Today we're going to be talking about SPF. SPF is a phrase you'll hear thrown around a lot when talking about sunscreens but what exactly does it mean? Let's break it down SPF stands for Sun Protection factor. It's a number that you'll find on the label of every officially sold sunscreen usually on the front in really big font How do they come up with this number? SPF testing is done on actual human volunteers in a laboratory. Two milligrams of sunscreen per square centimeter of skin is applied on to the volunteers.
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A UV lamp is shone onto Bare skin and sunscreened skin and the times required for redness or erythema are measured, and then fed into this formula. You divide how long it took for the skin to go red with the sunscreen by how long it took for the skin to go red without the sunscreen and the number you get is the SPF. MED stands for the minimal erythema dose in other words how much UV was required for the skin to turn red. For example if a volunteer normally gets burned in five minutes under the lamp with bare skin, a sunscreen that stops them from burning for 75 minutes or 15 times longer would be classified as SPF 15. So SPF tells you how many more times UV your skin can handle before burning with the sunscreen on compared to bare skin. So SPF tells you how much longer you can stay in the sun with the sunscreen on. doesn't it? Well no. This is a really common misconception. While the length of time before going red is used in SPF testing with UV lamps, it doesn't work this way in the real world.
The UV lamps used in testing put out a consistent amount of UV so it's proportional to time but in the real world the intensity of the UV radiation from the Sun that causes sunburn, known as erythemal UV, changes a lot throughout the day. In Sydney during summer here's what the UV levels look like. This is a graph of the UV index which shows you the levels of erythermal UV. The higher the UV index the more erythemal UV there is. At midday the UV level is 11. 9 which is over ten times the level at 7:00 a.m. which is 0.9 so if your bare skin normally gets burned after 20 minutes in the Sun at 7 a.m. at midday you'd only last 22. 7 minutes even with SPF 15 sunscreen on, not 15 times 20 which is 300 minutes or 5 hours. That's a massive difference. Even with SPF 50 sunscreen you'd only last 75 minutes at midday. Most of the time it will be even more complicated than this: clouds absorb UV and some parts of reflective surfaces like snow and water can actually double your UV exposure. Here's another complication the SPF of your sunscreen depends on the amount of sunscreen you apply. As I mentioned before the SPF of the sunscreen is measured at two milligrams per square centimeter on skin. Now 2 milligrams per square centimeter doesn't sound like that much but a square centimeter is pretty small. Most people use much less sunscreen than this. Even when they're told to apply a lot it translates to about 35 mL for an adult human body which is around a shot glass full. Most people end up applying between a quarter and a half of this recommended amount. According to a physical relationship called the Beer-Lambert law, protection should decrease exponentially with less sunscreen so applying half the amount of sunscreen would give you less than half the amount of protection. But luckily skin is a bit weird so microscopically it's quite bumpy and the relationship that most studies have found is close to linear.
This means that using half of the recommended amount of sunscreen would actually give you about a third or a half of the labeled protection. There's a common myth that says SPF 15 blocks 93% of UV, SPF 30 blocks around 97% and SPF 50 blocks 98%, so SPF 50 would only give 5% more protection than SPF 15 and only 1% more protection than SPF 30. However this isn't a good interpretation for a number of reasons. Firstly these percentages are only if you apply the full amount of sunscreen: two milligrams per square centimeter and as I mentioned earlier most people don't. Secondly it's more important to think about the UV that does pass through the sunscreen and gets to your skin rather than the UV that doesn't pass through. So a better way of looking at this: assuming that you apply the recommended amount is that SPF 15 lets in 7% of UV SPF 30 less than 3% and SPF 50 lets in 2%. So this means SPF 15 lets in almost four times as much UV as SPF 50. Here's an analogy: you earn an average Australians income $85000 a year. In other words you don't earn 99. 99999% of Australia's GDP 99. 99999% of Australia's GDP does not enter your bank account. Your boss tells you that an extra 1% of Australia's GDP will enter your bank account if you work an extra hour a week. You say no because it's only 1%. You then go off do some calculations and then kick yourself because it's 17, 000 million dollars which would make your income 20 million percent higher. A recent study found that SPF 100 plus sunscreen did in fact in practice protect more than SPF 50 sunscreen from sunburns. So yes going higher with SPF isn't going to be unnoticeable. In this study 199 people were told to apply two sunscreens, one on each side of their face.
They didn't know which sunscreen was higher SPF. After around six hours in thesun scientists looked at how red the people's faces were. More than half the people were more sunburnt on the SPF 50+ side while only 5% of people were more sunburnt on the SPF 100+ side. There's been a few other studies as well that have had similar results. The problem with going too high with SPF is that high SPF generally means more sunscreen actives, which usually means more greasy and more expensive products. This might mean you won't end up applying quite as much of that product as you would with a lower protection product, and so you'd end up with around the same amount of protection. So I would recommend going with the highest SPF sunscreen that you can considering your budget and the formula. There's also the fact that with higher SPF some people end up applying less sunscreen or reapplying sunscreen less often because they think they're more protected but you still need to reapply high SPF sunscreen regularly, because the sunscreen post will wear off and wear down over time. As well as SPF there's also UVAPF which is protection from the longer wavelengths of UV that don't cause burning. But they still cause skin damage and in particular this is aging and pigmentation. UVA is also tends to penetrate more deeply into the skin. To ensure protection against UVA you can either look for a numerical PPD or PA rating which works a lot like SPF. You can also look for the words broad spectrum or the UVA logo on your sunscreen which means that the UVA protection factor is at least one third of the labeled SPF. In some countries like Australia if the sunscreen has a certain SPF it's mandatory to be also broad-spectrum. It's important to remember that sunscreen isn't the only type of sun protection and it isn't even the most effective type of sun protection. You should be avoiding sun exposure as much as possible and also using different types of protection like hats clothes and sunglasses. These end up being more effective than sunscreen because you can't under apply them they won't wear off and you don't need to remember to reapply them. If you want to learn more about sunscreen I have a few other posts about sunscreen I talk about how much you need to apply, how to fit sunscreen into a routine and what the difference is between chemical and physical sunscreen ingredients. The links are in the description or you can just check out the rest of the posts on my My blog. That's all from me today! If you enjoyed this post about sunscreens and SPF I'd love it if you could comment to my My blog and like this post. If you want to learn more about beauty science you can also look at my blog which has tons and tons of stuff on the science behind beauty products. You can also follow me on Facebook and Instagram where I post regular tips. See you next time for more beauty and science.