The grading of sunscreens depends on whether UVA or UVB radiation is being considered.
this is given as the Sun Protection Factor - SPF
the SPF is the ratio of the amount of UVB radiation that is just enough to produce sunburn on protected skin compared to that which will produce the same effect on unprotected skin; hence, SPF 10 should be able to withstand a given intensity of UVB for ten times as long as unprotected skin before burning
this is given as a Star Rating
four stars represents the maximum protection from UVA
the use of this system is controversial as the measure is only relative to UVB protection and UVA protection is still only minimal