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.

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.

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.

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.

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!

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.

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.

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.

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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.

An SPF of 30 allows one-thirtieth or 3.3% of UV to reach your skin. This means it filters 96.7% of UV. With an SPF of 50, 98% is filtered and one-fiftieth or 2% gets through.

Terry Slevin works for Cancer Council Western Australia (CCWA), and works with Cancer Council Australia (CCA). Cancer Councils sell sunscreen. Less than 5% of Cancer Council income is derived from sunscreen sales. CCWA received government grants to run the SunSmart program in Western Australia. He is editor of Sun Skin and Health (2014 CSIRO publishing). Proceeds of both sunscreen sales and the book go to cancer research, education and support for people with cancer.

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.

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.

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.

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.

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.

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?

So while the difference between SPF 30 and SPF 50 sounds like a lot – it is a pretty modest (1.3%) - difference in protection.

Put another way, if your unprotected skin would take ten 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.

Adjunct Professor, School of Psychology, Curtin University; Education and Research Director, Cancer Council WA; Chair, Occupational and Environmental Cancer Committee, Cancer Council Australia