SPF 50
It doesn't seem very long ago that SPF 50 was thought of as being outrageously high for a sunscreen. However, today there are many sunscreens around that are way higher - some even over SPF 100. There is a certain amount of controversy about how high a sunscreen can really go and the USA, through the FDA (Food and Drug Administration), is even proposing to disallow a sunscreen to be labeled anything over SPF 50. Instead it must be labeled 'SPF50+'.
SPF and UVB protection
The figure below illustrates how as the SPF levels curve more sharply after SPF 15, the extra protection offered is marginal. Eg. SPF 30 provides 97% protection from the sun's UVB rays, while an SPF 60 only provides 98% protection. Higher SPF levels block out only a small fraction extra of ultraviolet radiation than the lower levels.
In fact, it is interesting to note that an SPF8 provides 80% protection. However, my experience of using sunscreens with an SPF8, is that when I am outdoors for a few hours at a time, SPF8 does not prevent me burning where an SPF30 will do an excellent job. Although not really a huge difference in percentages, the difference seems huge in actuality.
The SPF numbers can also tend to mislead people into thinking that using an SPF 30 is twice as protective as a SPF 15, or that a SPF 100 is twice as good as one with SPF 50. From the diagram we can see that this is just not true.
SPF Protection Curve
SPF labels the UVB protection only, but it is important to note that additional good protection from UVA rays is of utmost importance. The FDA in the US is proposing the implementation of a four-star system for rating the UVA protection of a sunscreen. As UVA rays are responsible for deeper skin damage and contribute to the development of skin cancer, it is important to make sure you choose a sunscreen with both UVA and UVB protection.
The higher the SPF the more important it is to balance your protection with high UVA protection. As it is quite difficult to obtain a high UVA level without the use of chemical sunscreens that are toxic, it is sometimes more sensible to choose a lower SPF. Otherwise you end up staying out in the sun longer than you normally would, as the SPF prevents you burning. Ie. Without your brain being told to get out of the sun as your skin is burning, the UVA rays are effectively being allowed to do more damage.
More important than a high SPF 50, is perhaps the correct application of your sunscreen, as well as reapplying when required. So make sure to apply your sunscreen liberally and evenly to all exposed skin. Reapplication is a good thing on average every couple of hours or after wiping off sweat, or swimming.
If you feel you're prone to burning easily, as I am, then you may wish to try getting a sunscreen with SPF 50 or even protective clothing that is labeled with a high SPF rating.
