Hands washed with regular soap were about as clean as those scrubbed down with the antimicrobial stuff, a finding that isn't novel. In fact, last year the FDA banned 19 additives that are put into soap to make them antimicrobial. In explaining the ban, which takes effect this fall, Janet Woodcock--director of the FDA's Center for Drug Evaluation and Research (CDER)--stated that, "Consumers may think antibacterial washes are more effective at preventing the spread of germs, but we have no scientific evidence that they are any better than plain soap and water. In fact, some data suggests that antibacterial ingredients may do more harm than good over the long-term." [...]

Each of the 20 subjects were actually tested four times--each using a different duration of soap lathering (5, 10, 20, and 40 seconds). "That was just the time people were lathering with the soap--it doesn't include rinse time--and we saw a statistically significant difference between 5 seconds and 20 seconds," says study author Donald Schaffner, a food risk analysis researcher at Rutgers University. "We did not see a difference between 10 and 20 seconds and 20 and 40 seconds.

In other words, five seconds of lathering up the soap isn't long enough to get your hands clean. Ten seconds is the magic number.

The Centers for Disease Control recommends hand washing for about 20 seconds, roughly as long as it takes to sing the lyrics to "Twinkle Twinkle Little Star". But since that time is supposed to include both lathering and rinsing, it easily gets us into the range that Schaffner found in his study.

Posted by at June 1, 2017 6:44 PM