A microbiologist used a germ-filled petri dish to show how a mask catches droplets that spray from your mouth when you talk, cough, sing, and sneeze

A microbiologist used a germ-filled petri dish to show how a mask catches droplets that spray from your mouth when you talk, cough, sing, and sneeze
Crystal Cox/Business Insider
  • A microbiologist's mask-wearing experiment is all over social media. It demonstrates how far a person can spread respiratory particles, which can transmit the coronavirus, with and without a mask.
  • Critics of the demonstration said it didn't use a large enough sample size, and that the researcher's use of bacteria wasn't helpful because the coronavirus is a virus.
  • But a virologist told Insider the demonstration makes a valid argument for mask-wearing as a vital public safety measure during the pandemic.

Face masks have become one of the most hotly debated coronavirus-pandemic topics, despite the CDC and WHO recommendations to wear them, so a microbiologist conducted an experiment that illustrated just how useful masks can be.

Rich Davis, the regional director of microbiology at Providence Health Care in Spokane, Washington, coughed, sneezed, and sang over a petri dish for 15 seconds. He did it a few times, half of the time while wearing a standard triple-layer hospital surgical mask, and half without a face covering.

He then looked at the petri dishes: the ones that he stood over wearing a mask had a few speckles of bacteria in them. The ones he used without a mask wear filled with germs.

Davis conducted his experiment from two, four, and six feet away from the bacteria-collecting dishes to see if distance made a difference, and found that the six-foot mark led to less bacteria spread both with and without a mask. But at all of the distances, Davis noted that mask-wearing substantially lessened particle spread.

The experiment used bacteria, not viral particles, but the results are still valid

Though the coronavirus is a virus, not bacteria, Davis told Insider his demonstration still shows how masks stop large respiratory droplets from traveling through the air. These droplets are the same kind that transmit COVID-19 particles.


"Even though individual viruses are many magnitudes smaller than individual bacteria, large droplets are what most respiratory viruses are contained in and these are blocked by masks," Davis told Insider.

He also noted that the sample size of one (Davis only performed the experiment using himself) was another caveat of the demonstration.

William Schaffner, a professor of preventive medicine and infectious diseases at Vanderbilt University Medical Center, told Insider Davis' experiment, despite its caveats, acts as a helpful visual example of why mask-wearing is so important right now.

A microbiologist used a germ-filled petri dish to show how a mask catches droplets that spray from your mouth when you talk, cough, sing, and sneeze
Richard E. Davis/Providence Sacred Heart Medical Center and Children’s Hospital

According to Schaffner, Davis' demonstration mimics previous studies that showed the effectiveness of mask-wearing to stop the spread of infectious particles.


"It's nice to have that physically reinforced in the current environment," Schaffner said.

He added that the differences between bacteria and virus spread are "so nuanced that they're not important. The major lessons about the general effectiveness of masks in interrupting transmission of infectious agents hold firm," in Davis' experiment, Schaffner said.

'What more do you need?'

Davis' demonstration was met with some backlash. Critics said his one-off trial was imperfect and didn't prove mask-wearing was necessary (though some people contacting him to say the experiment inspired them to finally commit to wearing a mask).

"The most frequent criticism I've seen is definitely a quick message of, 'Don't you know that bacteria are not the same thing as viruses?' I've really tried to provide all the necessary context and caveats for these really dramatic images people are seeing (both in the images themselves and in the discussion and explanation)," Davis said. "Because of that, it's easier to see a lot of these criticisms as people who maybe didn't take the time to read what's there already."

Schaffner agreed, and said those critics are missing the point. Although it's true masks are an imperfect solution to stopping coronavirus spread, they're a tool that can be combined with other methods to get the job done.


"So by using masks of various kinds, social distancing, or avoiding crowds, et cetera, those are the things that, taken cumulatively, have been shown very nicely to substantially reduce the risk of transmission if we all do them together," Schaffner said.

"What more do you need? Stop trying to be a scientist. Don't get into the details. The important thing is to wear a mask."