Everyone has heard of SPF, or sun protection factor.  It’s been around for years and is formulated into number of products, but mostly into sunscreens.  Simply, SPF is a measure of what higher dose of ultraviolet B you can tolerate before you burn once applied.  There are three important points here.  First, it only works if you apply it.  That is, it does not protect you if it stays in the bottle.  Second, it only works if you apply it correctly.  The teaspoon rule may be helpful for some—1 teaspoon on the face, head, and neck, 1 teaspoon on each arm, 2 teaspoons on the trunk, and 2 teaspoons on each leg.  Furthermore, you may need to reapply if you are in the sun for a long time, sweating it off, or swimming it off.  Water resistant sunscreens may help in the latter two scenarios. Third, it only works for UVB.  For this reason, we now have UVA (or broad-spectrum) sunscreens