California’s recent power shut-offs, meant to reduce the risk of potentially catastrophic fires, have had an unwelcome side effect. The blackouts have also cut power to many cellphone towers, blocking the main communications source for many in harm’s way.

“You don’t appreciate how essential cellphone service is until you lose it,” said Chris Ungson, deputy director for communications and water policy for the California Public Advocates Office, an independent agency within the state’s Public Utilities Commission. “It’s not just a matter of inconvenience; it’s a matter of public health and safety. It’s a lifeline to many, many people.”

[Update: California shop owners on how much will PG&E blackouts will cost them.]

Emergency calls to 911 are one indicator: The Governor’s Office of Emergency Services said more than 80 percent of such calls in California last year were made by cellphone.

For years, state and federal regulators have pressed the cellular companies to better reinforce their networks for emergencies. The Federal Communications Commission said Monday that it was conducting “a comprehensive review of the wireless industry’s voluntary commitment to promote resilient wireless communications during disasters.”