Couple of points:
There are many lip balms on the market that do not have any SPF, check the label.
The SPF factor, if at least 30, matters little compared to the actual usage.
Most people don't know that all these lotions are absorbed by the skin quite rapidly, perhaps 80% in 20 minutes, so you have little protection. For that reason, it is recommended that you slather on another coating within about 30 min of the first. This gives you FAR better protection.
SPF factors are a funny scale, sort of like the Richter scale.
It relates to the Standard Deviation of the UV that is blocked. The easy number to remember is that SPF 15 blocks 93% (~2 SD) of UV. One would think that SPF 30 blocks twice the amount, BUT IT DOES NOT. It blocks ~3 SD, which translates into 97%, which is only a 2% increase in blockage, but it DOES contain double the chemicals,
to which some people will react.
SPF 8, however, blocks only 1 SD, or ~60%.....which is 35% LESS protection, and that is a big difference!
So generally, authorities recommend nothing higher than SPF 30, but use it correctly, and it will do the job pretty well.
Here is a good reference:
http://www.epa.gov/sunwise/doc/sunscreen.pdf
