Math can be very confusing. And when it comes to sunscreen, it is often deceptive too. This can be dangerous as it may lead us into a false sense of security that prevents us from using sunscreen properly, leaving our skin exposed to sun damage. So, even though it may be boring, it’s very important that we try to make some sense of it. Of course, it’d be more convenient if we could simply rely on the labels, but these too can display information that, at first sight, can seem conflicting. Especially when it comes to reapplication.
Shouldn’t an application of sunscreen last for the whole day?
The label always recommends we reapply sunscreen every couple of hours, but how can we reconcile that to the length of time the SPF level promises to keep our skin protected? As we know, the SPF number determines the amount of time you can stay in the sun without burning, and it is calculated by multiplying this length of time by the SPF number. That means that, if your skin burn after 20 minutes, then by using SPF 30, you can stay safely in the sun for 10 hours (30 x 20 minutes = 600 minutes). If you burn after 30 minutes, you’re protected for 15 hours. That’s a whole day in the sun! And yet, we need to reapply sunscreen often anyway? Why?
Chemical sunscreens wear out overtime
Not all sunscreen ingredients are created equal. Chemical sunscreen agents, such as Avobenzone, Octinoxate and Mexoryl, work by turning UV radiation into heat, but in doing so, they get used up. Dr Ellen Marmur better explains this in her book Simple Skin Beauty: “Chemical sunscreens don’t absorb the UV radiation as much as convert it into something benign (heat) that dissipates from the surface of your skin before it’s able to reach the collagen in the dermis and the DNA in your cells. Chemical filters intercept that radiation and transform it into something much less dangerous.
“The problem is that this photochemical reaction uses up sunscreen. Imagine that each chemical sunscreen molecule gets zapped and disappears the second it gets hit by a photon and converts to heat. Rather than wearing off, it is actually used up, like gasoline used by a car or food consumed by your body. That’s why it must be reapplied. The more sun you are getting, the faster sunscreen is used up or breaks down.” This degradation can be slowed down (for instance technologies like Helioplex, found in Neutrogena sunscreens, and Octocrylene, can reduce the oxidation of Avobenzone), but as a rule, the more time you spend outdoors, the oftener you’ll need to reapply.
Physical sunscreen agents (Titanium Dioxide and Zinc Oxide), on the other hand, simply work by creating a shield on the skin. When UV rays hit it, they are reflected away from it. There’s no special chemical reaction here that will compromise the effectiveness of your sunscreen overtime, but that doesn’t mean that you can simply slather them on and forget all about it. Physical sunscreens need to be reapplied too, although less often.
Poor application, swimming and sweating
But that’s not all. There are other factors that can make your sunscreen less effective, if not useless, overtime. To start with, many people simply do not apply the recommended amount (1/2 teaspoon for your face and neck, and a shot glass for the whole body). As a result, they never reach the SPF level stated on the bottle, and, what little sunscreen they applied, degrades even faster.
In addition, during the summer, we sweat more as our bodies work to cool us down in higher temperatures. We sweat even more if we play beach volley or run around on the beach. And to get some relief, we may be tempted to go swimming in the sea. As much fun as this is, sweat and water (no sunscreen is 100% waterproof, doesn’t matter what the labels say) will remove sunscreen, leaving our skin unprotected. Therefore, we must reapply it.
Remember, just because your skin isn’t getting sunburnt, it doesn’t mean that it’s not getting damaged. Unprotected sun exposure also causes wrinkles and cancer, and, unlike a sunburn, these don’t develop straight away.
The bottom line
It doesn’t matter if the calculations don’t add up. Sunscreen agents degrade, and lose their effectiveness, overtime when they are exposed to sunlight, and can be removed by sweat and water. That’s why you need to reapply sunscreen often whenever you spend a lot of time outdoors, and always after a swim or a sweat. Enjoy the sun, but do it safely!
Do you reapply sunscreen as often as you should?
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