SACRAMENTO, Calif. —

California Gov. Gavin Newsom on Monday announced a voluntary smartphone tool to alert people of possible coronavirus exposure as cases soar higher, new restrictions are imposed and many people still say they won’t heed the pleas to stay home.

The tool — which has been used on a pilot basis on some state university campuses — doesn’t track people’s identities or locations but uses Bluetooth wireless signals to detect when two phones are within 6 feet (1.8 meters) of each other for at least 15 minutes, officials said.

California’s 40 million residents can opt in to the system starting Thursday. When someone who has activated the technology tests positive for the virus, that person will receive a verification code from state health officials that can be used to send an anonymous alert to other users who may have been exposed over the past 14 days.

“The more people that participate in it, the more that opt in, the more effective this program can be,” Newsom told reporters. “We are hoping there will be enough to make this meaningful.”

The technology comes as coronavirus cases are exploding in California and more than 80% of the state’s residents are under orders not to leave their homes for at least the next three weeks except for essential purposes. Sixteen other states, plus Guam and Washington, D.C., have already made available the system co-created by Apple and Google, though most residents of those places aren’t using it.

Andrew Noymer, a public health professor at University of California, Irvine, questioned how many residents would opt in due to privacy concerns and the value of the tool if they don’t.

He said people may find themselves paralyzed by a flood of information and it isn’t clear what they’ll do with it — especially if they take a coronavirus test after getting an alert and wind up negative, only to receive another alert.

“In a purely epidemiological perspective, uptake is everything. If about 10% of people do it, it’s useless,” he said. “Even if it does get takers. It’s still unproven. Because then, what do you do?”“

Over the past two weeks, California has reported a quarter of a million positive virus cases. The 7-day average for new virus cases on Monday neared 22,000, a 50% increase over the prior week, state data shows.

More than 10,000 people were hospitalized with COVID-19, including more than 2,300 in intensive care, Newsom said.

The state’s 400 hospitals are at about 80% capacity but there are hospitals in San Diego, Imperial, and Los Angeles counties with intensive care units that are full, said Carmela Coyle, president of the California Hospital Association. Hospitals are limited by staff shortages following a spike in virus cases around Halloween, she said.

“These numbers do not yet include the Thanksgiving holiday, and the gathering of families just a week or so ago so. We do expect that this will get far worse before it gets better,” she said.

Newsom’s administration issued the stay-at-home rules closing restaurant dining, salons and playgrounds in Southern California and a large swath of the state’s Central Valley agricultural region after more than 85% of beds in intensive care units were occupied in those regions. Five San Francisco Bay Area counties voluntarily joined the rules over ICU capacity concerns. Those restrictions will last until Jan. 4, a week longer than the state’s timeline.

Ten months into the pandemic, most of the state is now back to where it started with the stay-at-home rules. But unlike in March, when the pandemic was in its infancy and California was the first state to impose such rules, fewer people are likely to obey them.

Some business owners said they would keep their doors open and several law enforcement agencies say they won’t enforce the rules and are counting on people to voluntarily wear masks and practice physical distancing to protect themselves and their families.

Lu Garcia Reynoso, who owns a Southern California barbershop, told the Press-Enterprise he’ll stay open. She’s concerned salons may move underground to avoid being detected.

The recent rise in coronavirus infections began in October and is being blamed largely on people ignoring safety measures and socializing with others.

Under the new stay-at-home rules, retailers including supermarkets and shopping centers can operate with 20% capacity while restaurant dining and hair and nail salons must close.

Schools that are currently open can continue providing in-person instruction.

But Los Angeles Unified School District, the nation’s second-largest, on Monday announced the suspension of all school-based instructional and childcare programs and conditioning programs for student athletes due to the record number of virus cases.

Starting on Thursday, Californians will be able to activate the new “exposure notification” tool in their iPhone settings or on Android phones by downloading the CA Notify app from the Google Play store. Many residents will get a notification inviting them to participate.

Officials said the encounters are temporarily logged in a way that doesn’t reveal a person’s identity or geographic location.