Answers to Patients’ Sun Protection Questions

In an article by Kerri Wachter in Skin & Allergy News (March 2004), Dr. DeLeo, an associate professor of clinical dermatology at St. Luke’s-Roosevelt Hospital Center in New York provides answers to questions that patient’s typically ask about sun protection:

I understand that we get 75% of our lifetime exposure to the sun before we are 18 years old. So why bother to wear sunscreen now?
New data are debunking this misconception. Food and Drug Administration researchers concluded that Americans receive only 23% of their lifetime sun exposure by the age of 18 (Photochem.Photobiol.77[4]:453-57,2003)

I understand that the risk of developing melanoma is linked to sunburns in childhood and not in adulthood. Is this true?
This is true, and researchers in several studies have shown that sunburns early in life are associated with a greater risk of developing melanoma than sunburns later in life. But it's important to stress that researchers have also shown that sunburn at any age increases the risk of melanoma. So the danger of developing melanoma from sun exposure is lifelong. It's also not just acute sunburns that are associated with the development of melanoma. Cumulative sun exposure is associated, too. Patients often have a lot of questions about sunscreens. Physical blockers - zinc oxide and titanium dioxide - reflect sunlight from the body. Chemical sunscreens work by absorbing the sun's radiation and dissipating this energy in a way that is not damaging to the skin.

Is an SPF 30 sunscreen twice as good as a SPF 15 sunscreen?
That depends on how you're measuring "good", says Dr. DeLeo. An SPF 30 sunscreen will allow a person to stay out in the sun twice as long as an SPF 15 sunscreen with the same amount of protection from burning. But in terms of blocking burning rays, an SPF 30 sunscreen is not twice as good as an SPF 15 sunscreen. For example, an SPF 10 sunscreen blocks 90% of burning rays, while an SPF 40 sunscreen blocks 97.5%.

If I apply an SPF 15 sunscreen and an SPF 30 sunscreen, do I get an SPF 45 sunscreen?
No. The resulting mixture has an SPF of about 22.5 because the lower-level SPF sunscreen dilutes the greater protection of the higher SPF sunscreen.

How much sunscreen should I apply, and how often do I have to reapply it?
To match the assumptions made in determining SPF, the average adult should use about 2 tablespoons of sunscreen for one application to exposed skin. So, an 8-oz bottle of sunscreen should last a family of four about 2 days. Most people use between 25% and 75% of the recommended amount of sunscreen. An average family of four uses 1.5 bottles of sunscreen in a year. "The reason that people get sunburned at the beach is that they're not using enough sunscreen," he said. There are two ways that patients can ensure that they use enough sunscreen. One way is to apply sunscreen, wait 20 minutes and then reapply (J. Am. Acad. Dermatol. 45[6]:882-85, 2001). The other way is to use a higher SPF sunscreen. "If they use half as much of an SPF 30, they actually get an SPF 15." The American Academy of Dermatology recommends reapplying sunscreen every 2 hours.

Do darker skinned people have to wear sunscreen?
It's true that people with darker skin have much better photoprotection than do people with lighter skin. However, these patients commonly have problems with hyperpigmentation, particularly after an outbreak of acne, eczema, or any inflammatory process. Many dermatologists are now recommending that darker-skinned persons, especially those with facial inflammation or inflammatory diseases, wear sunscreen - with an SPF of 15 or greater and broad-spectrum protection - every day," said Dr. DeLeo. "I think in the future we're going to see marketing of sunscreens to darker-skinned individuals to protect from dyspigmentation."

If long-wave UVA plays a role in photoaging, how do I get UVA protection?
All sunscreens with SPF or greater provide some UVA protection. Patients can also look at labels for specific ingredients that protect from UVA rays. Look for Parsol 1789 (avobenzone) and benzophenones (primarily oxybenzone). The chemical-free physical blockers also offer good UVA protection equal to the UVB protection.

What about sunscreen and vitamin D?
Wearing sunscreen can diminish the body's production of vitamin D because it blocks absorption of UVB, which is needed to make vitamin D. However, there has been no clinical evidence so far of vitamin D deficiency associated with sunscreen use.