Sunscreen: Good for All Day?

Every sunscreen I have ever used recommends re-applying after 2 hours, or after heavy sweating or swimming.

SPF is Sun Protection Factor... It has nothing to do with the amount of time it is effective.

ETA...

http://www.rei.com/expertadvice/articles/sunscreen.html

SPF 30 DOES NOT MEAN 300 minutes... it means 95% UVB rays will be absorbed by the sunscreen instead of your skin... SPF 15 doesn't mean 150 minutes, it means that even less UVB rays will be absorbed by the sunscreen.

SPF is NOT a timer. It has nothing to do with time. :)

SPF doesn't matter with time. You should reapply every 2 hours, less if you are sweating or getting wet.

I'm not sure where you're getting these numbers. The SPF is the level of filtering the sunscreen has. Higher levels filter out more UVA/UVB rays. You don't just "add a zero" to figure out how long it lasts. That's not correct at all...and dangerous advice!

You should follow the directions on the bottle. But the American Academy of Dermatology says reapply every two hours. The bottles I have at home all say "80 continuous minutes."

So you'll need to reapply when you go back out, unless it's within the first two hours.

http://www.aad.org/media-resources/stats-and-facts/preven...

It always says to reapply every 2 hours, right?

ETA: You're right--that's where the SPF numbers come from....SPF 15 means you can stay in the sun 15x longer before burning that with SPF 0, etc.

SPF 30 isn't effective for 300 minutes, the 30 the factor by which your natural sun protection is increased. The amount of sun exposure is dependent on more than simply duration in the sun. Time of day, geographic location and weather conditions are also factors.

Regardless, you should reapply every two hours or after swimming or sweating for maximum protection. Also, be sure to use new sunscreen each summer, as SPF can decrease over time. And if you are using a chemical sunscreen, apply 20-30 minutes prior to sun exposure.

I would reapply before going back out into the sun.

Be sure it is broad spectrum. UVA/UVB. That blocks the rays that give you a sunburn and the rays that give you cancer. Reapply often regardless of SFP. Don't buy anything over spf 50. No sunscreen has been proven to be higher than spf 50.