Let us suppose your skin begins to burn after 10 minutes of sun exposure. If you were to liberally apply a SPF15 (about 1 ounce for a normal size adult), then, under "normal" circumstances, your skin should not start burning for 150 minutes.
There is a formula you can use to help you select a sunscreen. Let's say that your skin does not begin burning for 10 minutes after sun exposure; you choose a sunscreen with an SPF of 15.
Elapse time before skin begins to burn x SPF = Burn delay
10 minutes x 15 = 150 minutes
10 minutes x 25 = 250 minutes
20 minutes x 30 = 600 minutes
The sun warms our planet 365 days a year; with the sun, there is always a need for sunscreen. You may not notice sun damage to your skin now, but it is there. Try keeping the damage to a minimun with sunscreen.
