Is it important to wear [sunscreen] on the face? I wear mineral makeup that has titanium dioxide and zinc oxide in the base but there, of course, is no exact protection number for that.
In short, yes, you definitely do need to wear sunscreen on your face in addition to your makeup.
As you mentioned, many mineral makeup companies do not label their makeup with an SPF, presumably because the cost of testing sun protection ratings for FDA approval can cost up to $25,000. There are, however, tons of products from larger companies- from bronzer to liquid foundation- that do boast an SPF. In either case, makeup shouldn't be relied upon for its sun protection because the typical user doesn't apply nearly enough product to benefit from the protection the product advertises.
In the case of mineral makeup (or any other powder sunscreen), you'd have to wear about 1.2 grams on your face to get the SPF stated on the product's label. To put that in perspective, you'd have to use about 1/6th of an ENTIRE JAR of your average mineral makeup every time you apply to get the correct coverage. According to cosmetic dermatologist Dr. Leslie Baumann, the average woman applies only 0.085 grams at a time, so you'd have to put on 14 times the normal amount! Ick!
Even though Deb's question pertained to mineral makeup, I do want to point out that the same issue exists for liquid foundations. According to Baumann, "SPF on the label of a...facial foundation is not an accurate reflection of how much sun protection these products offer. This is because the average person applies much less sunscreen product than is used in FDA testing".
Chances are, the idea of layering on even twice the foundation (let alone 14 times the amount!) probably doesn't sound like a great solution, so before you apply your makeup, make sure you apply a broad spectrum sunscreen. I like CeraVe AM; you can read my review about it here. Thanks for the question Deb, and keep them coming!