Few people know that there are federal safety limits for exposure to the weak radiation emitted by cellphones and other wireless devices. There often is language about this embedded right in our phones, but finding it requires knowing where to look, wading through sometimes five or more steps and then making sense of the technical jargon.

Concerned about the lack of public awareness, Berkeley, California passed a law in 2015 calling for cellphone stores to post warning signs. The signs caution that if you carry a phone in a pocket or tucked into a bra when the device is on, “you may exceed the federal guidelines for exposure’’ to radiofrequency radiation.

That advisory is intended to sum up the information the Federal Communications Commission requires cellphone manufacturers to disclose in phones, owner manuals or packaging material, and it’s milder than warnings given by such organizations as the American Academy of Pediatrics. But eager to squelch unfavorable information, the wireless industry–led by the big trade group CTIA–has waged a high-powered legal battle to take down the signs. In court filings, CTIA has called the advisory “inflammatory” and has argued that the Berkeley law violates cellphone store owners’ First Amendment rights by compelling them to disclose misleading information.

Berkeley won the first round in federal district court, and then again in the 9th Circuit U.S. Court of Appeals. But it was dealt a setback on June 28 when the U.S. Supreme Court vacated the appeals court ruling, and sent the case back to the 9th Circuit for further consideration.

CTIA’s legal argument against Berkeley is emblematic of a widening effort by business groups to contest regulations affecting, among other things, drugs, tobacco and campaign finance by arguing that the rules violate free speech protections. Lawrence Lessig, a Harvard law professor who is representing Berkeley pro bono, said the case is not only about cellphone safety but ”the way the First Amendment has become a tool of corporations to stop regulation.”

First Amendment arguments have won favor at the Supreme Court, which ruled last month on free speech grounds that public employees could not be required to pay dues to their union locals. That decision prompted Justice Elena Kagan to declare, in a dissent, that the Court was ”weaponizing the First Amendment” and using it as ”a sword … against workaday economic and regulatory policy.”

Public health advocates also see the challenge to the Berkeley law as part of a broader strategy to keep the possible risks of cellphone radiation off the public radar. The effort has been largely successful, they say, partly because of industry pressure and legal tactics but also due to our overwhelming dependence on wireless devices and the reluctance to consider the possibility that they cause harm. Meanwhile, initiatives by regulators to update or expand public warnings have languished for years, as one government agency after another has hesitated to take action.

Whether radiation emitted from cellphones actually poses a risk of cancer or other harm remains an open question. Unlike ionizing radiation such as X-rays or gamma rays, which are known to be strong enough to damage DNA, radiofrequency waves emitted by cellphones are much weaker, and health research has produced ambiguous findings.

While the threat, if any, is probably very small for any single individual, the exposed population is so enormous that any risk at all could trigger a significant number of preventable illnesses. As a result, many health officials and advocates urge that, as a precaution, people take simple steps to limit their exposure such as using headsets and avoiding use of the devices when reception is poor, a situation that causes them to emit more radiation. Advocates also want to push manufacturers to engineer cellphones to reduce radiation.

All of these measures could be particularly important for children, whose brains have been shown to absorb more than twice as much radiation relative to their body size as adults and whose developing bodies may be more sensitive to radiation’s effects. Many children now also become cellphone users at an early age and have decades of exposure ahead of them.“How much proof do you need before you start to protect people? Are we going to go down road of tobacco and asbestos and say, ‘You’ve got to prove human harm before we take steps to protect people?’ I sure hope not,” said Devra Davis, founder of the non-profit Environmental Health Trust, which focuses its advocacy efforts on cellphone radiation.

Some studies, including recent federal government research, have linked radiofrequency radiation to a higher incidence of certain rare heart tumors in test animals. Just this March, an expert panel reviewed two recent draft papers based on a $25 million study commissioned by the FDA on the impact of cellphone radiation on mice and rats. The panel concluded, among other things, that there was “clear evidence of carcinogenic activity” in male rats exposed over a long period of time to low levels of radiofrequency radiation. Schwannoma, a rare tumor of the heart, appeared in significantly more exposed than unexposed animals.

The same rare tumor of the heart surfaced in separate research published this year by European investigators at the Ramazzini Institute in Italy. Male rats in that study developed the tumor at a significantly higher rate than animals that weren’t exposed. What’s more, hints of increased risks for a rare but lethal type of brain tumor, glioblastoma, emerged in both the FDA-commissioned study and a survey of human cases in England, also published in March.

The new findings come after the classification in 2011 by the International Agency for Research on Cancer, a part of the World Health Organization, of radiofrequency radiation as a “possible ” human carcinogen . For now, authors of the recent studies say more research is needed to confirm whether there are health risks for cellphone users.

However, many other studies, including most of those supported by industry, have shown no health effects. And until more is known, the wireless industry and some scientists and government officials warn against fostering unwarranted fears. “The last thing you want is to create a sense of helplessness and false alarm,” said Dr. Nathaniel DeNicola, an environmental health expert with The American College of Obstetricians and Gynecologists. In the absence of stronger data, he cautioned against jumping to any conclusions but, in the meantime, he discourages his patients from keeping their phones in direct contact with their skin.

Today, there are more cell phones in the U.S. than people. More and more, our productivity, our social lives and even our personal safety rely on our phones. And, not surprisingly, most of us — this journalist included — would rather talk, text and put phones in pockets without worry.

“There’s a lot of denial operating at every level — from industry to government to the general public,” said Joel Moskowitz, director of the Center for Family and Community Health at the University of California, Berkeley and one of the architects of the Berkeley law. “People do not want to know about these potential risks. That’s understandable because they are so dependent on this technology. It’s quite a morass.”

For its part, the cellphone industry says it is committed to protecting the public. “The safety of cellphone consumers is important to CTIA and the wireless industry,” said Justin Cole, spokesman for CTIA, which represents such companies as AT&T and Verizon Wireless. “We follow the guidance of the experts when it comes to cellphones and health effects.”

But some reports dispute the notion that the industry simply follows the science. An investigation in March by The Nation highlighted a 2017 review of research literature that concluded that industry-funded studies were 2½ times less likely than independent studies to find a health effect associated with cell phone radiation.

The report also focused on an alleged industry disinformation campaign against research providing evidence of possible health effects. One example involved research in 1994 that found DNA damage in the brain cells of rats exposed to low levels of radiation similar to that emitted by cellphones. Motorola sought to downplay the study’s significance and discredit its authors. An internal company memo leaked to the independent publication Microwave News, described the company’s plans to “war-game” the issue, including “recruiting individuals willing and able to reassure the public.”

The wireless industry also deploys what author Norm Alster in his 2015 book “Captured Agency” calls a “huge standing army of lawyers” to snuff out opposition. One example: When San Francisco in 2010 passed a law similar to Berkeley’s, CTIA challenged it on First Amendment grounds, and the city backed off after losing a federal appeals court ruling. “When they saw the San Francisco law happen, a bunch of cities were thinking about adopting similar laws,” Moskowitz said. “But once San Francisco got sued by CTIA, others didn’t want to get on board because they didn’t want to get into a lawsuit too.”

Berkeley, however, moved ahead with a narrower “right to know” ordinance. Its milder wording is closer to the FCC-mandated disclosures and avoided San Francisco’s requirement for retailers to tell customers that their phones could expose them to dangerous, possibly cancer-causing radiation.

CTIA, however, claims that the language required on signs by the Berkeley ordinance is flawed and, thus, it is unconstitutional to compel the signs. It has argued that the signs are “fraught with negative associations” meant ”to stoke consumer anxiety,” and are therefore not “purely factual.”

Heading their legal team is powerhouse attorney Theodore Olson, the former U.S. solicitor general who argued the case that put George W. Bush in the White House. (Olson recently turned down an offer to join President Donald Trump’s legal team.) CTIA did not comment on how much money they have spent on the case. But the group invests heavily to influence policy, including $89.4 million on lobbying in Washington since 2010, according to the Center for Responsive Politics. That sum doesn’t count the much larger sums spent by CTIA member companies.

Although the Berkeley law has survived so far, the CTIA managed to get one phrase stricken from the original warning: “This potential risk is greater for children.” U.S. District Judge Edward Chen agreed with the industry group that the statement lacked scientific support.

In fact, some health authorities believe the potential risk is greater for children. In guidelines on cell phone radiation published in December 2017, the California Department of Public Health notes that the radiation can reach a larger area of a child’s brain compared to an adult’s brain. The guidelines, posted on the agency’s website, also cite studies that have linked long-term, frequent cellphone use with cancer and low sperm counts, as well as learning, behavior, hearing and sleep issues.

That disclosure, however, wasn’t made until eight years after the health department drafted the guidelines – and it took a lawsuit by Moskowitz to get the information released. Asked why it sat on the information so long, the agency told FairWarning that its internal reviewers had determined that the agency’s document didn’t add anything beyond what already was publicly available.

The FCC is the only federal agency that, for years, has provided wireless radiation exposure guidelines – the ones that are the foundation of the Berkeley warning signs. The FCC cautions that radiation exposure from a wireless device could exceed its guidelines if carried closer than about a half-inch from the body. But those guidelines date to 1996, and dramatic changes have occurred since then in cellphone technology and how the devices are used. In the late 1990s, a cellphone clipped on a businessperson’s belt might have been a relatively common sight; one in the hand of a 10-year-old child would have been rare. What’s more, along with the 380 million cellphones now in use across the U.S., today devices such as iPads as well as Wi-Fi networks are producing low-level radiofrequency radiation exposure.

FCC spokesman Neil Grace said in a statement to FairWarning, “Scientific evidence always informs FCC rules” on radiofrequency safety. But, in 2012, the U.S. Government Accountability Office recommended that the FCC reassess its exposure and testing requirements for cellphones, which it suggested “may not reflect the latest research.”

The following year, the FCC began a review but, five years later, the process still is under way and no changes have been made. The FCC declined to comment on why the agency hasn’t revamped the requirements and whether industry influence has delayed action.

The Food and Drug Administration has responsibility for protecting the public from radiation-emitting electronic products and advises the FCC. The agency said its current position on cellphones and human health was explained in a February 2018 statement from Jeffrey Shuren, director of the FDA’s Center for Devices and Radiological Health: “I want to underscore that based on our ongoing evaluation of this issue and taking into account all available scientific evidence we have received, we have not found sufficient evidence that there are adverse health effects in humans.”

Still, De-Kun Li, a senior research scientist at Kaiser Foundation Research Institute, advises taking seriously the possibility that cellphones can cause human harm. He has co-authored a series of papers that highlight potential risks for poor semen quality, miscarriage, asthma and childhood obesity from exposure to magnetic field radiation — including radiofrequency from cellphones. He underscored the point that developing fetuses and young children may absorb more radiation relative to their body weight and be more vulnerable to health effects, given their rapid brain and nervous system development.

“My hope is that we’ll start to rethink the currently pervading assumption that there are no biological effects” for humans from radiofrequency radiation, Li said. “We don’t have to ban the use of cell phones. But we can give people a few simple precautionary measures they can take, just in case they turn out not to be safe.”