SARS-CoV-2 can spread by close contact through large droplet spray and indirect contact via contaminated objects. There is mounting evidence that it can also be transmitted by inhalation of infected saliva aerosol particles. These particles are generated when breathing, talking, laughing, coughing or sneezing. It can be assumed that aerosol particle concentrations should be kept low in order to minimize the potential risk of airborne virus transmission. This paper presents measurements of aerosol particle concentrations in a gym, where saliva aerosol production is pronounced. 35 test persons performed physical exercise and aerosol particle concentrations, CO2 concentrations, air temperature and relative humidity were obtained in the room of 886 m³. A separate test was used to discriminate between human endogenous and exogenous aerosol particles. Aerosol particle removal by mechanical ventilation and mobile air cleaning units was measured. The gym test showed that ventilation with air-change rate ACH = 2.2 h−۱, i.e. 4.5 times the minimum of the Dutch Building Code, was insufficient to stop the significant aerosol concentration rise over 30 min. Air cleaning alone with ACH = 1.39 h−۱ had a similar effect as ventilation alone. Simplified mathematical models were engaged to provide further insight into ventilation, air cleaning and deposition. It was shown that combining the above-mentioned ventilation and air cleaning can reduce aerosol particle concentrations with 80 to 90% , depending on aerosol size. This combination of existing ventilation supplemented with air cleaning is energy efficient and can also be applied for other indoor environments.

۱٫ Introduction

In the second week of 2021, the European Centre for Disease Prevention and Control reported 94,582,873 cases of SARS-CoV-2 including 2,036,713 deaths, world-wide [1]. It has been suggested that this virus can be transmitted by respiratory droplets and by contact routes [2–۷]. Direct transmission can occur when infective droplets produced by activities such as talking, laughing, coughing or sneezing reach the mucosae (mouth and nose) or conjunctiva (eyes) of another person. Indirect or contact route transmission can occur via handrails, keyboard buttons and other objects, where virus is deposited after contact with an infected person. There is mounting evidence that the virus can also be transmitted by inhalation of saliva aerosol particles because the virus has been found in small aerosol particles that can remain in the air for hours, and it has been shown to maintain viability in such aerosols [8–۱۲]. Therefore, precautionary measures should not only be applied for the direct transmission route and the contact route, but also for the airborne route.