By now, everyone is aware of the importance of physical distancing to prevent the spread of COVID. The six-foot barrier concept has been hammered into our brains. It’s simple, understandable, and (reasonably) easy to follow. But does this really keep us safe, or at least safe enough? The reality is more complex and is important to understand as our economy begins to open up.
Transmission of COVID-19 is impacted by many factors, including:
- How an infected person transmits COVID
- Duration of exposure
- The setting/environment
- Your susceptibility
- Physical distance
You need to keep all of these in mind if you are planning to go to a friend’s house, a restaurant, the salon, or any other public place.
First, some basic science. In order to get infected with COVID, you need to have inhaled a certain number of virus particles, called the infectious dose, before an infection can take hold. It’s different for every infection. Norovirus, a common cause of diarrhea, only requires about ten particles to cause infection, while E. coli, another diarrheal illness, needs about one million particles to cause infection. We don’t know about COVID-19 yet, but initial estimates for the infectious dose is about 1,000. This information is important because it can help us understand what we can do to limit your risk of infection.
The infectious dose can occur all at once, or over time. In other words, you can inhale 1,000 particles in one breath, 100 particles over ten breaths, or 10 particles over 100 breaths. As long as the previously inhaled virus particles are still alive, the result is additive. And right now, it is unclear how long the virus can live in our nasal passages before causing an infection.
COVID is spread mostly by respiratory droplets, which tend to fall to the ground after a few feet of breathing them out. This is where the six-foot distance arises. Based on studies prior to COVID, the risk of 1000 viral particles reaching you from an infected person beyond six feet is quite small.
However, there’s much more to this. Six feet is not a magic number, but more of a guideline. Your risk is lower at eight feet than six, and lower again at ten feet. There’s no magic number where you are completely safe, but the risk beyond six feet is considered acceptable. How high the risk is at different distances is unknown, but we do know that many factors impact the risk.
Duration of exposure plays a very big role. Your risk goes up the longer you are exposed to an infected person, even if you are six or more feet away. Let’s say only 10 particles can reach you after each breath from six feet. That’s a very small dose, and unlikely to cause an infection. However, if you take 100 breaths in that space, which is only about 10 minutes, suddenly you are at the threshold for COVID-19 infection. Some studies suggest the duration of exposure can be a bigger factor than distance, which would explain why COVID is more likely to spread among coworkers or housemates than in public spaces.
Also, the number of people you are exposed to increases your chances of getting COVID. We don’t know the true number, but estimates of the prevalence (percent of people with the disease) of COVID-19 in the U.S. is currently about 2%. That means there is approximately a one in 50 chance of anyone you encounter having COVID. That’s pretty low. But if you encounter 10 people, all of the sudden your risk jumps to 20%, which is starting to be uncomfortably high. This is why most states are banning crowded activities.
The setting of exposure is important as well. We now know that COVID is nearly 20 times more likely to spread indoors than outdoors, especially in smaller rooms. This has to do with many factors, such as wind dispersing droplets outside, and the sun killing the virus. Indoors, the air is stagnant, and infected droplets can linger in the air for some time, possibly hours.
We know that age is a factor in poor outcomes with COVID, but it also seems to be a factor in contracting the virus. The Coronavirus enters cells by ACE-2 receptors, which are hormone receptors that help control blood pressure. The older you get, the more of these receptors you have, which facilitates both getting the infection and the severity of the infection.
The method of spread is important, too. A single breath can release as many as 5,000 droplets, each potentially containing COVID. However, speaking releases about 10 times this amount, and singing likely even more. Coughing and sneezing are the largest emitters of respiratory droplets. So sitting quietly in a room with someone is much safer than having a conversation or singing along.
And obviously, masks and handwashing make a difference. By some estimates, if 60% of the population wore masks in public, the epidemic would end. However, most masks are ineffective within two feet of exposure, so physical distancing is still important even with a mask.
In summary, you can protect yourself from COVID if you:
- wear a mask
- wash your hands
- avoid crowds
- increase your distance and limit your length of exposure to others
- limit speaking or singing indoors with others outside your household
- go outside to interact with others
These guidelines may not be possible for you to follow all the time, but the more you follow, the less likely you are to contract the virus.
Good luck, good health, and stay safe!
Ken Zweig, M.D.