Exercise in times of the corona virus: Is running or biking outdoors an infection risk?

With lockdowns in place, most forms of exercise are not possible anymore: The gyms are closed and groups are not allowed to meet to play group sports. Individual sports seem to be the only solution: That may involve exercise at home such as Yoga or Pilates, or outdoor activities like running and biking. But especially for the later one may wonder, is it risky? And is if worth the risk?

Why exercise is so important right now

Exercise is beneficial in general: It keeps our body in shape and has a major positive influence on our overall health. But especially now, its positive influence on our mind is even more relevant: It gives us the opportunity to leave our home (if we are allowed to run outside!), get some fresh air and calm the nerves. It causes an effective reduction of stress which is what many of us need now with worries about health, family and friends as well as jobs. Studies [1] have shown that moderate exercise causes positive changes on the central nervous system: Stress hormone (e.g. Adrenalin) levels are modulated. Even more than that, exercise makes us happy: Serotonin is released which is a mood booster!

What are the infection risks during outdoor activities?

The corona virus SARS-CoV-2 which causes the COVID-19 disease spreads between people via droplets [2]. Infection can be related to touching a surface that droplets from an infected person are on or via inhaling of infected respiratory droplets.

In Germany, many outdoor gyms have been closed to limit the first cause for infection. Also, public advice to avoid touching your face aims to limit this risk. The disease will not be obtained from having infectious droplets on your hands only, but regular hand washing is still necessary to effectively minimizes that way of infection.

Inhaling infectious droplets on the other side is more difficult to avoid: If an infected person sneezes, coughs or even just exhales, he or she can emit droplets that contain the virus. Social distancing and keeping 1.5 meters to the next persons aims to give those droplets enough space to fall to the ground before the next person can inhale them. But this social distance has been determined for people standing: During running or biking, the velocity has significant aerodynamic influence on the droplets!

A recent study [3] showed how the speed of the person emitting the droplet will influence their behavior in air: The faster this person is moving (e.g. during running), the more meters will their emitted droplets remain in the air – risking others of infection at too low distances.

This data suggests that 1.5 meters are not sufficient to limit the infection risk for moving persons. Instead, the authors recommend to keep 5 meters distance when walking, 10 meters when running – and make sure to avoid direct slipstream.

While this study is based only on aerodynamic simulations of the behavior of droplets and does not show explicitly that infection is therefore possible over distances of several meters, the data strongly suggests this. Doing a clinical study with infected people running ahead of healthy ones and measuring how many get sick is not an option, therefore this data is the best we have. As a scientist, I know how well simulations in well-understood fields such as aerodynamics match reality – otherwise we would hardly be able to develop huge airplanes like the A380 that actually are ably to fly – therefore I consider this data [3] and their conclusions of high relevance.

How can I limit my infection risk while exercising outside?

The bottom line of all suggestions is simple: We should at all times keep as much distance as possible. Breaking it down into a few simple rules, we get:

• Keep the right distance: 1.5 meters when standing, 5 meters when walking, 10 meters when running.
• Avoid exposing others or yourself with staggered arrangements.
• Avoid touching surfaces and your face.
• Use masks whenever possible. During exercise, this might not be an option as it makes breathing difficult, but a simple cloth will already lower your infection risk and help you remember to avoid touching your face!
• Exercise is areas with fewer people to avoid the risks.

As a runner living in a big city, I find it increasingly difficult to keep the correct distance lately with too many people walking on my usual running tracks. Therefore I am investigating new routes, which is also a fun use of my new free time. How did you change your exercise routine due to the corona crisis?

References:
[1] https://dx.doi.org/10.1186%2Fs40303-015-0010-8
[2] WHO/2019-nCoV/Sci_Brief/Transmission_modes/2020.2
[3] http://www.urbanphysics.net/COVID19_Aero_Paper.pdf