But, how well is this rule being enforced, and is everyone getting the message about wearing it correctly?
Obviously, covering your mouth and nose is the ideal way to do it.
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Having your mask just under your nose can be OK, according to USF researcher Jeremy Howard. Howard has been gathering data on mask compliance and is a staunch advocate of wearing masks.
Howard says most droplets come out of a person's mouth, so it's unlikely that someone could transmit the virus through their nose.
However, the person wearing the mask under the nose is less protected. Wearing a mask around your chin or off to the side is the worst way to do it.
NY Governor Andrew Cuomo admonished protestors in one of his recent press briefings, urging them not to wear their masks like a chin guard.
At SFO, ABC7 easily spotted seven people within minutes, either wearing their mask under their nose, off to the side, or not at all.
"Every time I go out I see at least four out ten people wearing a mask incorrectly," said Azelious Adams, a traveler.
However, Doug Yakel, spokesperson for SFO said that their first priority is to make sure people have masks. In that regard, their compliance rate is at about 95%.
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"Our real focus is for people not wearing a mask at all, that's the starting point, if we get to the point where people have a mask on but they're not wearing it properly, I'll view that as a sign of success," said Yakel.
SFO has been running a pretty aggressive information campaign to ensure that their travelers know that masks are required at the airport.
There are several stations at different terminals with airport staff giving out masks.
SFO recently launched an ambassador program where airport staff roam the terminals, approaching people without masks. Instead of issuing citations, airport staff educate and/or give out free masks.
"We're trying to remove those barriers of understanding and barriers of availability," says Yakel.
Getting serious on enforcement is exactly what all major US airlines are doing when it comes to masks.
United Airlines announcing on Monday that passengers who don't comply could be placed on an internal travel restriction list. Howard agrees that compliance begins with enforcement. For example states without mask rules - less than 50 percent wear them.
RELATED: Airline passengers who do not wear masks could have flying privileges revoked
"What the surveys are suggesting is that in states that require masks, compliance is pretty good, it's up 70 or 80 percent," says Howard.
Mayor London Breed agrees that education is what it will take to make sure everyone wears their masks as reopening continues. She urged the public not to take matters into their own hands.
"If you are not the police, please don't act like the police," said Breed as she walked the streets of Noe Valley, encouraging residents to shop at their local neighborhood stores.
And as a final reminder.
"The important thing is that covers your mouth," said Howard.
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