The release of AirPods, those sleek wireless earbuds from Apple, is again spurring debate over the safety of radiation-emitting devices—including cell phones, WiFi routers, and wireless headsets. While U.S. regulatory agencies and some scientists believe the risk from these devices is either low or unproven, there are experts, including a UC Berkeley public health researcher, concerned about their safety.

Joel Moskowitz, who directs the Center for Family and Community Health at the School of Public Health, warns that radiation from the Bluetooth technology in the AirPods (and other headsets) poses health risks.

More than a dozen studies have shown that low-intensity microwave radiation similar to that emitted by Bluetooth can open up the blood-brain barrier, a layer of cells that block pathogens and toxins in the body from reaching the brain. That could lead to conditions such as autism, dementia, and brain cancer, Moskowitz says.

“When I heard about the AirPods, it caused me great concern,” he says. “We can’t say with certainty that these devices are dangerous, but based on the research that has been done on similar types of radiation, there’s good reason to think this is going to be problematic in the long term.”

Bluetooth uses radio waves in the 2.4–2.48 GHz frequency range to wirelessly transmit signals between devices. That range is also used by microwaves, WiFi, cell phones, and other devices.

Announced on September 7 along with the iPhone 7, the AirPods will be available starting in late October for $159. They will work with iPhones and iPads running iOS 10 and above, the Apple Watch, and Macs running the latest operating system.

In response to Moskowitz’s concerns, Apple spokesperson Alex Kirschner said in an email: “Apple products are always designed and tested to meet or exceed all safety requirements.”

The AirPods’ specific absorption rate (SAR), which measures how much radiation is absorbed by the body, is 0.466 watts per kilogram, well below the U.S. legal limit of 1.60 w/kg set by the Federal Communications Commission. It’s also below the 1.58 w/kg you would get from holding the iPhone 7 itself against your head or body.

Jerry Phillips, a biochemistry professor at the University of Colorado who has studied health effects of radiation frequencies similar to Bluetooth, says that standard isn’t robust. The SAR safety levels were based on the assumption that radiation from sources like Bluetooth, WiFi, and cell phones was safe unless it heated tissue. “That’s been shown to be absolute nonsense. Biological effects have been shown at very low SARs to the point where there is no measurable increase in heat,” Phillips says. He adds that we don’t know whether effects of exposure could add up over time.

Based on his own research, Phillips is concerned that, along with permeating the blood-brain barrier, AirPods and other devices could damage DNA. Some of his research was funded by Motorola, which he says asked him not to release his results and stopped funding his work after he published a major paper on the topic.

“What bothers me the most about AirPods is it’s taking cell phones one step deeper into the head. It’s a significant amount of power being delivered even closer to the brain. It just doesn’t make sense,” he says.

Not all scientists agree. Kenneth Foster, a bioengineering professor at the University of Pennsylvania, says that many studies did not find that radio-frequency radiation affected the blood-brain barrier. “The health agencies that have evaluated this simply haven’t found any problems at exposure levels that were even higher than Bluetooth,” Foster says. “If someone is concerned for any reason, good or bad, they should take precautions, but I don’t think the evidence is strong enough for a health expert to stand up and say we should be cautious.”

John Moulder, who taught radiation oncology at the Medical College of Wisconsin and published a paper about the health effects of WiFi with Foster, says AirPods are unlikely to pose health risks, especially because the Bluetooth transmitter hangs down from the ear. “I don’t see any real possibility of brain exposure at all, let alone an exposure high enough to get in the credible range of the blood-barrier effect,” he says. “There’s no biological or physical basis for concern about exposure levels this low, especially if it’s not transmitting most of the time.”

(Kirschner at Apple confirmed that the AirPods, like other Bluetooth headsets, will constantly transmit signals while they are in your ears.)

Foster and Moulder have received research funding from the Wi-Fi Alliance and the Mobile Manufacturers Forum, trade groups whose members include Apple, Samsung Electronics, and other cell phone manufacturers. They said the funders did not influence their research. Moulder has also served as an industry consultant.

U.S. regulators, including the Food and Drug Administration, the Federal Communications Commission and the Centers for Disease Control and Prevention, say that available science has not definitively linked exposure to radiation from Bluetooth, WiFi, and cell phones with health problems. “If there is a risk from being exposed to radio-frequency energy from cell phones—and at this point we do not know that there is—it is probably very small,” according to the FDA website.

Major recent studies have sent mixed signals. In 2010, a 13-country study from the World Health Organization found no overall elevated risk for two types of brain tumors after 10 years of cell phone use. However, it did find an increased risk of glioma, an aggressive brain tumor, in the very heaviest users.

In 2011, after looking at dozens of peer-reviewed studies, the WHO’s International Agency for Research on Cancer classified cell phone radiation as “possibly carcinogenic” to humans. The same year, one of the largest studies of cell phone users ever, which included nearly 360,000 people in Denmark, found that long-term use did not increase the risk of brain tumors, but it did not rule out an elevated risk for those who used cell phones for more than 10 or 15 years. The study looked at the length of cell phone subscriptions rather than actual use.

In May, the U.S. National Toxicology Program released preliminary results of a two-year study that showed exposure to cell-phone radiation increased the risk that male rats developed tumors on their brains and hearts (the results didn’t show up in female rats). The study has been controversial. Michael Lauer of the National Institutes of Health, one of its reviewers, wrote that he didn’t accept the results because of the likelihood of false positives. But Otis Brawley, chief medical officer of the American Cancer Society, called the study “good science,” and the American Academy of Pediatrics issued recommendations to reduce children’s exposure to cell phones in response to the results.

“The findings of the NTP constitute important signals that there are very serious health issues tied with microwave radiation from cell phones and other devices. At this point, the question is not whether cell phone radiation causes cancer, but how we can best reduce exposures,” said Ronald Melnick, a former senior NTP scientist and an advisor to advocacy group Environmental Health Trust, in a statement responding to Apple’s release of the iPhone 7 and AirPods.

What to make of the varied findings? The CDC concludes that, “More research is needed before we know if using cell phones causes health effects.” Moskowitz and Phillips agree more studies should be done, noting that research funding on the topic has largely dried up in the U.S. No studies have looked at effects of cell phone use for longer than 15 years.

Meanwhile, Moskowitz has tried to bring attention to the issue. In 2009, he and colleagues published a meta-analysis that found “possible evidence” of a link between cell phone use and tumor risk and urged further study. (Today he says the link is “highly probable.”) Last year, Moskowitz was among five scientists who organized the International EMF Scientist Appeal, which urged major international bodies to develop stronger guidelines and educate the public on health risks. It garnered 222 signatures from radiation researchers in 41 countries. In July, he sent an open letter to the FCC asking the regulator to reassess research on radio-frequency radiation and strengthen standards.

He has also advised Berkeley officials on a new law that requires cell phone retailers to post a warning about possible radiation risks in stores. The law went into effect earlier this year but is currently being challenged by a cell-phone trade group in a federal appeals court.

Moskowitz himself doesn’t own a smart phone and usually keeps his cell phone in his briefcase. He recommends using a wired headset or speaker phone, rather than holding the device against your head. He also discourages keeping a cell phone in your pocket, bra, or anywhere close to your body, and suggests turning your phone off or keeping it in airplane mode when possible. Children, pregnant women, and the elderly should be particularly cautious, he says. In general, he says, “distance is your friend”—keeping the phone farther from your body dramatically decreases radiation exposure.

For his part, Phillips only uses a speaker phone and tries to keep his phone off, sitting either in his bag or on the far corner of his desk. He never puts it in his pocket. He says: “[Cell phone companies] don’t have a bunch of biomedical researchers testing this stuff as engineers propose it. When engineers are in a room trying to come up with bigger or better things to do, I don’t think there’s much thought given to what the work means in terms of effects to a real person.”