A Staff Report from the Straight Dope Science Advisory Board

What's more important in cleaning, soap or hot water?

July 5, 2001

Dear Straight Dope:

My wife and I have been having an argument about the cleaning properties of water. Does hot water clean surfaces, either counters or skin, better than cold water if soap is involved? I maintain that soap is the more important ingredient since it helps lift off the dirt, while my wife maintains that hot water always works better than cold water for cleaning the aforementioned surfaces. Can you help settle this argument?

From what you've said, it sounds like you're both right. Yes, soap is important, and yes, hot water is better than cold.

Is soap the most important ingredient? Hard to say, since you need water to do the actual washing; otherwise you'd have a big soapy mess. But, yes, soap is of critical importance in separating dirt from the dirty article and, equally important, helps kill germs.

In his recent book, Eat, Drink and Be Merry, Dr. Dean Edell talks a bit about washing up. He says, "The one universal and most important preventive in the spread of infectious disease is washing your hands." He cites research showing that a hand-washing group of kids missed 23% fewer school days due to infectious disease. Oh, sure, tell kids they'll get to go to school more often--that'll certainly make them wash up.

Regarding the hot vs. cold issue, Edell says warm water is best, but cold is better than none at all. He also says you can't just dab on a quick bit of soap--you've got to lather up to make it worth while. I can't tell you how many times I'll see people at work "washing their hands" after going to the bathroom where they just barely wet their fingertips, as if that actually does something. But, considering that studies have shown that a large number of people don't even bother at all, I guess anything is better than nothing.

Let's take a look at what else Edell has to say: "The art of hand-washing is in the technique and the timing. You should wash before and after you prepare food (especially raw meat, chicken, and fish), eat, go to the bathroom, have sex, wipe your nose, cough, or sneeze. That should remove 95 percent of the germs." So if you have sex with a chicken while sneezing, you had better scrub until those hands bleed.

Edell even addresses drying, saying that paper towels are best (contrary to what you might read on hot-air dryers) but you need to make sure your hands really are dry --   this goes for countertops, too. Wet hands (or counters) hold on to germs. So even though you need the water for washing, you also need it to go away in order to finish the job.

As Cecil Himself noted in an earlier column about washing your hands after going to the bathroom, "soap gets rid of the skin oil that the bacteria stick to," so you definitely don't just want to use plain water. Edell noted much the same thing (obviously, he copied from Cecil) and added that regular soap is fine, "you don't need expensive antibacterial fancy-pants soap."

So, in summary: Hot water is better than cold. Cold water is better than none. Soapy water is better than plain water. And you and your wife need to find some hobbies if this is what you're arguing about.

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