How Covid Spreads and How to Prevent Covid: It’s becoming more apparent now that this covid spreads not just through contact and respiratory droplets that fly through the air like ballistics, but also it’s being transmitted through the airborne route, meaning through aerosol, meaning the virus lingers in the air, and then someone inhales the virus. This is known as airborne transmission.
Let’s face it; there is a reason why hospitals with designated COVID areas require everyone to wear an N95 respirator mask, as well as eye goggles. That’s because we know that this virus has the potential for airborne transmission. During normal breathing and speech, tiny particles are emitted mainly from the mouth. These particles can range in size, with the smallest being less than a micron (1 um) and the biggest being over 500 um in diameter.
To put some perspective on that, the average diameter of human hair is about 80 microns). Typically droplets that are less than 5 um are considered small, and it’s these tiny droplets can be suspended in the air. Droplets over 100 um are considered large, and between 5 and 100 microns is intermediate. But the reality is, it’s a range of sizes; it’s a continuum, from less than 1 um to over 500 um. And more and more particles are emitted when someone is breathing heavier, such as with exercise, or if someone is coughing or sneezing, or if someone is shouting or singing.
How Covid Spreads
Due to gravitational forces, particles more prominent than 5 microns tend to settle, meaning they fall on surfaces such as the floor and relatively close to the source, typically within 6 feet. This is why the CDC recommends 6 feet for social distancing. But here’s the thing: sometimes these larger particles travel further than that, especially if someone is breathing heavy, shouting, singing, coughing, or sneezing.
Typically they fly no further than 12 feet in these situations. But we’re also spraying particles smaller than 5 microns, and it’s these tiny particles don’t act like ballistics; they act more like a gas cloud, where they float in the air and travel up to 27 feet. The ones that are less than 1 um evaporate within milliseconds of hitting the air, while the particles that are more than 100 um can take up to a minute to evaporate.
What happens when the droplets are less than 5 microns? What if they are spewed from someone who is infected with the virus, and all of a sudden, in midair, they evaporate? Well, they dry out, and you’re left with a virus floating in the air. These are called droplet nuclei, aka aerosols.
There are lots of factors that determine how long aerosols remain in the air. It depends on the person who emitted the particles, how they emitted them, the temperature and humidity of the environment. Lack of airflow means this cloud will persist longer. And when this moist cloud finally does dissipate, you’re still going to have droplet nuclei that stay airborne for about 3 hours, based on that NIH study.
At this point, we might not have 100% conclusive evidence that proves airborne transmission, but there are now several studies that strongly suggest that to be the case. Just because we know that this virus spreads through the airborne route, that’s not to say that it doesn’t spread through contact and respiratory droplets, meaning the bigger droplets that act like ballistics. It spreads by all 3 of these mechanisms.
So hand washing is still important as it is not touching your face or mask with dirty hands. And maintaining 6 feet apart is a good thing, but it’s not good enough for specific situations. Remember earlier how I said when someone sneezes, that moist cloud containing aerosols can travel up to 27 feet? And the virus can linger in the air for 3 hours.
Some rooms have adequate ventilation that supplies clean outdoor air and minimizes recirculated air: the better the ventilation, the less likely the spread of aerosols. And even cracking open a window can make a huge difference, and having a fan blowing is good too. Other measures can help, too, like having an air purifier with high-efficiency air filtration and germicidal UV lights.
Doctor Mike Hansen, MD
Internal Medicine | Pulmonary Disease | Critical Care Medicine
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