Employers will be required to provide the masks, and employees will be required to wear them, according to the order.
The county Department of Public Health issued the new order amid a staggering increase in COVID-19 infections as the highly-contagious omicron variant continues to spread rapidly.
On Wednesday alone, the county reported more than 26,000 new COVID-19 cases and 27 additional deaths.
Under the new order issued Wednesday, employers must start providing employees who work indoors next to others "with well-fitting medical grade masks, surgical masks, or higher-level respirators, such as N95 or KN95 masks."
The order comes as new research indicates that simple cloth masks do not do a sufficient job at blocking out virus particles and stopping the spread of infection.
"These upgraded masks are better at blocking virus particles from going through the mask," the county order states.
How effective are cloth masks?
A study released in November by researchers at Yale University, Stanford University and others found that villages in Bangladesh where surgical masks were worn had 11% fewer cases of COVID than in villages where no masks were worn. In villages that wore cloth masks, infections were reduced by only 5%.
The University of Southern California is also now requiring medical-grade masks on campus.
The university issued a memo on Wednesday stating: "Per these revised guidelines for workplaces, individuals on USC campus premises in locations where masking is required are now required to wear medical grade masks, which at minimum are surgical masks and may also include higher grade respirator masks (N95, KN95, or KF94)."
New rules for cardrooms
The county issued several other modifications to its existing health order on the use of masks.
Patrons at cardrooms, who are already required to wear masks while indoors, may no longer consume food or drinks while playing. Such behavior typically involves removing a mask while still in close contact with others. Instead, they may only consume food and drink in designated dining areas.
The county is also recommending, but not requiring, that large venues, including concert halls and movie theaters, only allow food and drink to be consumed in designated areas.