As two trusted groups go into battle on SPF ratings, microbiologist Siouxsie Wiles explains what the numbers mean, and how they’re measured
The Cancer Society and Consumer NZ recently got in to a bit of a fight after Consumer NZ tested the SPF of a whole bunch of sunscreens and found that the Cancer Society’s Kids Pure Low Irritant Sun Lotion didn’t meet its claim of being SPF50+. Instead it rated 41. That wasn’t the only sunscreen that failed Consumer NZ’s testing – just four out of 15 matched or exceeded the SPF claimed on their label. So, who’s right? The sunscreen manufacturers or Consumer NZ? Turns out the answer may well be all of them. Let me explain.
SPF stands for Sun Protection Factor and is a measure of the level of protection against mainly UVB radiation, the rays that cause sunburn. The higher the SPF, the greater the protection. I’ll get into why thinking of protection in that way is a bit misleading in a minute But first, let’s talk about how you go about testing the SPF of a sunscreen.
There’s an internationally accepted method for sunscreen testing which in Australia and New Zealand is called AS/NZS 2604:2012. Basically, you take a group of people with what’s known as Fitzpatrick skin types I-III* (people who either always burn, usually burn, or sometimes burn), cover some of their skin with 2 mg of sunscreen per cm2 and then expose that sunscreen-lathered skin to increasing doses of UV radiation. You do the same to some unprotected areas of skin too. About 16-24 hours later, you measure the smallest dose of UV light needed to cause redness in both areas and use this to calculate the SPF. In other words, the SPF is entirely dependent on the skin types of the test volunteers, the skill of the tester, and, potentially, the batch of sunscreen being tested.
In 2016, researchers in France looked at the results of tests carried out in at least three different labs on 44 sunscreen products. They found quite some variation between the results from different labs. In one case, a sunscreen could have been labelled either as SPF30 or SPF50+ depending on which lab’s result the company used. The researchers argue that it’s misleading to let companies use results from tests carried out in just one lab. They should be having their sunscreens tested in three or four different labs to get to the ‘real’ SPF value, and should regularly be testing different batches to ensure the results are the same, they say.
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There are two really important things to remember about SPF. The first is that it’s a measure of how much UVB radiation the sunscreen can block when used correctly. That means when you slather the sunscreen on at 2 mg per cm2 of skin. For a full body application that amounts to about 35 ml. And loads of studies show we don’t use anywhere near that amount. If you used 35 ml every day, a 500 ml bottle would only last two weeks. How long does your sunscreen normally last?!
The second important thing to know is that the relationship between SPF and the amount of UVB radiation being blocked isn’t linear. An SPF of 4 blocks 70% of UVB rays, an SPF of 15 blocks 93%, and an SPF of 30 blocks 97%. An SPF of 50? That blocks just 1% more UVB than an SPF 30. In other words, the relationship is one of diminishing returns, and plenty of studies have shown that people who use very high SPF sunscreens stay out in the sun longer than they should because they think they are more protected than they are.
Whatever sunscreen you use, make sure you apply enough of it, and reapply every couple of hours. Stay sun safe everyone!
* Here’s a fun fact I learned from Wikipedia: the Fitzpatrick scale is the basis of emoji skin colour.
The Spinoff’s science content is made possible thanks to the support of The MacDiarmid Institute for Advanced Materials and Nanotechnology, a national institute devoted to scientific research.