“Gio, do I really need to reapply sunscreen every 2 hours? I’m sitting in my office all day, it’s not like I have the chance to soak up the sun in a bikini…”
I hear ya. Reapplying sunscreen every two hours is cool at the beach, but do you need the hassle in your everyday life? Touching it up when you’re wearing a full face of makeup is something you’d rather NOT do – if you have the chance.
Do you have the chance?
Here’s the real reason behind the 2 hours reapplication rule:
Shouldn’t an application of sunscreen last for the whole day?
You’ve probably heard that the SPF number determines the amount of time you can stay in the sun without burning. You calculate it by multiplying this length of time by the SPF number.
Let’s say your skin burns after 20 minutes without sunscreen. If you’re using SPF 30, you can safely stay in the sun for 10 hours (30 x 20 minutes = 600 minutes). That’s almost the whole day! Why the heck do you need to reapply sunscreen at all?
FYI, 10 hours in theory. Remember, SPF is measured by scientists using a lamp that emits a constant stream of UV light. In real life, UV rays are more intense during certain hours of the day. Even with the same SPF, you’d burn faster at noon than in the early morning.
But wait, doesn’t that mean your sunscreen would last you even longer, then? Not so fast…
Chemical sunscreens wear out overtime
Remember the good old days when you could rely on math? 2+2=4 and all that jazz?
Well, sunscreen math is not so straightforward – or reliable. Just because in theory SPF 30 should last you 10 hours, it doesn’t mean it will. Chances are, your sunscreen becomes useless way before day.
Here’s the deal: as they neutralise UV rays, UV filters become ineffective. The more UV rays they absorbs, the faster their death.
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.”
At the beach, it usually takes around 2 hours to make your sunscreen completely useless. At the office? It’s impossible to tell.
It really depends on whether you’re working near big windows that let the sunshine through, how much you’re spending out of the office/house on any given day, what time of the day you’re exposed to all this sunlight…
As a rule, the sunnier the day, the more often you need to reapply it.
FYI, even though Dr Marmur is talking about chemical sunscreens, all sunscreens act this way. Technically, mineral UV filters (zinc oxide and titanium dioxide) are chemicals too. They break down as well, only more slowly.
Poor application, swimming and sweating
UV filters degrade overtime, but that’s not the only thing that makes your sunscreen useless within a few short hours. Here are the other culprits to look out for:
- Poor application: if you just apply a thin layer, it’ll get used up fast. Always apply the recommend amount to give your sunscreen the best chance to last as long as possible. That’s at least 1/4 of teaspoon for the face alone and a shot glass for the entire body.
- Swimming: no sunscreen is 100% waterproof. Some can last a little longer in the water, but you still need to apply them again once you get out.
- Sweating: if you’ve been working out at the gym, playing volleyball with your friends at the gym or just been sweating a lot (heck, some summer days are so hot, you’re sweating buckets even if you don’t move), the sunscreen will be melting away with the sweat. Reapply it often.
The bottom line
Sunscreens don’t last forever. UV filters get used up while doing their job and wiped away when you’re swimming or sweating. Reapply yours as often as you can.
Do you reapply sunscreen as often as you should? Share your thoughts in the comments below.
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