Scientists know that the Covid-19 can spread through droplets that spew from the nose and mouth after every cough. But there has been controversial back and forth as to whether droplets are also released when we're just talking — let alone yelling.
In a video attached to a letter published in The New England Journal of Medicine, researchers used fluorescent imaging to track just how far spittle can spray when phrase "stay healthy" is uttered. Below, you can see how the darkened box lights up with spray — especially when the scientist lingers on the "th" sound in the word "healthy."
Overall, droplets between 20 microns across and 200 microns across were created when the scientists spoke (for reference, a single human hair is about 90 microns across). The louder the scientists spoke, the more spit was sent flying.
Meanwhile, when they covered their mouths with a slightly damp towel, the amount of spit released was reduced significantly (you can see the "mask" condition above).
The team recorded the video with an iPhone 11 Pro video camera and estimated the size of the droplets from ultrahigh-resolution recordings. The aim: "provide visual evidence of speech-generated droplets and to qualitatively describe the effect of a damp cloth cover over the mouth to curb the emission of droplets."
While speaking makes the video light up with spit like a garish Christmas tree, there is still an ongoing debate as to whether the virus can actually live in particles of certain sizes
A traditional droplet that can spread the virus is around 5 microns in size, a relatively large droplet when it comes to spit. These are the droplets that emerge when we cough, sneeze, or, snot-rocket. The droplets that emerge in this video range from far smaller to far larger (up to 200 microns).
The World Health Organization's site reads that such heavy virus-laden droplets can't linger in the air, and instead, fall to the ground unless they come into contact with another person within one meter. The WHO has been adamant that the virus is not "airborne", but has noted that the virus can be spread when a patient speaks.
However, on April 1, Harvey Fineberg, the President of the National Academies of Sciences released a letter concluding that the virus can be spread through aerosols generated by "patients' exhalation." Another letter published in the New England Journal of Medicine on April 16 found that SARS-CoV-2 could linger on aerosols less than 5 microns in size.
In response, the World Health Organization, in a review, stated that the experimental conditions in that New England Journal of Medicine letter didn't mimic real "human cough conditions."
Still, Warner Greene, an expert in immunology and microbiology at the University of California San Francisco, tells Inverse that "the data for the coronavirus is pretty clear." Greene is also director of the Gladstone Center for HIV Cure Research.
"They can be transmitted in aerosol forms — droplets that are less than 5 microns," he says.
That said, even if transmission on these small particles is possible, we don't know whether or not the amount of virus present on those small spittle packages is enough to cause a severe infection. For instance, a study from China published in The Lancet suggests that viral load (getting a big dose of the virus) is linked to having more severe symptoms.
Greene explains that likely, even a small amount of contact — even via a small aerosol — poses some risk of infection, generally. How each person's immune system responds to that virus can differ.
"I think any amount of virus potentially — if it gets deposited on a mucosal surface — is enough," Greene says.
While research is ongoing, the evidence suggests that it's just not worth taking the risk of spreading the virus. If you must leave quarantine, wearing a cloth face covering can help protect yourself and others.
If you must scream into the void, try to ensure there isn't someone lurking on the other side. Or at least, wear a mark while doing it