In this sort of situation, I don’t reapply. This is if I’m not going to get much sun – for example, if I go to work in the morning, get mostly indirect sun exposure within the first 2 hours, then at the end of the day I go home and most of my commute is in the dark. I’ve divided my approach to reapplying sunscreen into 3 general situations which work for me: Situation 1: Minimal Sun There isn’t really a way to avoid this compromise. If you’re reapplying sunscreen on top of makeup, you’ll have to mess up your makeup to some extent, or apply less sunscreen, or apply less evenly. Like I said in my post on wearing sunscreen indoors, how much protection you need depends on both how much UV you’re exposed to, and how concerned you are about the effects of UV (the main issues for most people are skin cancer, wrinkles and hyperpigmentation). The higher the SPF you start off with, the more likely it is that you’ll have more protection left at the end of the day. We can’t confidently say that these results will be the same for every sunscreen since different sunscreens have different lasting power – water resistant sunscreens stay on skin for longer even when they’re being disturbed, and some chemical sunscreens do absorb into the top layers of skin which helps them last. But it’s likely that there’s still a significant amount of protection on your skin at the end of the day from most sunscreens, if you don’t move around much.Īs an indication, if you take 30% away, SPF 50 becomes SPF 35, SPF 30 becomes SPF 21, and SPF 15 becomes SPF 10 ( SPF varies approximately linearly with the amount of sunscreen on your skin). After 4 hours it was 7% less than that, and then after 8 hours it was another 4.5% less – a total of almost 30% less dye, which means probably somewhere in the region of 30% less sunscreen coverage. Here’s a graph of what happened (purple line is the average, grey shaded area is the range):Īfter 2 hours, there was an average of 16% less dye. They didn’t use any other products on their face, and they were allowed outside for up to an hour. Unfortunately there isn’t that much info on how long sunscreen lasts on your skin if you aren’t moving around much. There’s only one study directly on this topic so far that I could find (thanks to Kind of Stephen).Ģ0 people applied 2 mg/cm 2 of sunscreen with a dye in it, so the researchers could easily photograph where the sunscreen went every 2 hours. So after you apply sunscreen, the film wears down and you end up with less and less sun protection as time goes on.īut how much less? How much protection is left after 2 hours? Related post: Chemical vs Physical Sunscreens: The Science (with video) That means they don’t have this specific problem, but they’re still affected by the first two issues, so they’ll still get less effective over time. Sunscreens these days are usually formulated to be photostable – they use photostable organic or inorganic filters, or they use photounstable filters in ways that increase their photostability. This means they break down after absorbing too much UV. The final reason, but probably the one that most people know, is that some older organic “chemical” sunscreens aren’t photostable. This changes the absorption properties of the film for the worse. The sunscreen film also gets thinner as time goes on, because it evaporates and is absorbed into the skin. Related post: Foundation Clumping Science, and How to Fix It (with Video) ![]() The microscope photo on the left is right after application, and the photo on the right is after a few hours. You can see that it clumps up and moves around, migrating into pores and creases on your skin over time. These photos aren’t sunscreen but foundation – it’s another product that’s designed to stay on your skin in an even film for as long as possible, so it uses some of the same technologies, and it’s much easier to see. The layer also breaks up and clumps into bits as your skin moves around – when you talk and eat and yawn. It’s worse if there’s any heavy sweating. Secondly, when you first apply sunscreen, if you apply enough, it’ll form an even, continuous film that protects your skin. But over time, the sunscreen film breaks up as it interacts with the oil and water that your skin produces throughout the day. ![]() Related post: What Does SPF Mean? The Science of Sunscreen ![]() The less sunscreen you have on your skin, the less protection you get. If you look on your sunscreen, you’ll notice that it says to reapply regularly, usually every 2 hours. That’s because sunscreen gets less effective over time.įirstly, sunscreen comes off if you brush against it – with your hands, your clothes and your hair. 6 Which product should I use for reapplication? Why do you need to reapply sunscreen?
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