Q. After trying different sunscreens, I discovered that I'm allergic to PABA. And judging by the number of PABA-free sunscreens on the market, many other people are, too.

What options do I have among sunscreen products? Why is PABA a problem for me, since I'm not allergic to anything else? How can I tell if I will or won't react to a PABA-free sunscreen?

A. Most sunscreens contain chemicals that help block ultraviolet-B (UVB) rays from the sun. Compared with UVA and UVC, UVB radiation is the most damaging to the skin.

PABA (para-amino benzoic acid) comes in several forms, including octyl dimethyl PABA, also called padimate O. Because some people are allergic to PABA, most sunscreens on the market no longer contain it.

PABA allergy shows up as a red, itchy rash. Besides having the potential to cause a contact dermatitis, PABA can also stain clothing yellow.

If you react to PABA, it's unlikely that you'll react to other sunscreens. However, people who are allergic to certain medications are more likely to react to PABA.

PABA isn't the only sunscreen product that causes allergic reactions. Other sunscreen chemicals--benzophenones (oxybenzone, dioxybenzone) and cinnamates (cinoxate, ethylhexyl methoxycinnamate, octocrylene, octylmethoxycinnamate) have also caused problems.

Before deciding that you're allergic to a particular sunscreen product, make sure you're not having what's known as a photo-sensitivity reaction to some medication you're taking. The reaction typically shows up as a redness in the areas most exposed to sunlight, particularly the face, neck and arms.

Medicines that can lead to a photo-sensitivity reaction include antibiotics; diabetes medicines; NSAIDs; birth control pills; antihistamines; and certain heart medicines. Besides medicines, cosmetics, plants and soaps can also cause photo-sensitivity reaction in some people.

If you have a photosensitivity reaction to a certain medicine and you need to continue taking it, it's important to use sunscreens and avoid excessive exposure to sunlight. For maximum protection, make sure you're using a sunscreen that protects against both UVB and UVA (such as those containing a benzophenone or avobenzone).

In addition to chemical sunscreens, you can also use physical or barrier-type sunscreens. Physical sunscreens contain titanium dioxide, red petrolatumor zinc oxide. However, because these sunscreen are thick and occlusive, they can block your pores and lead to heat rash or folliculitis (hair bumps).

To help protect against skin cancer and photo-aging changes in the skin, use a sunscreen with a SPF of at least 15. SPFs above 30 don't add much additional protection. For maximum benefit, apply the sunscreen 30 minutes to 2 hours before exposure. And don't make the mistake of staying out longer in the sun just because you have sunscreen on. That defeats its purpose and negates any benefit you might have had.