Do I Really Need to Apply More Sunscreen Every 2 Hours?

Is Sunscreen Reapplication A Must?

Woman sunbathing
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Question: Do I Really Need to Apply More Sunscreen Every 2 Hours?

While I was reading about higher SPF levels in sunscreen, I had a disconnect when I got to the part that describes the strength of a particular SPF level. You say that SPF indicates how long the sunscreen will last; however, you then go on to say that one should reapply sunscreen every hour or two.

Assuming I use a SPF 15 sunscreen, this means that I will be protected for 150 minutes, clearly longer than 1 or 2 hours. And if I were to use SPF 100, that's 1000 minutes, more than 16 hours! Even if conditions weren't perfect, that should protect me for the whole day! Why would I then need to reapply every hour or two?


This is just part of a wonderful email that I got from a reader named Vincent. He brings up such a good point, and my guess is, so many others might have a disconnect with our recommendations of applying sunscreen every 1 1/2 to 2 hours that I would share what I wrote back to Vincent.

I am so glad that you are researching sunscreen. You're absolutely right, when it comes to which sunscreen to choose, how to apply it, and how often to reapply things can get down right confusing.  I am happy to help you clear up your confusion.

There are many factors that can affect a sunscreen's ability to be effective as long as they are supposed to be.  First and foremost, almost no one applies enough sunscreen.  In addition, after applying sunscreen people sweat, get in the pool, kids are in the sprinkler, etc. Then you add on top of that the sunscreen labels that are telling you they are waterproof. (They're not.)  People get a false sense of security when it comes to sunscreen.  If you are spending the day outdoors, sweating, or are in the water applying just once in the morning is simply not enough in order to be fully protected and not get sunburned.  That's why we say it's a good idea to apply every 1 1/2 to 2 hours, even if the calculations don't add up.

If you look at a bottle of sunscreen, no matter the SPF level, it will say something along the lines that it retains good coverage when in the water for up to 80 minutes. Less than 1 1/2 hours. Most people sweat while outside in the summer. Likely not profusely, but sweat none-the-less. Furthermore, sunscreen labels have to say if they offer protection in the water for 80 or 40 minutes.  40 minutes is less than an hour - even if it's SPF 50!

So many people go to water parks with good intentions, buying the "waterproof" sunscreen with a high SPF and put it on a couple times during the day. And so many of them wind up with a sunburn, confused because they thought they were doing things correctly. The label says so, right? But when they see the 80 minutes on the label, they're surprised. Imagine what people think when they see 40 minutes on the label.

So, call me a little sunscreen obsessed (I am), but I have read time and time again by different Dermatologists that people just don't use enough sunscreen and then are surprised when they get burned after a day in the sun.  So be sure to apply your sunscreen very liberally before going out for the day. In order to really be protected wear a wide brimmed hat and sunglasses as well.  Don't forget to apply sunscreen on the back of your neck and the tops of your ears.  

There is one exception when it comes to this rule about reapplication, since sweating, sunlight, and water breakdown the effectiveness of sunscreen if you are are spending the day indoors applying sunscreen once in the morning before leaving your home will protect you for much longer than 2 hours.  You always need to apply sunscreen - no matter the weather or the season -  before leaving home though reapplication during the day if you are in the office or just at home may be unnecessary.