I've been reading America's Forgotten Pandemic: The Influenza of 1918, which is about the 1918 Spanish Flu pandemic. One of the most interesting parts is about the use of surgical masks in San Francisco.

You could buy a mask from the Red Cross for around $1.75, or from a “gouger” for around $8.75, in 2020 dollars. Levi Straus, the jean manufacturer, helped produce masks.

At first there was merely social pressure to wear a mask. The San Francisco Chronicle wrote:

It will soon be impolite to acknowledge an introduction without a mask and the man who wears none will likely to become isolated, suspected and regarded as a slacker.

Then masking became compulsory. The law read, in part:

Every person appearing on the public streets, in any public place, or in any assemblage of persons or in any place where two or more persons are congregated…shall wear a mask or covering except when partaking of meals, over the nose and mouth…

Compliance with this law was initially widespread, but it fell off once the pandemic appeared to be under control:

An increasing number of people slipped their masks down under their chins or didn't wear them all. The police arrested hundreds whom the courts subjected to punishments ranging from a five-dollar fine to thirty days in jail…[One night] the police raided the lobbies of all the downtown hotels and arrested 400 mask-slackers, most of whom had slipped off their masks to sneak a quick smoke…

Some doubted that masking laws could be enforced effectively. The mayor of Denver said, “Why, it would take half the population to make the other half wear masks."

The masking law was rescinded when it looked like the pandemic was nearly over. Then the number of flu cases started to increase again.

The mayor of San Francisco tried asking people nicely to please go back to wearing masks:

The Board of Health feels it necessary to resume the wearing of masks and I, as Mayor of San Francisco, hereby respectfully ask you to do so immediately.

Most people were unmoved. There was talk of re-instating the masking law. This prompted the opponents of masking to come out of the woodwork:

Specific opponents of masking included, as one might expect, the Christian Scientists. They had complied, albeit reluctantly, with the fall masking ordinance, but now opposed any revival of that regulation was “subversive of personal liberty and constitutional rights.” Civil libertarians…agreed: “If the Board of Health can force people to wear masks, then it can force them to submit to inoculation, or any experiment of indignity.” What businessmen hoped would be the most profitable Christmas shopping season ever had just opened, and they opposed masking on the grounds that it would frighten and depress the public and diminish sales.

Supervisor Nelson…pointed to the danger that enforcement of the mask ordnance would mean “the stilling of song in the throats of singers” and the arresting of musicians “as they blow their horns going down the street.” An octogenarian asserted that he would not wear a mask and defined the authorities to arrest him.

Someone, apparently upset by the possibility of the re-instatement of the masking law, mailed a bomb to the mayor's office.

Despite the opposition, the masking law ended up being re-instated. Hundreds more were arrested for non-compliance.

Opponents of masking banded together to form the Anti-Mask League. Its first meeting went badly:

[It] ended in a shouting match between the moderates, who wanted to circulate a petition calling for an end to masking, and the extremists, who wanted to initiate recall proceedings against Hassler [the mayor]. Pandemonium reigned until someone announced, “I rented this hall and now I'm going to turn out the lights."

It is unclear whether there were any other meetings of the League. But it was presumably short-lived: within weeks the masking law was rescinded for a second and final time, as the pandemic began to wind down for good.