The government said there has been a sharp rise in skin cancer patients in the past few years, maybe without knowing it, and thinking your a big man for taking a bit of a burn is responsible for that. Or possibly the use of UVA tanning bothes which is literally a cancer box. Who knows.
I don't burn easily, but i still put on 15-20 because i stay out in it all day, i think a lot of the comments on here are bragging.
On a similar note, the sun produces 3 types of UV rays a,b,c. C is blocked almost entirely, whilst B is a little, and A has the highest strength. Sun tan lotion SPF is a rating of protection against B only, so 15-20 is only telling you your resistance against B, so the protection its provides against A is minimal. I can't remember the name, but there is a specific chemical that is best for protection against A.
A is what tanning salons produce and is highly dangerous, and penetrates below the first few layers of skin, into the very sensitive lower areas.