Starting at midnight last night, it became illegal for anyone to be in a public space in Sonoma County without a facial covering. And now starting on Wednesday, a similar order is taking effect in both Marin and Contra Costa County.

Marin's order doesn't apply if you are out on a walk by yourself, but if you are in a public space or walking anywhere with others, face masks or coverings will be required. Children under the age of six are exempt. This takes effect at noon on Wednesday, April 22, and specific language regarding fines, etc., has not yet been published.

Speaking to the Chronicle Friday, Contra Costa County's Director of Public Health Dan Peddycord said, "If we’re going to move to a place where we relax community mitigation measures, we have first got to get a mask on everybody. It makes sense to put out messaging about mandatory masking several weeks ahead of lifting some community mitigation measures."

And San Francisco and San Mateo counties are both said to be readying announcements of their own, likely to take effect Wednesday along with Marin and Contra Costa, per the Chronicle.

Similarly this week, the city of Fremont issued an order requiring facial coverings inside all essential businesses, like grocery stores and pharmacies.

In Sonoma County, the face-mask order was issued earlier this week and took effect Friday morning at 12:01 a.m. It requires everyone to wear facial coverings at all times in public — and specifies the possibility of fines, imprisonment, "or both" for those who do not comply. "Even a small reduction in community transmission could make a major difference to the demand on healthcare system," writes County Health Officer Dr. Sundari R. Mase.

A similar order took effect today in New York State, which has come to be seen as the primary nexus of the virus in the United States. Statewide people will now be required to wear face masks in public, and Governor Andrew Cuomo earlier this week said, "Stopping the spread is everything. How can you not wear a mask when you’re going to come close to a person?"

Los Angeles and Miami are also under similar orders, but San Francisco so far has not required masks or facial coverings by legal order — despite the fact that many residents are wearing them to grocery-shop nonetheless.

Most health experts now agree that the virus can be spread through aerosolized droplets that can hang in the air in an indoor space for hours after an infected person was there. These tiny, airborne virus particles can be expelled just through talking or exhaling — not just through coughing or sneezing. In one noted case from early March, 45 out of 60 people at a choir practice in rural Washington State apparently became infected in one night after being in the same room singing for two hours.

Wear a mask!

Related: Sonoma County Makes Wearing Face Masks the Law