Still, these studies have led some people to choose physical sunscreens over chemical ones.
"I have a tendency to opt for the natural stuff, the physical sunscreens," Friedman says, "but overall, the positives outweigh the negatives for all sunscreens" by reducing skin cancer and aging risks.
In addition, the negatives aren't limited to chemical sunscreens. The titanium dioxide in many physical sunscreens also degrades in light and can generate free radicals in the skin.
What's the best way to avoid the negatives? According to Hanson, the key is to use sunscreen properly, so it doesn't degrade in the sunlight.
"It all comes back to the need to reapply," she says. Most sunbathers don't reapply their sunscreen, or they apply it too infrequently. Hanson urges anyone venturing into the sunshine to follow the Skin Cancer Foundation's guidelines, which call for one ounce (about two tablespoons) of sunscreen for an adult's entire body, reapplied every two to three hours. The reapplications replace light-degraded sunscreen, preventing some of the free radical formation and more consistently protecting the skin, Hanson says.
Both the Skin Cancer Foundation and the American Academy of Pediatrics recommend that babies younger than 6 months of age be kept out of the sun and receive only very small amounts of sunscreen when sun exposure can't be avoided. They say that infants' skin is more likely to absorb or react to the ingredients in sunscreens.
When shopping for sunscreen, Hanson suggests seeking out brands that contain antioxidants such as Vitamin E (often listed as Vitamin E acetate) or Vitamin C (often listed as sodium ascorbyl phosphate), which are added to many physical and chemical sunscreen formulations to counteract free radical formation.
"Look for sunscreens that have antioxidants highest up on the list of ingredients," she says, "as that means they have a higher antioxidant content."