Earlier this month, the California department of health released guidelines warning residents to avoid putting mobile phones up to their heads.

“Keep your phone away from your body,” the state health department writes. “Although the science is still evolving, some laboratory experiments and human health studies have suggested the possibility” that typical long-term cell phone use could be linked to “brain cancer and tumors of the acoustic nerve,” “lower sperm counts,” and “effects on learning and memory.”

Mobile phones emit radiation, which is measured in radio-frequency (RF) energy. In the manual that comes pre-installed on your iPhone, Apple explicitly tells you to use a hands-free option like speakerphone or headphones while talking in order to “reduce exposure to RF energy.” The manual also notes that cell phones are currently tested for radiation assuming the devices would be kept at least 5 mm (0.2 in) away from the body while being carried. That’s a lot more than the thickness of pocket fabric. (On an iPhone 6 and above, you can find this information by going to Settings > General > About > Legal > RF Exposure.)

Previous iPhone manuals were even more conservative: The manual for the iPhone 5 says users should carry their iPhones a full 10 millimeters (or 0.39 inches) away from their bodies at all times.

US law demands that all cell phones function in such a way that they cause less than 1.6 watts of radiation to be absorbed by the human body, per gram of body tissue (known as specific absorption rate, or SAR 1.6). The Federal Communications Commission (FCC) tests all mobile phones coming onto the market for compliance. But that rule is designed only to prevent harm from excess heat that can be generated by RF waves. It doesn’t consider (pdf) other potential biological effects, like DNA damage or altered protein expression—the FCC calls these all “ambiguous and unproven.”

Further, while cell phones are tested against a simulated human head in the “talking” position, they are not tested against the body (or in a pocket) in the “carrying” position. Instead, the tests assume the user is holding the phone away from the body whenever the phone is broadcasting at full power. And since RF energy exposure increases sharply if the phone is in contact with your body, and falls off rapidly at a distance, some worry that FCC testing is missing a lot of actual exposure.

Experts and organizations like the nonprofit Environmental Working Group have expressed concern over these rules, citing studies that show links between cell phone radiation exposure and heightened cancer risk. In 2011, a World Health Organization report classified radiation from cell phones as “possibly carcinogenic to humans”; in particular, the WHO noted that cell phone use correlated with an increased risk for glioma, a malignant type of brain cancer.

Jonathan Samet, the dean of the University of Colorado-Denver School of Public Health, and the lead investigator on the working group that produced the WHO report, told the Verge that there is “some indication” of risk, but it is still unclear how much.

“I’m not saying there’s a public health crisis by any means,” Samet told the Verge, “but I’m saying here’s a very widespread exposure in our society and we should make sure we understand it.”

Keep your phone away from your body

The California guidelines have been in the works since 2009, but after 27 draft versions the state abandoned the document without publishing it. It took a lawsuit from a Berkeley professor to get the guidelines released. Joel Moskowitz, the director of the Center for Family and Community Health at UC-Berkeley’s School of Public Health, sued the state in 2016 under the California Public Records Act after learning the document existed. Although lawyers for the state that the guidelines would cause unnecessary panic, according to the San Francisco Chronicle, a judge eventually compelled them to publish.

The guidelines include a long list of precautions people should take to limit radiation exposure, noting that, because radiation drops off sharply with distance, “keeping your phone just a few feet away from you can make a big difference.” Here’s the first part of the list:

When you talk on your cell phone, avoid holding it up to your head—use the speakerphone or a headset instead. Wireless (Bluetooth) and wired headsets emit much less RF energy than cell phones. Send text messages instead of talking on the phone. If you are streaming or if you are downloading or sending large files, try to keep the phone away from your head and body. Carry your cell phone in a backpack, briefcase, or purse; NOT in a pocket, bra or belt holster.

The guidelines also note that when your wireless signal is weak, and when you are on a fast-moving car, bus, or train, the phone has to work harder to make a connection, which means it emits higher levels of radiation. One option to improve safety is to turn your phone to “airplane mode” when service is bad or nonexistent. Phones emit no RF energy in airplane mode.

Other steps one should take to reduce exposure, according to the health guidelines:

To watch movies or listen to playlists on your phone, download them first, then switch to airplane mode while you watch or listen. Don’t sleep with your phone in your bed or near your head. Unless the phone is off or in airplane mode, keep it at least a few feet away from your bed. Take off the headset when you’re not on a call. Headsets release small amounts of RF energy even when you are not using your phone Don’t rely on a “radiation shield” or other products claiming to block RF energy, electromagnetic fields, or radiation from cell phones. According to the US Federal Trade Commission, products that interfere with the phone’s signal may force it to work harder and emit more RF energy in order to stay connected, possibly increasing your exposure.

Children, especially the very young, may be more at risk

The guidelines have a separate warning about cell phones and children. “RF energy can reach a larger area of a child’s brain than an adult’s brain,” the California health department writes, simply because the former is smaller in relation to a phone. In addition, because the brains and bodies of children and teens are still developing, the effects of exposure may be “more harmful and longer lasting” than in adults.

Plus, any kid who uses a cell phone “will have many more years of exposure to RF energy in his or her lifetime than someone who started using a cell phone as an adult,” the document reads. “There is not a lot of research about the effects of cell phone RF energy on children or teenagers, but a few studies have shown that there may be hearing loss or ringing in the ears, headaches, and decreased general well-being.”

The California health department document, though, is essentially just a warning. Any actual changes to manufacturing standards would have to originate at the FCC. In 2013, the American Academy of Pediatrics urged the FCC to begin taking child users of cell phones into account. “Children are not little adults and are disproportionately impacted by all environmental exposures, including cell phone radiation,” their letter to the FCC reads. But the FCC still maintains that “currently no scientific evidence establishes a causal link between wireless device use and cancer or other illnesses.”