I spent today on the beach. Before I went into the sun I liberally slathered myself with sunscreen (Coppertone SPF 15, to be precice). I spent about an hour in the sun. So why the hell do I have a sunburn now? This is making me crazy.
SPF describes the increased allowable time of sun exposure before your particular skin type burns.
SPF15 means that you can expose yourself to the sun fifteen times longer without burning.
are you sure it was only an hour?
My experience with this (fair-skinned Scots/Irish lass that I am) is that SPF 15 means I can stay in the sun about 15 minutes without burning. My sister, on the other hand, can use SPF 15 and not burn for about an hour.
You don’t mention if you did a lot of running around/swimming/sweating - all of which will decrease the efficacy of sunscreen.
My recommendation - buy waterproof, and buy nothing below SPF 30. But that’s just me. 
Whoa, wait a minute. SPF= sun protection factor
SPF15 means you get 15 times YOUR regular protection. Doesn’t mean you can stay in the sun 15 times longer.
Try SPF30 or 35 Forget anything above that.
I’ve heard that putting sunblock on 15 to 30 minutes before going in the sun is optimal. I believe this works, I remember still getting burnt when using sunblock of at least 15, but I used to always put the stuff on as soon as I sat down on my towel at the beach. You must remember to use very generous amounts. And reapply often.
I have extremely fair skin, and burn all the time. I went to the shore last week on a very hot and sunny day, with no umbrella to sit under or even a hat on, and came home not burnt! Thanks to my early sunblock application and reapplying about every 1/2 hour (or less).
yes, handy, it does. if you have fair skin that will burn in 3 minutes, using SPF15 will let you stay out for 45 minutes without being burned.
Seconding the following:
You must put it on lavishly. You may not have used enough. I have read suggestions of 1 oz per child as an average, so an adult would need perhaps close to 2 oz for beach exposure?
You must put it on 20-30 min before exposure. I have also read that it has a cumulative effect, so fair skinned people who are planning on spending a lot of time in the sun can get even more protection by putting it on the night before (and, of course, reapplying the next day.)
If you are sweating or in the water it will wash away. Even the waterproof formulas will need to be reapplied, but not as often.
Also, the stuff does degrade over time, so if it was a bottle left over from last summer, it was probably not as effective as it should have been.
I just wear clothes or a full wetsuit for protection…
from howthingswork.com:
"Sunscreens (see also this article) block or absorb ultraviolet light. You can block UV with opaque creams like the white
zinc oxide cream that you see lifeguards putting on their noses. You can also absorb UV radiation in much the same
way that melanin does. The first and most common of the absorpsion chemicals is PABA (para-aminobenzoic acid). It
absorbs UVB. Other chemicals include:
Cinnamates absorb UVB.
Benzophenones absorb UVA.
Anthranilates absorb UVA and UVB.
All sunscreens are labeled with an SPF, or Sun Protection Factor. The SPF acts like a multiplying factor. If you would
normally be OK in the sun for 10 minutes and you apply and SPF 10 sunscreen, you will be OK in the sun for 100
minutes. In order for the sunscreen to work, however, you have to apply plenty and it has to stay on. You should apply
it about half an hour before going out in the sun (or the water) so it can bind to your skin - if you don’t, then it is very
easy for the sunscreen to wash off.
The SPF rating, by the way, applies only to UVB radiation."
If it makes you feel any better, I have tried all methods listed above (and even some others like refrigerating the sunblock, combining specific brands, etc) and I have never not gotten sunburned when out in the sun. I get burned no matter what if I have skin exposed. I always cover up.
Hmm, well, I didn’t do any moving around, just sat there. I’m pretty sure my time of exposure was between 45 minutes and 1 hr 15 minutes. The sunscreen I used might have been old, which I’m told can limit its effectiveness. I did lavish it on, for what it’s worth. Now its four days later and I’m still more lobster than bluejay. Melanoma, here I come…
Mmmmm. Look, a chance to be really pedantic and helpful at the same time. Should I just keep my mouth shut? Nah.
The SPF number really rates one sunscreen against another, and is not supposed to be an increased burn time for an individual.
The US of A leads the world in so many things…what did Australia line up for when the ‘what to lead the world in’ prizes were being given out? Skin cancer. Hence our anti-cancer societies have done a lot of work in trying to explain to people that an SPF number is not, repeat not, an increased burn time, it is an increased protection time.
Here’s a very shortened version of how the SPF system works:
For each product, a number of people (the number varies from country to country) are used as a panel of test subjects. A precise and equal amount of product is applied to each test subject, and spread over a precise and equal test area using a non-absorbent applicator, to form a thick layer. The test subjects are irradiated with a calibrated amount of UV of specified wavelengths for several different lengths of time, and the time it takes for each to subject to burn is measured. The results are averaged. The average is, roughly speaking, the SPF number.
So, if you are up the pointy end of the burning curve, an SPF 15 might give you 10 times the burn time of unprotected skin, if the product hasn’t expired, hasn’t been kept in a hot glovebox, and you’ve put it on thickly and often enough. And someone at the other end of the curve might get 20 times the burn time.
So if one SPF 15 gives you 10 times the burn time, so (allowing for differences in chemicals and formulae) will the other products labelled SPF 15. And a product labelled SPF 30 (2 x 15)will therefore probably give you 20 (2 x 10) times the burn time.
Yes you need to keep reapplying the stuff while you’re exposed. But reapplying doesn’t extend your protection time, it just allows you to get the most protection time your skin type will allow.
Which frankly doesn’t sound like much. You have my sympathies - my husband is much the same. He’ll put on an SPF 30 and burn in about the same time as I will burn with no sunscreen. This ain’t no joke for a balding man!
Note:
I spent the weekend riding in the MS-150 bike tour Pittsburgh to Lake Erie. During those two days I spent about 16 hours (9 Saturday/7 Sunday) in direct sunlight. I used SPF 45 with transparent Aluminum and despite much sweat I am happy to say that except for a few small hard to reach places on my back I am sunburn free.
SPF 45 is a miracle. Who would have though that they could create a lotion that you could put on your skin that you cannot see but which keeps the sunlight from burning your skin. Marvelous.