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Name: Heather
Status: Student
Age: 13
Location: N/A
Country: N/A
Date: 2002

How much (percentage) of bacteria on the human body is on the hand? How much is killed when washing?

I cannot give you an exact percentage, but the majority of bacteria on or inside our body will be found in the intestines. It will be less than 0.01% of those that are present on our skin. Of those present on the skin, I would say the proportion on the hands would be roughly correlating to the proportion in skin surface. Hands will be more exposed to bacteria than other skin surfaces, as they touch more 'dirt', but the hand palms have sweat glands to keep the skin surface salty, and that is an unfriendly environment for bacteria.

How many bacteria are killed by washing? That depends on the time, and, to a lesser extend, to the soap you use. Surgeons who have to clean their hands as good as they can, use antibacterial soap and must wash for at least 5 minutes. Our normal hand-wash routine usually takes not even 1 minute. That is insufficient to kill all bacteria, but do not worry: it is completely safe to have bacteria on your hands. Normal hygiene practice is completely sufficient under normal conditions. As long as you are not a doctor treating vulnerable patients, a normal hand wash with normal (not even anti-bacterial) soap is all you need to prevent trouble. Too much soap and hand washing can lead to problems, like skin problems (and this can lead to infections!) or psychological damage, as some people become obsessed with bacteria. Remember, our body is perfectly capable to cope with normal bacterial load.

Trudy Wassenaar

Dear Heather,

A very similar question has recently been answered by Trudy Wassenaar and is in the NEWTON Molecular Biology Archive:

The only point that question does not address is the actual "(percentage) of bacteria on the human body on the hand". This would partially depend on the hand to body surface area ratio, but the hand is often more readily exposed to contamination, as well. So it is very likely that the hand has more bacteria per unit area than the rest of the body, but I do not know that anyone has actually done a quantitative analysis since this would require somehow controlling for exposure, which would be difficult.

Jeff Buzby, Ph.D.
Children's Hospital of Orange County
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