The current pandemic has bestowed many restrictions on both professional and amateur sporting activities and led to the postponement of major sporting events like the European Soccer Championship and the Olympic Games from 2020 to 2021.
While during the early phase of the pandemic, the focus of infection control was still mainly on smear infections, it has become clear that infections through aerosols, i.e. the inhalation of airborne viruses are much more dominant (8). Viruses become airborne when they are exhaled by an infectious person. It is well known that with every breath, humans exhale small liquid aerosol particles in a size range of < 0.5 µm (9) that mainly consist of surfactant (11). Particles exhaled during other activities like talking and singing are larger with average sizes of > 1 µm (2). These liquid aerosol particles, often erroneously termed “aerosols” , are formed in the alveoli and, if exhaled by an infectious person, contain viruses. The concentration of exhaled particles depends on the age as well as the lung health status of a subject. It was found that a healthy person exhales only a few particles per liter of air, whereas a super-emitter can exhale up to several hundred thousand particles per liter (7). What makes the aerosol infection path so insidious is that these particles are too small to be seen by the naked eye and that they almost do not settle and can remain airborne for at least several hours (5).
Two different routes of aerogenic transmission can be distinguished, the direct and the indirect route. Direct transmission or infection occurs when a person directly inhales the highly concentrated aerosol cloud exhaled by an infectious person or the larger droplets emitted during coughing or sneezing. The risk of direct infections can be significantly reduced by keeping distance and properly wearing efficient masks. An indirect transmission or infection occurs when the viruses exhaled by an infectious person remain in an aerial space and are subsequently inhaled by another person. It should be noticed that due to their size, exhaled particles almost perfectly follow air flows, since they are too small to be affected by particle inertia and too large for a significant effect of Brownian motion (5). The problem with indirect transmission is that this infection can happen, even if the infected person has already left the room, because airborne SARS-CoV-2 can remain viable for more than an hour (13). Whereas direct infections may happen both indoors and outdoors, indirect infections may only occur indoors, where the viruses are exhaled into a room and remain within a confined space, where they can accumulate. Consequently, the infection risk outdoors is significantly lower than the risk indoors (4). The risk of indirect infections can be reduced by lowering the virus concentration in a room through proper ventilation, technical air cleaning systems, short residence times and the wearing of masks.
When an infectious person occupies a closed room, the virus concentration increases linearly with time. Other persons in the same room constantly inhale the increasing virus concentration and thus the dose of inhaled viruses increases with the time squared, spent in the in indoor environment, i.e. twice the time results in four times the dose. The viral dose, necessary to cause an infection is not finally known, but a value between a few hundred and a few thousand virus copies is currently discussed (6). The time necessary to accumulate such an inhaled dose in a closed room depends on the infectivity of the infected person and the volume of the room. It was shown that with a super-emitter (emitting 100,000 virus copies per minute) in a closed room with a volume of 100 m³, other persons can get infected within the order of minutes (10). It is therefore important to ensure good ventilation and/or air cleaning in indoor environments in order to keep the virus concentration and thus the risk of indirect infections low. The risk of transmission can be mitigated significantly by masks covering nose and mouth. Masks can substantially decrease the number of aerosol particles inhaled and therefore decrease the likelihood of infection or the severity of disease, because the viral load is decreased. Masks should consequently be used in indoor facilities, during meetings and travel depending on the state of the pandemic (12).
Outdoors, any motion of a subject and/or air (wind) causes immediate dilution of the exhaled particle and virus concentration. In addition, the exhaled breath is usually transported upwards due to buoyancy effects of the warmer exhaled compared to surrounding air. The air volume outdoors is nearly infinite, so that the particle/virus concentration cannot accumulate, and in that way making indirect infections almost impossible outdoors. Outdoor infections, particularly between two or more persons in motion, are therefore much less likely than indoor infections.
What does this mean for sporting activities and large sports events like the upcoming Olympic Games in Tokyo? We should clearly distinguish between indoor and outdoor activities as well as between direct and indirect infections. In any outdoor sporting activities, indirect infections are very unlikely, as mentioned earlier. Direct infections may occur during outdoor contact sports, although there are not many such disciplines. Direct infections in outdoor non-contact sporting activities like athletics, cycling or tennis are almost impossible. Even in disciplines like rowing or marathon, where the athletes are in closer contact to each other, the infection risk is low, because firstly, the exhaled breath will be diluted rapidly due to buoyancy and the movement of the athletes so that the number of accidentally inhaled viruses would be too low to cause an infection and secondly, the athletes are not face to face so that a direct inhalation of the exhaled air by another athlete is not possible.