Nearly 10 percent of all sunscreen sales in the U.S. occur on Amazon.com, but many of the most popular sun protection products sold there don’t meet standards set by medical professionals, a new report says.

According to the American Academy of Dermatology, sunscreens should provide broad spectrum protection against ultraviolet (UV) A and B rays, be water resistant and have a sun protection factor (SPF) of at least 30. But less than two-thirds of the most popular sunscreens on Amazon met those standards, researchers report.

“One of the things I get asked the most from patients is what sunscreen should they use,” said lead author Dr. Steve Xu, of Northwestern University Feinberg School of Medicine in Chicago.

Dermatologists make recommendations and offer guidelines, but they can’t know what patients do after they leave the doctor’s office, Xu told Reuters #Health.

To get a better idea of what people look for when buying sunscreen, the researchers turned to Amazon.com. In a paper in JAMA Dermatology July 6, they say nearly three quarters of people in the U.S. have reported buying beauty and personal care products on Amazon, and they estimate that 9 percent of all sunscreen sales occur there.

Using Amazon.com’s customer rating system, the researchers analyzed the website’s top 1 percent best-rated sunscreens.

Overall, there were 6,500 sunscreens sold on the website. Of the 65 products the researchers analyzed, prices per ounce ranged from $0.68 to $23.47.

Forty percent – 26 of the 65 top-rated sunscreens – did not meet the AAD standards.

Approximately 90 percent of the sunscreens claimed to have at least a 30 SPF and roughly the same proportion were broad spectrum. Only 62 percent of the sunscreens claimed to be water resistant, however.

Customers were most likely to cite cosmetic elegance – feeling, color and scent – as their main factor for leaving a positive review for the product. The second most common factor in positive reviews was product performance.