Sunscreen

Last updated: July 6, 2007.

Even when we're outdoors, we're inside—inside an amazing, flexible, self-repairing container called skin! Few things are quite so amazing. Skin stretches as you move and grows as you get older. Like a hi-tech waterproof fabric, it lets perspiration flow out but stops water and dirt getting in. It keeps us warm on cold days and cools us down on warm ones. It protects the inner organs of our body from minor damage and it can even regenerate itself after quite serious accidents and illnesses. But skin is surprisingly vulnerable too. One of the biggest threats to our skin comes from sunlight: the harmful ultraviolet energy in sunlight can make skin age prematurely and cause a life-threatening condition called skin cancer. Sunscreens (also known as sun creams and sun blocs) are a handy way to protect ourselves. What are they and how do they work?

Photo: Suncream in action. Put plenty on, apply it evenly, and don't rub it in too much.

Hidden dangers

Have you noticed how you can get sunburned on a cloudy day, and even when you're swimming underwater? That's because sunlight contains a whole range of different forms of energy. Put simply, there's the sunlight we can see that lights the world up. And there's the sunlight we can't see—invisible energy called ultraviolet light, which is like a light so blue that our eyes can't see it. Ultraviolet is what does the damage to skin.

There are actually three different kinds of ultraviolet in sunlight: UVA, UVB, and UVC. UVC is the most harmful, because the energy waves it contains have the highest frequency and the most energy. Fortunately, it's entirely filtered out by Earth's ozone layer and none of it reaches Earth, so we don't have to worry about it.

What we do have to worry about are the other two types of ultraviolet light. UVB is less harmful than UVC, because it's energy waves are less energetic, but it's still more energetic and harmful than UVA. Both UVA and UVB can pass through the ozone layer, though much of the UVB is filtered out and most of the ultraviolet light that finally reaches Earth's surface is UVA.

Why is this a problem? UVB causes sunburn (a painful first degree burn with some reddening and swelling of the skin) and other surface skin problems such as skin cancer and premature ageing. Scientists think UVA (which penetrates deeper) is important in causing damage to the deeper layers of our skin.

Generally speaking, all you need to remember is that ultraviolet light (both UVA and UVB) can be harmful to your skin. Because you can't see the damage you're doing until it's too late, ut's a good idea to protect yourself by staying out of the midday sun (the safe times depend on the time of year and where in the world you are), and wearing loose-fitting clothes, sunhats, and sunscreen.

Photo: Suncreams like this promise protection against both UVA and UVB. The large number on the bottle is the sun protection factor (SPF). Suncreams with the highest numbers offer most protection. They allow you to stay out in the Sun for longer with less risk of damaging your skin.

Natural sunscreens

Nature's a pretty clever thing. It solved the sunburn problem for us a long time ago with two very handy mechanisms.

First, we have the ozone layer—Earth's natural sunscreen. Ozone is a special kind of oxygen gas that gathers in the stratosphere, the highest layer of Earth's atmosphere that is about 19-48 km (12-30 miles) above sea level (that's way beyond the height at which jet airplanes fly, but still not quite high enough to be in space). As you've probably heard in the news, people managed to do great damage to the ozone layer—making some huge holes in it—by using so-called "ozone-depleting" chemicals in aerosols and refrigerators.

Holes in the ozone layer are very serious, because they mean more harmful ultraviolet light can get through and damage people's skin. A few years ago, the US Environmental Protection Agency (the government body responsible for helping to avoid threats to the US environment) estimated that a 40 percent loss of ozone by the year 2075 could lead to 154 million extra cases of skin cancer and 3.4 million more deaths.

The other natural sunscreen we have is built into our own skin. If you're a white-skinned (Caucasian) person, you'll have noticed that your skin darkens very gradually in sunlight: you get a sun tan. What happens is that sunlight stimulates skin cells under the surface called melanocytes. These produce a pigment called melanin, a natural coloring that moves toward the surface of your skin, giving it that attractively brown, tanned appearance. After a while, the melanin in your skin provides a certain amount of protection against ultraviolet light. People with naturally darker skin have more melanin in their upper skin layers and naturally greater protection from ultraviolet light.

Artificial sunscreens

Sunscreens are designed to stop harmful ultraviolet light penetrating into your skin and they work in one of two ways. Some of them absorb (soak up) the ultraviolet light as it tries to pass through them. Until recently, many sunscreens contained a complex, organic (carbon-based) UVAbsorbing chemical called paraaminobenzoic acid (PABA), but it was found to cause skin allergies and is barely used at all now. Many sunscreens now use other organic, UVAbsorbing chemicals instead, such as benzophenones, but these have another drawback. Scientists think they are endocrine-disrupting chemicals: when they wash into the waste-water system (when people take showers after going to the beach), they build up in rivers and inland waters, causing fish to change sex.

Other suncreens work by reflecting UV light away from your skin using inorganic chemicals like titanium dioxide and zinc oxide. Titanium dioxide is the brilliant white chemical in sun creams; it's the same substance used to whiten paints and toothpaste. When it sits on your face, it works like the white paint on greenhouse windows—a kind of natural mirror that bounces away the harmful energy in sunlight so it does less damage to your skin. For reflective sunscreens to work properly, you need to apply them liberally and leave them forming a barrier on your face and body—in other words, don't rub them into your skin but let them dry on top, even if it makes you look like a ghost! Some reflective sunscreens use incredibly tiny nanoparticles of titanium dioxide. Smaller than human hairs, they give greater skin protection, with less risk of allergic reactions, and less of the ghostly pallor.

Photo: This sun cream contains several dozen different ingredients, including titanium dioxide. If you're the kind of person who doesn't like long lists of chemical ingredients, you might find this list rather scary!

Further reading

Favorite websites

• The Science of Suntans: A brief BBC article about the health risks of going out in the Sun. Includes an animation showing how suntans work.
• How does sun cream work?: The Naked Scientists show you how to test the effectiveness of sun cream.