What does SPF actually mean? Doctor reveals the truth about sun safety

Many believe the rating number refers to the amount of “safe” minutes you have before you get burnt. Dr Sam Hay shares the facts. 

When it comes to sun safety, there are a lot of misconceptions floating around. Does the SPF rating actually refer to the minutes your skin is protected after you get burnt? 

SPF Demystified: The science behind sunscreen protection

Sun rays constitute a broad spectrum of electromagnetic radiation emitted by the sun. This spectrum encompasses ultraviolet (UV), visible, and infrared rays.

Exposure to sunlight is vital for human health as it plays a crucial role in the synthesis of vitamin D, essential for bone health and immune function. However, it is essential to be mindful of the potential risks associated with excessive sun exposure, particularly to harmful UV rays.

Ultraviolet B (UVB) rays, prevalent in sunlight, stand out as the primary culprits behind sunburn and skin cancer. There is also a risk with UVA exposure, which contributes to skin aging and some risk of cancer.

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What does SPF actually mean? Image: iStock

What's in sunscreen?

Any tube of sunscreen you buy has three main components.

The slippery creamy part called an “emulsion” that allows you to rub it over your body, plus two active ingredients.

UV Absorbers. These are chemicals that absorb the UV rays and convert them to heat - so low you won’t even notice it.

UV Reflectors.  These create a physical barrier on the skin to reflect to sun, and are mostly made up of oxides, like zinc oxide and titanium dioxide, that absorb and scatter UV radiation.

Different tubes of sunscreen have different amounts of absorber and reflector components in them. In general, we like to use high ‘reflector’ products over highly sun-exposed parts of the body like the face - just like Warney and his zinc cream!

What does SPF stand for?

“SPF” is short for Sun Protection Factor. You’ll also see numbers that refer to ‘ratings’ from SPF15 up to SPF30+ or SPF50+. But what do these ratings even mean, and how did they get them?

The SPF rating is an indication of a sunscreen’s ability to ‘screen’ out UV rays - which means most get blocked and some get through.

For example, SPF30 sunscreen blocks 96.7% of UVB rays, allowing 3.3%, or one-thirtieth to reach your skin.  SPF50 sunscreen blocks 98% of UVB, allowing 2% or one-fiftieth through.  So, a higher SPF rating means less UV rays reach the skin.

Now, theoretically under these SPF ratings, if your skin would take ten minutes to show signs of burning, then a SPF30 sunscreen increases that timeframe by a factor of 30; and a SPF50 by a factor 50.  That's an increase from 10 to 300 or 500 minutes isn’t it!?!

Sort of, but not really.  It’s this myth that really gets parents confused - many believe the rating number refers to the amount of “safe” minutes you have before you get burnt. I’m sorry mums and dads - this is not true. Because how much sun you’re exposed to - the ‘dose’ -  in any given time frame is influenced by many factors. Not just the clock. 

• Applying the sunscreen correctly in the first place. For sunscreen to ‘work’ as directed’ it has to be ‘used’ as directed!
• The UV index.  The UV index is a fancy computer-generated rating for the intensity of sun radiation on any day.  The higher the index rating, the higher the ‘dose’ of sun exposure you’ll get in any given time - this means not all days are equal.
• Swimming and the use of towels will wash or wipe sunscreen off faster.
• Exercising causes sweating which may lead to the sunscreen wiping off easily.

Bottom line, when it comes to sun protection, we need to use sunscreen properly the first time and keep using it.

RELATED: Which sunscreen is best for you baby?

Is every SPF50+ the same?

If it’s sold in reputable shops in Australia - then yes: all sunscreens are what they say they are. Australian regulations are incredibly stringent, and therefore we have the best sunscreens in the world.

However, travel abroad, and who knows what you’re getting! Their numbers might not be the same as ours - so best to take your sunscreen with you.

Truth about the ‘+’ sign.  The ‘+’ sign simply means more than - so whatever sunscreen you’ve got, then protection is better than the number rating displayed. In reality, in Australia, to reach a 50+ rating it actually has to have a 60 or better rating by our standards.

How much sunscreen do we need?

Keep in mind, no sunscreen can block 100% of UVB rays, so it's essential to follow the Slip, Slop, Slap routine for maximum protection. And keep an eye on the daily UV index, because you’ll need sunscreen if you’re spending time outdoors during days with a rating of 3 or higher.

Broad-Spectrum Protection. Look for sunscreens labeled "broad-spectrum," meaning they shield against both UVA and UVB rays. UVA rays penetrate the skin more deeply and are associated with premature aging, while UVB rays cause sunburn.

Sun protection doesn't just stop at sunscreen. Image: iStock

RELATED: What you need to know about children and sunburn

Application Technique. The key to effective sun protection is applying enough sunscreen. The Cancer Council of Australia recommends a simple rule of ‘teaspoons’:

• Head: one teaspoon.
• Front body: one teaspoon.
• Back body: one teaspoon.
• Each limb: one teaspoon.

That's 7 teaspoons all up, about 35ml - the equivalent of a shot glass full.

Always put it on 20 minutes before going outside, which allows it plenty of time to soak in and maximise protection.

Reapply every two hours. Sunscreen must be reapplied every two hours, and consider sooner if you’re swimming or exercising more.

A shot glass full (approximately 35ml) is the recommended amount for an average-sized adult. For your little Aussie tikes, a proportional amount is needed based on their size.

Babies. Cancer Council does not recommend the use of sunscreen in babies under six months. The main forms of sun protection for babies should always be protective clothing, hats and shade.

Scientific Evidence: Sunscreen and Skin Cancer Risk Reduction

Now, let's talk evidence because as an Aussie parent, you want the facts.

Multiple studies, including a 2018 publication from the University of Sydney, have highlighted the significant impact of sunscreen use on reducing the risk of skin cancer in young people. Daily application has been shown to cut the risk of melanoma – the most serious type of skin cancer – by a whopping 40% compared to people who rarely used sunscreen.

Sunscreen, while essential, is just one piece of the sun protection puzzle. Embracing shade and appropriate clothing is equally crucial. Whether it's finding refuge under trees or opting for indoor activities, the spectrum of sun protection ranges from short sleeves and shorts to full-body zip-up lycra space suits. It's a balance between safeguarding against harmful rays and ensuring our kids have enjoyable outdoor experiences.

With this understanding, you can confidently shield your kids from the sun's harsh effects while still relishing outdoor activities safely. The degree of aggressiveness in sun protection is a personal choice, but adhering to the Slip, Slop, Slap mantra and appreciating the science of sun protection are paramount.

Applying sunscreen daily, avoiding intense UV periods, and ensuring your kids always wear a hat are non-negotiable elements of a comprehensive sun safety strategy.

Originally published as What does SPF actually mean? Doctor reveals the truth about sun safety