California's brings back indoor mask mandate: What you need to know

California on Monday announced a statewide mask mandate for indoor public spaces.

Here is what you need to know:

What is the mandate?

Beginning Wednesday, wearing masks in indoor public settings is required.

"Even a 10% increase in indoor masking can reduce case transmission significantly," said Dr. Mark Ghaly, the California health and human services secretary, in explaining the new rules.

The coronavirus is airborne and can also spread silently from infected, asymptomatic people.

How long does the order last?

It is set to expire on Jan. 15.

Who is covered?

The order will affect roughly half the state's population, including San Diego and Orange counties, the Inland Empire, the Central Valley and rural Northern California. The statewide indoor mask mandate order will last a month and will expire on Jan. 15.

Los Angeles County, Ventura County and most of the San Francisco Bay Area have their own indoor mask mandates that were implemented in the summer. While those jurisdictions have issued criteria by which officials would end the local mask mandates, there is no specific end date to the others in those areas.

Why is this happening?

California has seen the beginning of a winter surge of the Delta variant. And there are worries about the new Omicron variant.

Coronavirus case rates have risen by 50% in the last 2½ weeks, and county health officials across the state say they suspect they may be seeing the start of a winter jump in coronavirus cases. The U.S. Centers for Disease Control and Prevention considers California as having a high level of transmission of the coronavirus, the worst tier in the federal agency's four-tier scale.

California's announcement came on the same day New York enacted its own statewide mask requirement in indoor public spaces, exempting only settings where everyone inside must be vaccinated.