Masks work, see for yourself
A study released by the CDC in October concluded that masks (among other mitigation measures) helped reduce COVID-19 incidence in Arizona by 75 percent. Back in the spring, Governor Ducey not only refused to put a statewide mask mandate in place, he signed an order preventing local governments from doing so. Infections soared, as shown in the this screenshot of positive test percentage by day (from the very useful ASU Biodesign dashboard; Xs are daily values, the red line is a 7 day trailing average, and the grey line is cumulative positive tests):
Then in late June, after Arizona had been the worst place in the world for coronavirus infections for several weeks, the governor rescinded the order, local ordinances were enacted, and it became a rare sight to see anyone in a store or other indoor public space without a mask (at least in the Phoenix area). As you can see from the graph, a couple weeks after this change the positive test rate declined as fast as it rose.
The primary transmission method for the virus is through droplets expelled in your breath when you vocalize, cough, or sneeze. Masks block these droplets. You can see it for yourself in this video of someone counting to ten unmasked and with three types of mask (h/t to Bryan for bringing this to my attention):
The larger droplets travel a short distance and fall onto the floor or another surface where they can come in contact with your hands, then be introduced into your nose, mouth or eyes through touch. Back when the pandemic started, scientists thought this was the primary means of transmission, so handwashing (not masks) was the recommended mitigation measure. With the smaller droplets, the saliva can evaporate (especially in a dry desert climate) leaving airborne viral particles that float in the air for some time, where they can be inhaled by a new victim.
There is still disagreement about the primary way coronavirus spreads, and there is controversy about the role of the airborne route. On one hand, the CDC says that if airborne transmission were a main infection method then SARS-CoV-2 would spread as easily as measles, which it does not. On the other hand, different scientists claim overwhelming evidence that airborne transmission is a major infection route and that it is heavily involved in superspreader events.
One thing is completely certain: All three known methods of transmission — contact with contaminated surfaces, droplets, and airborne particles — originate with people spraying spit as shown in the video above. As you can also see from the video, masks do a good job of keeping that spray out of the air.
