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Dozens of over-the-counter acne products had very high levels of the known carcinogen benzene, even when they were taken right off the store shelves, researchers found.

Among 111 benzoyl peroxide (BPO) products purchased from various retailers across the U.S., about a third (34%) had benzene above the "conditionally restricted" FDA limit of 2 ppm, reported Kaury Kucera, PhD, of Valisure, an independent laboratory in New Haven, Connecticut, and colleagues.

Concentrations ranged widely -- from 0.16 ppm to 35.3 ppm -- and studies have shown there's "no safe level of exposure to benzene," they noted in the .

Additionally, the researchers found that the products rapidly degraded benzene when exposed to UV levels similar to those from sunlight, at a rate of about 4.36 ppb per hour. "This suggests reasonable sunlight levels of UV exposure may generate substantially more benzene than elevated temperature reasonably expected in transportation," they wrote.

They also conducted cold testing on one prescription product -- benzoyl peroxide 5% cream (Epsolay), the first BPO cream approved by the FDA to treat rosacea -- which "confirms the expectation that cold storage stabilizes BPO formulations and supports from the American Acne and Rosacea Society which suggests refrigerated storage of BPO products."

"Our research demonstrates that BPO products can generate benzene at typical room and store shelf temperatures, while cold storage significantly reduces this formation," co-author Christopher Bunick, MD, PhD, of Yale University in New Haven, Connecticut, said in a statement.

"These findings suggest a need to recommend refrigeration of BPO products throughout the supply chain -- from manufacturing to patient use -- to limit benzene exposure," Bunick continued. "Until formulations are developed to prevent benzene formation, refrigeration may serve as a practical solution to minimize unnecessary exposure."

He added that dermatologists "should continue to advise patients on the appropriate use of BPO, including potential risks associated with UV exposure."

Valisure finding benzene in BPO products at body temperature and higher temperatures expected during storage and transportation. Based on those findings, the company sent FDA a citizen petition calling for the agency to take action such as revising industry guidance, requesting recalls, and suspending sales.

This current study goes further by conducting an analysis at room temperature, and by testing UV light conditions. It also found that with the prescription cream, benzene formed linearly over time when tested at temperatures of 50ºC (122ºF).

That's despite this product being encapsulated -- a method used to stabilize BPO products -- suggesting, however, that it may not work, they reported.

The instability of BPO products is similar to the case of ranitidine products, the researchers said, which Valisure previously showed degraded high levels of nitrosodimethylamine (NDMA) both on store shelves or after being exposed to heat.

Valisure also discovered that some products degraded into NDMA, prompting recalls. It has detected benzene in other consumer health products, including hand sanitizers, sunscreens, deodorants, and dry shampoos.

Richard Gallo, MD, PhD, of the University of California San Diego, who was not involved in the study, said in a press release that the research was "carefully done" and "should put to rest the question of whether benzene is present in skin care products that contain BPO."

"It is now important that further studies be conducted to determine if the presence of this potential carcinogen in drugs with BPO translates into any increased risk of cancer," Gallo said.

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