When you apply sunscreen, your goal is usually to protect your skin from harmful ultraviolet rays from the sun. Physical sunblocks can include zinc oxide and titanium oxide, which reflect all ultraviolet (UV) radiation before it gets to your skin. More commonly marketed, though, are chemical sunblocks, which absorb into the skin, and then in turn absorb some of the UV radiation before it can affect your skin. Both physical and chemical sunscreens are marketed in both dedicated lotions, and as part of other cosmetics such as facial moisturizers, hand creams, and foundation makeup. The American Academy of Dermatology (AAD) recommends a sunscreen that includes as much of the UV spectrum as possible, with a sun protection factor (SPF) of at least 15 [source: AAD].
Suncreen has specific chemicals that could provoke allergic reactions in some people. The FDA has approved more than 16 compounds as UV filters for sunscreens sold in the United States, so there's a wide variety of ingredients for manufacturers from which to choose. The University of California, San Francisco, recommends that if you have an allergic reaction to a specific sunscreen, such as severe itching or a rash, try another sunscreen with a different combination of chemicals [source: UCSF].
One of the most common allergic reactions to sunscreens is to para-aminobenzoic acid, or PABA. Though PABA is an essential nutrient in some animals, its internal benefits for humans are still under research. Topically, though, it is known to be an effective sunscreen. If you experience rash when using a sunscreen that has PABA, stop using it and try one of the "PABA-free" sunscreens to see if your allergic reaction is prompted by the PABA.