One such misconception is that high SPF give higher protection. This is not the case.
the amount of protection that a sunscreen provides has been derived by the formula
(SPF-1)
--------- *100 = % UV Protection
SPF
Looking at this formula at this moment no mater how high an SPF you buy you can never reach 100% protection. So no sunscreen provides complete protection.
Now, going back to the formula and using five popular SPF ratings currently in use, SPF 15, 30, 50, 70, and 100. The following table of data can be made.
| SPF | 15 | 30 | 50 | 70 | 100 |
| % UV Protection | 93.33 | 96.67 | 98 | 98.57 | 99 |
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So what does that SPF number mean?
The SPF is determined by the Minimum Erythelose Dose (MED) times the amount of UV light the skin can handle.
So what is an Minimum Erythelose Does (MED)?
An MED is the amount of time that the skin is irradiated by light before it shows the first sign of reding. Most people are in the twenty minute range.
I've made another table using our favorite five SPF's. I've listed all the times in hours.
| SPF | 15 | 30 | 50 | 70 | 100 |
| MED |
| hours spent in the sun before the SPF limit is reached
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| 10 | 2.5 | 5 | 8.33 | 11.67 | 16.67 |
| 20 | 5 | 10 | 16.67 | 23.33 | 33.33 |
| 30 | 7.5 | 15 | 25 | 35 | 50 |
| 40 | 10 | 20 | 33.33 | 46.67 | 66.67 |
Now we know from the first table that the sunscreen protect relatively the same and here we see how long they can protect us at the five SPF levels we've looked at earlier. Looks pretty impressive. Until you put the number in a perspective.
In Dallas, Texas on June 20 from sunrise to sun set the city will have 14 hours and 20 minutes of sun. And since most people are an MED of 20. An average person will be able to use an SPF 50 stay our in the sun all day and be protected, and that is only ONE application!!! In theory.
There are three problems with this table. Issue one, most people will not use enough sunscreen and issue two, you will move around and you will sweat and move and rub off that precious sunscreen. Issue three that is a ridiculous amount of time to be spent in the sun to only use one application of sunscreen!
Yet all table 2 shows us is the amount of time we "CAN" sped in the sun. But don't let those numbers fool you. For it is the water resistance that matters! The FDA has two types of ratings when it comes to water resistance. The first is (wait this the shocking part) water resistant and Very water resistant (tax dollars at work people). (It is the same test to determine if a product is sweat resistant or very sweat resistant). What that means is, to be considered water resistant the SPF must remain the same for 40 minutes after being submerged in 75 degree F water. For a sunscreen to receive a rating of very water resistant it must last for 80 minutes in water then tested. That's it that is as long as a sunscreen has to last according to the FDA. At the end of the 80 minutes you are suggested by the FDA to reapply. So, even the FDA doesn't believe that you need to be wearing a high SPF. Also, why would you want to cover yourself in so much chemical?
Application: How do you know you are using enough? The best method I've ever seen I like to call the gun method. Lay you hand flat palm up and hold your hand like you're shooting a gun with your pointing and middle fingers. Then fill the space between the pointing and middle finger with a half inch wide line of sunscreen. Filling the space from the tips of your fingers to the base of your fingers. That is enough sunscreen to cover your face, chest and arms.
Conclusions:
According to the FDA you need to reapply sunscreen every 80 minutes that's when the water resistance wears off. Most (99%) of us don't need a sunscreen with an SPF of 30 or more (see above for explanation). The amount of protection an SPF 15 can give compared to an SPF 100 is less than 6% and a SPF 15 gives over 92% protection. Not applying enough is just as bad as not applying at all so reread the application step before you get to the beach. Please, buy smart, reapply every hour and a half and tell everyone you know how to be sunscreen safe.
