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Monday’s medical myth: you have to wash with hot water to kill bugs

Water alone won’t kill germs – it’s the soap that counts. Flickr/CafeMama

Despite decades of medical breakthroughs and growing health budgets, the simple act of washing our hands remains one of the most important things we can do to protect ourselves from disease.

The principle of hand washing is simple: disease-causing germs (bacteria, viruses, fungi and parasites) get onto our hands and can cause infection if they’re transferred to the mouth, nose or eyes – depending on the type of organism, the dose and individual’s susceptibility.

Cleaning your hands with soap or detergent can significantly reduce the load of these germs and the risk of passing on illness, particularly after using the toilet and before preparing food.

But does it matter whether we use hot water or cold to wash our hands?

The role of the water in hand washing is more controversial than the role of soap. Although you might wash off the dirt, and your hands may look clean enough, water alone isn’t that effective in removing germs, even when it’s hot.

Germs grow rapidly in our bodies, which maintains a temperature of around 37°C. So the average hot tap temperature won’t kill disease-causing germs. Neither will water simply wash them away.

Most germs are attached to the surface layer of the skin, which is formed from acidic fats, oils and cellular debris. To dislodge the germs, you have to dissolve this surface layer, then mechanically rub them off. This is why you need soap.

And water is helpful in three aspects of cleaning.

The first is solvation – the chemical process that leads to the formation of tiny soap micelles, which allows otherwise insoluble grease, fats and oils to disperse.

The second is lathering and lubrication (or soaping). Most liquid soaps also need a small amount of water to help the product spread across the surface of the hands, where massaging, rubbing, and friction is used to create a lather.

It’s not clear whether lathering itself is required to dislodge germs from your hands, but it’s an excellent marker for having done the hard work required.

The third reason to turn on the tap is to rinse the soap/detergent (in which many germs are now suspended) from the skin.

In each of these roles, the effectiveness of water bears no relationship to its temperature.

But when given the choice, most of us still reach for the hot tap, even on the hottest of days. Maybe we’re simply creatures of comfort and prefer the feel of warm water.

And if this means more people are likely to wash their hands more often, and transmit less disease, I’m happy for this myth to hang around a little longer.

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