If you still need evidence that wearing a mask works and helps prevents you from spreading COVID-19 if you are infected, look no further than Dr. Rich Davis's Twitter feed. The microbiologist performed a demonstration to show just how effectively masks block respiratory droplets from spreading to other people, and the results were astounding (and yeah, a little gross).
In his now viral (no pun intended) thread, Dr. Davis explains what he did. He sneezed, sang, talked, and coughed toward agar culture plates with and without a mask and put the culture plates in an incubator for 24 hours. In this process, bacteria colonies will appear on the agar culture plates anywhere respiratory droplets land. The conclusion? "A mask blocks virtually all of them."
Look at those bacteria colonies! They're most present on the plate where he sneezed without a mask on. Yet, none showed up when he sneezed with a mask on. That says a lot.
And if you're wondering (we all were), he explained that the song that he sang to the culture plates was "Dear Theodosia" from Hamilton. He sang at the top of his lungs. Turns out if you do that with a mask on, it's fairly safe!
In a second demonstration, Dr. Davis did not wear a mask but stood at a distance from his culture plates while coughing. He wanted to show how many respiratory droplets travel certain distances. Then, he put a mask on and repeated the demo. The results speak for themselves.
He coughed into the plates ("hard, for 15 seconds) from two, four, and six feet away with a mask on, and then without a mask on. Bacteria pretty much only showed up on plates where he wasn't wearing a mask. Predictably, the most showed up when he was two feet away, fewer at four feet, and the least at six feet.
Dr. Davis clarifies in following tweets that he knows this simple demonstration "isn't how you culture viruses or model spread of SARS-CoV-2." But he says that the colonies of normal bacteria that do show up on these plates indicate the spread of "large respiratory droplets, like the kind we think mostly spread #COVID19."
He's very careful to note what the demonstration shows and what it does not. But from the results of this exercise, it's not out of bounds to conclude that a mask works to prevent the spread of the mask wearer's respiratory droplets.
This demonstration does show that "particles of liquid" come out of your mouth when you are talking, singing, coughing, or sneezing. It also shows that these droplets have the capability of carrying microbes like bacteria.
The third conclusion he draws is the most important one: Masks blocks most of those droplets. He then explains very plainly what this demonstration does not show or prove.
This demonstration was limited in scope. It does not show the "number, size, and distribution of these respiratory droplets. Some could have landed outside the scope of the dish. It's also impossible to count them or detect their size based on the bacteria that grew in the culture plate.
This demonstration in and of itself also doesn't prove that a mask will block droplets carrying the virus. However, as he points out, we can infer that this is true based on many other scientists' and experts' reports.
Dr. Davis's conclusion is a simple one: "Masks as a political / social litmus test or used to shame those who won't (or disabled folks who truly can't!) wear them is a travesty. We wash hands after using the bathroom and wipe noses on tissues. Masks / face shields need to be just another normalized act of hygiene. #MasksWork"