A: There are two general classes of sunscreening agents. The chemical sunscreen and the physical sunscreens called sun blocks. Chemical sunscreens absorb UV radiation and convert it to heat or fluorescence. Physical sunscreens mainly block and reflect the sun's rays. These include titanium dioxide and zinc oxide--that's the white stuff that the lifeguards put on their noses. There are also sun blocks now that have microfine particles in them; they have a slightly whitish tint, but they're not white and are more cosmetically acceptable to use.
Q: Do they both work equally well? And which active ingredients are necessary or best for protection? Or are they all the same?
A: Chemical sunscreens fall into many categories. There are the paba derivatives. The one most people know is padimate O. Other categories include the salicylates (homosalate and octyl salicylate), the benzophenones (oxybenzone, sulisobenzone and dioxybenzone), the cinnamates (cinoxate, octyl methoxycinnamate), the dibenzoylmethanes (avobenzone, also called parsol 1789), and the anthranilates (menthyl anthanilate). Since these products are effective for various UV wavelengths, manufacturers often use a combination of these agents in their sunscreen products.
Q: But is one better than another?
A: There really isn't just one or two particular ingredients that people should look for. Each of the chemical sunscreen ingredients block specific portions of the UV spectrum and differ from one another in their capacity to do so. One can't really say which is "better"; they're just different.
Q: Why do most sunscreens seem to be so greasy? How can consumers buy one that doesn't leave their skin [feeling] like an oil slick?
A: Manufacturers often add emollients and moisturizers to sunscreens to lubricate and moisturize the skin. Consumers may want to try [sun block] lotions or gels if they find a particular product too greasy.
For information, contact the American Academy of Dermatology at http://www.aad.org or the Skin Cancer Foundation at http://www.skincancer.org.