Sunscreen — how does it work, what is SPF, and can I still tan with it on?

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.

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.

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.

SPF stands for sun protection factor. It's the measure of how much UV gets through the screen. The higher the number, the less UV passes through.

An SPF of 30 allows one-thirtieth — or 3.3 per cent —of UV to reach your skin. This means it filters 96.7 per cent of UV.

A girl jumps off a jetty into emerald green water at Coffs Harbour.

With an SPF of 50, 98 per cent is filtered and one-fiftieth — or 2 per cent — gets through.

So while the difference between SPF 30 and SPF 50 sounds like a lot — it is a pretty modest (1.3 per cent) — difference in protection.

Put another way, if your unprotected skin would take 10 minutes to show signs of burning, then properly applying SPF 30 sunscreen would slow the rate of burning to the point where it would take 30 times longer, or 300 minutes in total. SPF 15 would take 150 minutes, while SPF 50, 500 minutes.

But this is perfect world stuff. If you extend your stay in the sun for 500 minutes (over eight hours!) only relying on sunscreen, you will very likely still burn!

When and how do I put it on?

At a microscopic level, the skin is a series of peaks and troughs. Layering on sunscreen around 20 minutes before going into the sun allows the product to flow into the troughs and bind properly to the skin.

Many sunscreens recommend reapplying every two hours. But another way to look at it is like painting a wall of your house. The first coat gets a reasonable coverage, but a reapplication 20-30 minutes after being in the sun — after the first coat has "dried" — gets you much more reliable coverage. And this will cover the bits you may have missed, or covered too thinly, on first pass.

Know your UV

  • Three types of UV radiation: UVA, B and C.
  • Ozone layer absorbs all UVC, almost all UVB.
  • Cancer, sunburn are likely to be result of UVB.
  • Wrinkles are mostly the result of UVA.
  • UV won't burn you through a window, but will age you (glass absorbs all UVB, but not all UVA.)
  • You can get sunburnt on a cloudy day as UVB radiation can penetrate clouds.

Also, use it generously. Most people use too little (between a quarter and three-quarters) of the amount of sunscreen necessary to achieve the sun protection claimed on the label. A teaspoon per limb is a good rule of thumb. Add another teaspoon for your face, front and back. This comes to seven teaspoons (35ml) in all if you are at the beach in board shorts or a bikini.

Layer it on and spread it around. Reapply every two hours or more often if you are active (sweating, towelling off, skin making physical contact with anything that might rub it off), even if the bottle claims four-hour water resistance. And a good idea is to check if the lotion hasn't passed its use-by date.

Use other things to protect your skin too. Hats, shade, clothing and even staying indoors at the highest UV periods. The closer to solar noon, usually between midday and 12.30pm, the higher the UV.

The World Health Organisation recommends protecting skin from the sun when the UV Index is 3 or above. The Bureau of Meteorology reports on the UV Index around Australia and the SunSmart App allows you to get live readings on your smartphone.

How long can I stay in the sun with sunscreen on?

It's wise to stay in the sun no longer than is necessary to do your planned activity. Staying out longer just because you have the sunscreen "suit of armour" (which it is not) is a bad idea.

Even following all the best advice, the normal daily activity — wiping water from your eyes, scratching an itch, cuddling the kids, brushing against a tree or your best buddy — will remove sunscreen and diminish its performance. And remember it is screening, not blocking the sun.

And will you still get a tan if you put on sunscreen properly?

Well, no. If sunscreen is properly applied to do its job of reducing UV radiation exposure, it prevents the biological process of tanning.

Top 5 Sunscreen Myths – Cancer Council Australia
Myth 1 Sunscreen shouldn't be used on a daily basis as it's not safe
FALSE: Sunscreen and sunscreen ingredients are strictly regulated by the TGA to ensure it is safe and effective. It can be worn on a daily basis without harming your health and should be used alongside other forms of sun protection, whenever UV levels are 3 or above.
Myth 2 Using sunscreen will stop you getting enough vitamin D
FALSE: A number of studies show sunscreen use in real life has minimal impact on Vitamin D levels. In summer, most Australians get enough Vitamin D through incidental sun exposure — for instance while walking to the shops at lunch. Even those who are Vitamin D deficient shouldn't sunbake or skip sun protection.
Myth 3 If you have a good sunscreen it's enough to protect you from the sun
FALSE: Sunscreen should always be used in conjunction with protective clothing, seeking shade, a broadbrim hat and sunglasses. Sunscreen is not a suit of armour and shouldn't be used to extend your time in the sun.
Myth 4 Using a water resistant SPF50+ means you can stay in the sun longer without having to reapply
FALSE: Any sunscreen should be reapplied every two hours, or after swimming, sweating or towel drying, regardless of the level of water resistance advised on the bottle.
Myth 5 You only need a little bit of SPF50+ to be protected
FALSE: To get the correct level of SPF you need to apply the right amount of sunscreen. This should be at least one teaspoon per limb, one for the front of the torso, one for the back, and one for the head. This is seven teaspoons (or 35ml) in total.

Terry Slevin is adjunct professor in the School of Psychology at Curtin University and chair of the Occupational and Environmental Cancer Committee Cancer Council Australia.

This article first appeared in The Conversation.

Original Article

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