(CNN) It took just one day of use for several common sunscreen ingredients to enter the bloodstream at levels high enough to trigger a government safety investigation, according to a pilot study conducted by the Center for Drug Evaluation and Research, an arm of the US Food and Drug Administration.
The study, published Monday in the medical journal JAMA, also found that the blood concentration of three of the ingredients continued to rise as daily use continued and then remained in the body for at least 24 hours after sunscreen use ended.
The four chemicals studied -- avobenzone, oxybenzone, ecamsule and octocrylene -- are part of a dozen that the FDA recently said needed to be researched by manufacturers before they could be considered "generally regarded as safe and effective."
So, should you stop using sunscreen? Absolutely not, experts say.
"Studies need to be performed to evaluate this finding and determine whether there are true medical implications to absorption of certain ingredients," said Yale School of Medicine dermatologist Dr. David Leffell, a spokesman for the American Academy of Dermatology. He added that in the meantime, people should "continue to be aggressive about sun protection."
"The sun is the real enemy here," said Scott Faber, senior vice president for government affairs at the Environmental Working Group, or EWG, an advocacy group that publishes a yearly guide on sunscreens.
"It's not news that things that you put on your skin are absorbed into the body," Faber said. "This study is the FDA's way of showing sunscreen manufacturers they need to do the studies to see if chemical absorption poses health risks."
According to the Skin Cancer Foundation, more Americans are diagnosed with skin cancer each year than all other cancers combined. Around the world, melanoma ranks as the 19th most common cancer in both men and women, the World Cancer Research Fund says.
In the United States, sunscreens were originally approved as an over-the-counter solution to sunburn. They came in two types: one using chemical combos to filter the sun, the other using minerals to block the sun such as titanium dioxide or zinc oxide, which leave a telltale white coating. With many people not wanting to sport a white tint, the popularity of the chemical sunscreens soared.
Because of the way they were used at the time, there wasn't a lot of concern about a potential health impact. But that soon changed, and the FDA began to ask the industry for safety testing, said David Andrews, senior scientist at the EWG.
"They were originally used in small quantities to prevent sunburn on vacation," Andrews said. "Now they recommend applying these every day, applying them to large parts of your body. And the FDA began raising concerns."
The new FDA study enrolled 24 healthy volunteers who were randomly assigned to a spray or lotion sunscreen that contained avobenzone, oxybenzone or octocrylene as ingredients or a crème sunscreen that contained the chemical ecamsule.
The volunteers were asked to put their assigned sunscreen on 75% of their bodies four times each day for four days. Thirty blood samples were taken from each volunteer over seven days.
Of the six people using the ecamsule cream, five had levels of the chemical in their blood considered statistically significant by the end of day one. For the other three chemicals, especially oxybenzone, all of the volunteers showed significant levels after the first day.
"Looking through the results tables of the study, one thing about oxybenzone stood out," Andrews said. "Oxybenzone was absorbed into the body at about 50 to 100 times higher concentration than any of these other three chemicals they tested."
In 2008, the US Centers for Disease Control and Prevention analyzed urine samples collected by a government study and found oxybenzone in 97% of the samples. Since then, studies have shown a potential link between oxybenzone and lower testosterone levels in adolescent boys, hormone changes in men, and shorter pregnancies and disrupted birth weights in babies, but researchers caution about assuming association.
A Swiss study found oxybenzone or one of four other sunscreen chemicals in 85% of breast milk samples, sparking concern that newborns could be exposed.
And Hawaii, the Pacific nation of Palau and Key West recently banned sunscreens containing oxybenzone and octinoxate because they cause coral bleaching and are dangerous to marine ecosystems.
The European Union has mostly replaced oxybenzone in its sunscreen products with newer, more protective substances that block out more of the dangerous UVB and UVA rays. But those newer products have not passed the safety tests needed for FDA approval. So oxybenzone remains in use; in fact, a 2018 report by the EWG estimated that it was in two-thirds of all chemically based sunscreens sold in the United States.
In an editorial accompanying the new study, former FDA Chairman Dr. Robert Califf assured readers that just because the research found chemical levels "well above the FDA guideline does not mean these ingredients are unsafe."
The Personal Care Products Council, the national trade council for sunscreen, cosmetic and personal care products, agreed in a statement.
"The presence of sunscreens in plasma after maximal use does not necessarily lead to safety issues," said Alex Kowcz, chief scientist for the council.
"It's important for consumers to know that for the purpose of this study, sunscreens were applied to 75% of the body, four times per day for four days -- which is twice the amount that would be applied in what the scientific community considers real-world conditions," Kowcz said. The council was concerned, she said, that the FDA's study might confuse consumers and discourage the use of sunscreen.
When going outside, the American Academy of Dermatology recommends applying at least 1 ounce of sunscreen to all exposed skin every two hours or after swimming, including "back, neck, face, ears, tops of your feet and legs. If you have thinning hair, either apply sunscreen to your scalp or wear a wide-brimmed hat. To protect your lips, apply a lip balm with a SPF of at least 15," the academy says, adding that since UV rays are always present, sunscreen should be applied to exposed skin even on cloudy days and in the winter.
Califf said next steps would be appropriately designed clinical trials by industry to test safety and determine the optimal dose to prevent skin cancer while balancing risk and benefit.
In addition, he said, "an urgent question involves absorption in infants and children, who have different ratios of body surface area to overall size and whose skin may absorb substances at differential rates."
The Personal Care Products Council's statement said the industry has offered "state-of-the-art toxicological safety approaches as alternatives" to the FDA's testing method. "We look forward to our continued work with the FDA to ensure that consumers have access to products containing a broad variety of sunscreen active ingredients," Kowcz said.
While science continues to answer questions about sunscreen, Califf and other experts call for the public to continue to protect their skin from the dangerous rays of the sun.
The Environment Working Group recommends choosing a mineral sunscreen containing titanium dioxide and zinc oxide when possible, while the American Academy of Dermatology recommends talking to a board-certified dermatologist if you are concerned about the safety of the sunscreen's ingredients.
Both organizations say there are ways to protect yourself and your family other than sunscreen. Seek shade, especially between 10 a.m. and 2 p.m. when the sun is at its hottest, and whenever your shadow is shorter than you. Use protective clothing, such as long-sleeved shirts and pants and a hat with a wide brim, and don't forget the sunglasses.
"It's seeking shade, using clothes and when necessary using sunscreen," Andrews said, "but not using sunscreen to prolong your time in the sun."