The sixth-floor rooftop terrace of 333 Baker St. in San Francisco has panoramic views of the city, but to enjoy those vistas the apartment building’s 200 residents, most of them elderly, must first pass a series of alarming signs.

“Notice. Radio Frequency Exposure Area,” reads one sign. “Radio frequency fields beyond this point may exceed FCC exposure guidelines.”

The exterior of the building, a converted 1908 hospital now called Mercy Terrace, a for-profit independent-living facility, is peppered with six cellphone antennas — with more planned to help feed the city’s ever-expanding appetite for technology. Mercy Terrace will be paid to host the installation, but neither the carrier, T-Mobile, nor the building manager would say how much.

Earlier this year, a majority of the tenants — worried that cellular signals might interfere with pacemakers or metallic elements in medications — signed a petition asking the city’s Planning Commission to prevent installation of the additional antennas. Their request was denied.

One resident, Jock Wayne, 79, who attended a Planning Commission hearing about the installation, said both the city’s Department of Public Health and the Federal Communications Commission said the antennas were safe.

“Then why all the warning signs?” Wayne asked. (Federal law requires carriers to post the warnings.)

The residents of 333 Baker St. are not alone in trying to sort out what appears to be mixed signals here about cellphone safety.

San Francisco has become the center of a growing — some say alarmist — debate over the health impact of cellphone technology. While there is no consensus in the scientific community about ill effects, and the city continues to approve additional cellphone infrastructure, city leaders have begun a high-profile effort to alert the public to potential dangers, risks they admit are unknown.

Earlier this month, Mayor Edwin Lee signed into law an ordinance, reportedly the first of its kind in the nation, that requires cellphone retailers to post and distribute notices about “how to limit exposure to the radio frequency energy emitted by cellphones.”

In recent years, the city, citing health concerns, has made many cellphone antenna installations more burdensome, requiring more permits and hearings. The city has more than 1,000 antennas, with 350 more proposed.

“We are out front on this issue,” said John Avalos, a mayoral candidate and city supervisor who championed the recent ordinance. “We don’t know what long-term exposure can mean.”

Experts say it will be at least a decade, and possibly longer, before any health impact of cellphones is known.

Scott James/The Bay Citizen

A warning sign at Mercy Terrace

“Our knowledge is growing, but is still very much incomplete,” said Renée Sharp, California director of the Environmental Working Group, a nonprofit organization that addresses environmental health concerns. She noted, though, that the World Health Organization had raised questions about a potential cancer link to cellphones.

The cellphone industry strongly disagrees, and so far has the law on its side. Last week the CTIA-The Wireless Association, a trade group, indicated it would mount a legal challenge to San Francisco’s ordinance.

“It conflicts with the conclusions of the expert federal agencies, including the National Cancer Institute, the FDA and the FCC, that the weight of scientific evidence has not linked cellphones with any health problems,” the association’s vice president, John Walls, said in a statement to The Bay Citizen.

In fact, despite the city’s new ordinance, the Planning Commission was required to follow federal laws, which allow for cellphone antennas, when it overruled the concerns of the residents of 333 Baker St., according to commission members. “We can’t take into consideration health impacts,” the commission’s president, Christina Olague, said.

Avalos agreed that the city was bound by federal rules. “The telecommunications industry has an enormous amount of influence over the FCC,” he said. Cellphone companies “will do whatever they can to protect the bottom line.”

For some residents of 333 Baker St. signing the petition was an act of courage. Half of the tenants are elderly immigrants who lived in Russia during the Soviet era, when signing one’s name to a list challenging authority meant a trip to the gulag.

Yet the residents signed anyway, an indication of how strongly they felt. Little did they know that it would not matter. For planning, the city cannot consider health worries about cellphones, even though it has drawn up a new ordinance that raises the same concern in stores.

To paraphrase a popular TV commercial: Can you hear me now? Uh, never mind.

This article also appears in the Bay Area edition of The New York Times.