Every weekday morning, Bret Bocook sits in a cozy Starbucks in downtown Los Altos. He sips coffee and reads the paper. But mostly, he watches people as they chat on their cell phones.

Then he walks over to deliver a message.

“I was observing you on your cell phone,” Bocook told a woman after she wrapped up a lengthy call on a recent morning. “I used a cell phone and I got a brain tumor.”

Startled, the woman politely listened. Bocook tends to command attention, and not just because he has the tall, broad build of a former competitive rower. The 49-year-old Los Altos man limps with a cane, the result of a surgery that removed a malignant brain tumor about four years ago but left him with shaky motor skills.

His right temple is indented where the tumor had once been. It’s also, he says, where he held his cell phone when he was a real estate agent, racking up an estimated 1 million minutes over two decades as he talked to clients.

Bocook is now among a growing number of people who believe beyond doubt that cell phones are a life-threatening health hazard. Some medical experts have also begun to raise concerns about the devices.

Scientifically, there is no consensus on whether, or to what extent, cell-phone radiation causes harm to humans. Some recent studies have tied phone use to cancer, decreased sperm count, impaired brain development and other maladies, but other research has found no such evidence.

Bocook needs no further study to convince him of the dangers of cell-phone use. In 2009, he was diagnosed with a cancer known as anaplastic astrocytoma. “As soon as I found out I had a brain tumor in this location,” he said, “it was just obvious.”

This month, Bocook appeared with a panel of scientists and physicians at the Commonwealth Club in San Francisco, arguing that phone users should gab with caution. Their point seems to be gaining traction.

The Federal Communications Commission announced this year it would review its maximum Specific Absorption Rate for cell phones. SAR is a measure of the rate of radio waves absorbed into the body of someone using a cell phone. A phone certified by the agency and sold in the United States cannot exceed a rate of 1.6 watts per kilogram, which critics say is possibly too high and based on outdated information from 1996, when the standard was set.

But regardless of whether an SAR is low, an individual’s exposure can increase when a phone is held close to the body for long periods of time. Apple now warns customers in its iPhone manuals to keep the device at least five-eighths of an inch away from their bodies while using its phones to avoid risking exposure to radiation that exceeds FCC guidelines. And Green Swan, a Novato company, makes an app that yelps a warning when talkers hold the phone too close to their heads.

‘Possibly carcinogenic’

And two years ago, the World Health Organization’s International Agency for Research on Cancer classified cell phones as “possibly carcinogenic.” The American Cancer Society and the National Cancer Institute say studies have been inconclusive.

The debate particularly resonates in San Francisco, where, three years ago, the Board of Supervisors passed a first-in-the-nation ordinance that would have required retailers to inform customers about cell-phone radiation. The legislation was blocked in court, however, in response to a lawsuit from the CTIA, the wireless industry’s trade association.

“The peer-reviewed scientific evidence has indicated that wireless devices, within the limits established by the FCC, do not pose a public health risk or cause any adverse health effects,” John Walls, the CTIA’s vice president of public affairs, said in a statement.

But Devra Davis, a visiting scholar at UC Berkeley whose 2011 book “Disconnect” warns about the dangers of cell phones, doesn’t think “the absence of definitive human harm should be held up as proof of safety.”