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Gio of Beautiful With Brains suggested that I do a post on this after an Instagram conversation about SPF, and how it doesn’t actually tell you how much longer you can stay in the sun without burning. So what exactly does a sunscreen’s SPF mean in terms of sun protection?
I’ve touched on this topic a few times in my other posts on sunscreen like How SPF Changes With How Much Sunscreen You Use and my video on Why DIY Sunscreen Doesn’t Work, but I’ve never tackled it directly before. So here it is!
How sunscreen SPF is tested
SPF testing of sunscreens is performed on actual human volunteers in a laboratory. The sunscreen is applied at 2 mg per cm2 (equivalent to around ¼ teaspoon for the face, or a shotglass full for the entire body). A UV lamp is shone on bare skin and sunscreened skin, and the times required for redness (erythema) to show up are fed into this formula to work out SPF:
(MED stands for minimum erythemal dose, i.e. how much UV was required for red sunburn to occur.)
For example, if a volunteer normally gets burned in 5 minutes under the lamp, a sunscreen that stops them from burning for 75 minutes (15 times longer) would be classified as SPF 15:
(There’s ongoing research to replace this into an in vitro test that doesn’t require exposing people to cancer-causing UV rays, but right now they’re not reliable enough.)
So SPF means how many times more UV your skin can handle before burning with sunscreen on, compared to nothing at all.
So SPF tells you how much longer you can stay in the sun… doesn’t it?
Well, no. It’s a common misconception that SPF refers to how much longer you can stay in the sun with the sunscreen on before getting sunburnt, but this isn’t accurate. In the laboratory, the UV output from the lamp is consistent, so the UV dosage is proportional to time, and the SPF equation works when we substitute in the times required for burning.
However, in the real world, the UV wavelengths from the sun that cause sunburn (erythemal UV) vary quite a lot throughout the day. The atmosphere absorbs erythemal UV, so if the sun is lower in the sky, less erythemal UV will hit you.
Here’s a graph showing the UV Index, a number directly proportional to the amount of erythemal UV present, over Sydney in summer. The higher the UV index, the more UV there is.
At noon on this particular day in Sydney, the UV level is over 10 times the level at 7 am (UV index of 11.9 vs 0.9). So if you normally get burned after 20 minutes in the sun at 7 am, at midday you’d only last 22.7 minutes at midday even with SPF 15 sunscreen. This is significantly less than the 300 minutes or 5 hours you’d predict with the “how much longer you can stay in the sun” interpretation! Even with SPF 50 sunscreen you’d only last 75 minutes.
In reality, it’s even more complex than this – clouds block out UV as well, and if you’re standing in the wrong place, reflective surfaces like snow and water can double the amount of UV you receive.
Will high SPF sunscreen be enough?
SPF is one of the most important things to look for in a sunscreen, but it isn’t everything.
Here’s a graph showing the UV wavelengths that cause sunburn (in the red line, with the actual output of UV from the sun in blue. Note that this is a log scale, so the graph should actually be far steeper on the left than it appears):
Note that the UV wavelengths that cause sunburn are mostly shorter wavelengths of UV, termed UVB. For UV, shorter wavelengths are more energetic but penetrate the skin less deeply. However, longer wavelength UVA is also harmful (read all about that here: Why you should protect yourself from UVA), and SPF doesn’t really take this into account. To ensure protection against UVA, look for either a PPD rating on your sunscreen or the words “broad spectrum”. In some countries like Australia, sunscreens with certain SPFs have to be broad spectrum, but in the US that isn’t the case.