SPF chart

SPF is another confusing part of sunscreen use. SPF stands for sun protection factor and is usually found in numbers like 10, 15, 30, 40, and some others. Basically, the higher the number, the longer it will protect your skin. This is because SPF is a multiplying factor. So, if you would usually be safe in the sun for 10 min, with an SPF of 10, you would be safe for 100 min, and so on.

Also, there are certain rules that should be followed for proper sunscreen application. These include re-applying during windy days, if you are sweating, or if you have been in the water. Also, you should apply suntan lotion 30 minutes brfore you go into the sun to allow it to properly bind to your skin.

However, there is another defense against the sun: our skin. Skin can only go so far, which is why we have sunblock, but it still has a great many defenses. First of all, the skin repairs itself at night. Certain cell target the sun-mutated cells and kill them off. Also, our skin produces cells called melanocytes.
Melanocytes absorb 70% of all UVB rays entering the skin, and the remaining 30% that penetrate trigger the production of melanin. Melanin darkens the skin (a sun tan) which provides a barrier against the sun. Also, the skin thickens which also protects us.

Melanocyte