FAMILY MEDICINE® COLUMN

By John C. Wolf, D.O.Associate Professor of Family Medicine Ohio University College of Osteopathic Medicine

“AT HOME” TESTS CAN HELP READER PLAY MEDICAL DETECTIVE

Question: After being out in the sun, I developed a bumpy itching rash only on the parts of my body that were exposed to the sun. My mother thinks it is a reaction from the sunscreen I used. I don’t think it is. How can I tell if I’m sensitive to the sunscreen without getting a rash again?

Answer: Your question is an important and timely one since the summer sunshine has once again drawn those of us in northern latitudes out into the sun. Your use of sunscreen indicates that you are aware of the risks of sun exposure. As you probably know, the greatest health risk produced by sun exposure is an increase in the chance of developing skin cancer. The risk increases as the amount of sun exposure increases, and this is particularly true for individuals with fair skin. Those with skin that burns easily instead of tanning are at the greatest risk.

The rash you described sounds like what we doctors call “photosensitivity dermatitis” when we are using our professional vernacular. This is a fancy way of saying that a rash is the direct consequence of sun exposure. Some medications, classified as “psoralens,” are actually designed to cause photosensitivity. This is useful in the treatment of psoriasis and other skin conditions. More frequently, however, photosensitivity is an undesirable consequence of the use of a medication. There are a number of drugs that can produce this kind of rash. Here are the most common examples:

• tricyclic antidepressants;
• specific antihistamines;
• some high blood pressure medications;
• tetracycline, and certain other antibiotics; and
• ibuprofen and some other anti-inflammatory medications.

Perhaps your rash occurred because you have been using one of these products and then went out in the sun.

The chemicals in sunscreens can also cause photosensitivity reactions. Aminobenzoic acid and the chemically similar compound, para-aminobenzoic acid (PABA) along with benzophonones are notorious offenders and are no longer used in most sunscreen formulas. Avobenzone, cinnamates, homosalate and methyl anthranilate are now more common, but they still occasionally cause photosensitivity.

It is also possible that your rash is actually an allergic reaction to one or more of the chemicals in your sunscreen product. The easiest way to determine if you are allergic to the sunscreen is to apply a small amount of it to an area that will not have sun exposure, such as the inside of the upper arm. Leave it on for 24 to 36 hours. A rash will appear in this time if you are allergic to any of the components of the product.

If that doesn’t give you the answer, try applying sunscreen to a small area that will receive sun exposure, such as the back of one arm, before spending time in the sun. If a rash develops only in this area, your photosensitivity is caused by one of the chemicals in the sunscreen.

I’d like to remind you of a few misconceptions I frequently encounter about sun protection products. First, here’s how to interpret those “SPF” numbers. If you normally burn after 20 minutes in the sun, a SPF of 15 means you can stay out about 15 times longer, or about 5 hours, before you start to burn. For most individuals in most situations, an SPF of 15 is adequate. A product with a higher number isn’t necessarily any better.

Once you’ve been out in the sun for a while, reapplying a sunscreen with an SPF of 15 will not extend the amount of time you can stay in the sun without burning. It only ensures that you have sufficient sunscreen to provide the stated SPF of 15. This reapplication is still a good idea, however, if you have been swimming or perspiring heavily, even if you have used a “waterproof” product. Also, be sure to apply enough sunscreen to get full benefit. Most products recommend about one ounce, or one fourth of the standard bottle.

"Family Medicine" is a weekly column.

To submit questions, write to: John C. Wolf, D.O., Ohio University College of Osteopathic Medicine, Grosvenor Hall, Athens, Ohio 45701.

Past columns are available online at http://www.FamilyMedicineNews.org.