ST. LOUIS (KMOV.com) -- Friday, masks will be required in all public places in St. Louis City and St. Louis County.
However, the mask mandate only applies to children nine and older.
The age cutoff isn't arbitrary, however.
COVID-19 spreads rapidly through droplets traveling through the air when people cough, sneeze or talk.
Doctors say the lungs of children under nine are typically not strong enough to generate large droplets to spread the disease.
They say a mask is also not effective if a child is routinely touching it.
"So at the age of nine, we'd expect most kids would be able to developmentally keep a mask on, not touch it all the time," said Dr. Alexis Elward, an infectious disease specialist with Washington University. "Then there are the kids who are older than nine who are probably going to be generating deeper coughs and more forceful aerosols."