Explainer: How Does Sunscreen Work, What is SPF and Can I Still Tan with it On?
Terry Slevin, 10 Jan 18
       

Sunscreen protects from skin cancer, burning and from the sun’s ageing effects. PRONicki Dugan Pogue/Flickr, CC BY

Sunscreen use not only reduces the risk of skin cancer and sunburn, it also reduces the ageing effect of the sun.

But whenever summer rolls around, it’s easy to forget the basics – like, how should I apply sunscreen? How long should I wait after applying it to go in the sun, and how long can I stay in the sun with it on? And how does it work anyway?

How does sunscreen work ?

There are two main parts to all sunscreens. The active ingredient and the emulsion.

Sunscreens either absorb UV radiation or reflect it. from shuttersrock.com

The active ingredient does the sun protection work. These come in two categories: UV absorbers and UV reflectors.

UV absorbers are chemicals that absorb UV radiation and convert it to a very low level of heat. So low most don’t notice it, but a small proportion of people do report sunscreens make them feel uncomfortably warm.

UV absorber chemicals are also called “organic”, because they contain carbon atoms, a basis for all organic matter.

Some absorb the UVB part of the spectrum, which is known to cause sunburn and contribute to skin cancer risk. Others absorb the UVA part of the spectrum. Recent research suggests the longer UVA wavelengths not only penetrate to deeper layers of the skin but contribute to skin cancer through compromising immune response to DNA damage.

For that reason, sunscreen labelled “broad spectrum” is recommended as it offers the best protection.

Broad action sunscreen is recommended. from shutterstock.com

UV “reflectors” are mostly made up of metals, like zinc oxide and titanium dioxide, that scatter UV radiation. The tiny flakes of metal act like mirrors to reflect the UV away from the skin.

There is normally more than one and often up to six or more active ingredients in most sunscreens.

The emulsion – the lotion, milk, cream, oil, foam or gel – is what carries the active ingredient. It is usually made up of some combination of oil and water, plus other goodies. These are important as they preserve the product so it lasts on the shelf or in your cupboard. They also help with water resistance, influence how the sunscreen feels and smells, and how well it binds to the skin.

What does SPF mean and how is it measured?

Sunscreen provides a screen, not a block. Think of a fly-screen door: air gets though but flies don’t. In the same way, the sun lotion or potion of your choice allows some small amount of UV radiation onto your skin.

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