What is SPF?
The Sun Protection Factor, or SPF, is a measure of how well a sunscreen will protect skin from UVB rays, which causes burning.
What does the SPF number on sunscreens mean?
The SPF rating determines the time it would take you to sunburn if you were not wearing sunscreen as opposed to the time it would take with sunscreen on.
Percentage of Protection from damaging UV rays:
SPF 15 = 92% absorption
SPF 30 = 97% absorption
SPF 40 = 97.5% absorption
As you see from the percentages of protection above, the amount of protection between SPF 15 and SPF 30 is great (5%) but from SPF 30 to SPF 40 its minimal (0.5%)- that rate remains constant the higher you go.
In some cases, higher SPF sunscreens will cause the skin to become more sensitive leading it to burn. Sunscreens with SPFs higher than 75 are very misleading and don’t offer greater protection than those with a lower number.
How to Choose What Number to Use:
SPF Number X Amount of Time in the Sun before Burning without Sun Protection = Amount of Time Protected from Sun
Example: If your skin burns within 10 minutes of being in the sun without any sun protection, and you applied a sunscreen with an SPF 15, you would have 15 x 10 = 150 minutes (2.5 hours) before you would burn.
After your calculated “time protected” is up, your skin will absorb all the UVB rays it can handle before it will start to burn. Even if you reapply at this point, you’ll still burn, so get out of the sun and give your skin some rest!