The ABC of the SPF
The Sun Protection Factor is a simple way of understanding the level of protection a particular sun screen offers. It's not a guarantee and there are some technical flaws in the way it works, so you will need to exercise caution and common sense to avoid sun damage.
The technical meaning of SPF is the ratio between how much ultraviolet radiation it takes to burn a particular person's skin with the sun screen on compared to not having the sun screen. Usually people simplify this with time references, for example saying that if it takes you an hour to burn without sun screen, then it will take 10 hours if you user SPF 10 sun screen.
In reality this is misleading as UV levels vary throughout the day. If, for example, it takes you half an hour to burn in the morning or evening with sunshine, using SPF10 sun screen won't give you five hours of protection in the middle of the day.
It's also important to remember that "burning" in this context simply means showing the first visible signs such as reddening, not waiting until you are in pain.
As a very rough rule, most people should be fine with SPF15 sun screen, while those who suffer from eczema should use at least SPF25. It may be worth using a higher SPF for areas such as the nose and shoulders that are very exposed. Whatever sun screen you use, you should still be careful to limit exposure during the middle of the day.
Not using adequate sun screen can lead to anything from mild cosmetic damage to serious medical problems. Fortunately most of the cosmetic damage can be treated through a range of techniques from skin peels and microdermabrasion to laser resurfacing. You can talk this over with a skin expert, though you should also see your doctor if you are concerned about moles that have increased in size.