I live in Boulder, Colorado, one of the outdoor exercise capitals of the world. These past few days, social media has been on fire with an impressive looking, but pretty unscientific simulation purporting to show the widespread and deadly transmission of coronavirus from someone exercising. So let’s take a look and actually dig into some science that supports that this viral video is more paranoia than real.
How Coronavirus Spreads
The novel coronavirus can spread through coughing and water droplets. Larger water droplets can project further and smaller droplets of about a micron in size can hang in the air (3). This is the genesis of the CDC’s recommendation of 6-foot social distancing.
The Viral Video
The video is here: https://twitter.com/i/status/1247540730425249799 It seems to show that various types of outdoor activities like running behind someone could expose the downwind running partner to the coronavirus. I have some stills above. However, there are some problems here.
First, this is a computer simulation performed by an engineer and NOT an actual experiment performed by physician-scientists. Meaning actual experiments would have used a coughing machine with coronavirus in droplets and looked at the ability of those droplets to be gathered by a breathing machine at these distances and relative positions. Meaning, the researchers would have needed to be able to grow the virus captured by the breathing machine. THIS DID NOT HAPPEN IN THIS SIMULATION.
In addition, the proper way to manage a situation where you have some new scientific data on coronavirus is to put it up on a pre-publication website and let everyone see your methods. That was NOT done here. Meaning, not even the most basic scientific method was followed on having others review whether the way the researchers set this model up was accurate.
The Actual Science
When you’re indoors, coughing can spread small water droplets containing virus particles that can result in the ability to pick up a viable virus in those water droplets dispersed in the air of the room (1). Notice how carefully I wrote that sentence. That’s because the study I quoted doesn’t actually say that those water droplets containing virus particles will infect someone. Why?
Doing these studies is really hard. Here’s one recent study that can serve as an example. In this study, they had college students talk into a cone that looked like an old megaphone to collect the water particles. Now that’s about as artificial an environment as you can set up, as how many of us walk around talking in a capped megaphone and then have someone else inhale at the other end of that device?
As you can likely gather from the discussion above, the issue is that the virus in the water particle not only needs to get from point A to point B, but it has to be intact, viable enough, and in enough quantity to make the other person sick. Meaning that this is not a situation here that if a single virus particle makes it intact to the other person that that person will become infected with virus. That all depends on how much virus the other person gets exposed to, which is a concept called viral load or dose.
This is the problem with all of these studies. The only, truly valid study looking at how the novel coronavirus can infect someone else while running outdoors would be to run a trial of infected people running near uninfected people who were otherwise quarantined and then see if the running partners got infected and sick. THAT HAS NEVER BEEN DONE.
Pilots often use what’s called the “big sky theory” to avoid collisions. Meaning, even a 747 is tiny compared to the volume of the sky in which it lives. We’re talking about water droplets here that are MUCH MUCH smaller than the period at the end of this sentence. Hence, these particles also live in a large volume outside, so the chances of getting exposed to many of them is small.
To reduce your chances of infection, just use common sense and try to stay as far apart from each other as possible while outside. Remember, the small viral loads that you get in these situations are also likely helping you build small amounts of immunity. Infection and illness happen when enough virus enters your system to overwhelm your immune system. Hence, getting random exposures to small amounts of virus are unlikely to cause illness in most people.
The upshot? There’s nothing like a cool looking viral video to freak people out! Just use common sense out there. If you see someone coughing, stay away. If you see a runner coming at you, just steer clear and allow that person to pass at a distance. Your likelihood of getting infected while exercising outdoors is much smaller than being infected while indoors.
(1) Lindsley WG, King WP, Thewlis RE, et al. Dispersion and exposure to a cough-generated aerosol in a simulated medical examination room [published correction appears in J Occup Environ Hyg. 2013;10(2):D25]. J Occup Environ Hyg. 2012;9(12):681–690. doi:10.1080/15459624.2012.725986
(2) Yan J, et al. , EMIT Consortium. Aerosol shedding of infectious influenza virus. Proceedings of the National Academy of Sciences Jan 2018, 115 (5) 1081-1086; DOI: 10.1073/pnas.1716561115
(3) Zayas, G., Chiang, M.C., Wong, E. et al. Cough aerosol in healthy participants: fundamental knowledge to optimize droplet-spread infectious respiratory disease management. BMC Pulm Med 12, 11 (2012). https://doi.org/10.1186/1471-2466-12-11