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Many people go to the gym to work-out or take part in classes to remain in good physical health. During the COVID-19 pandemic, gyms in most countries across the world were obliged to close to prevent the virus from spreading during indoor work-outs. Indeed, the risk of aerosol virus transmission during high intensity exercise is significantly increased due to the increased aerosol production of a potentially infected person and the increased breathing volume of susceptible individuals. However, doing physical workouts is of great importance to stay healthy and enhance the immune system, especially during the pandemic, since it is known that overweight people and people with underlying diseases are at increased risk. Therefore, the objective of this project was to design a ventilation and air cleaning system that significantly reduces the risk of aerosol virus transmission in indoor sport environments. The project aims to contribute to establishing requirements for indoor sport facilities in terms of ventilation and air cleaning such that, during a next lockdown, gyms that fulfil these requirements can remain open. To achieve this objective, a literature survey was conducted to map the current knowledge on the aerosol transmission of COVID-19 and the characteristics of different types of exercise. Based on that, requirements were set for different intensity group classes to maintain the CO₂ concentration below 1200 ppm and the theoretical Wells Riley-infection risk below 5%. In addition, the importance of ventilation efficiency to dilute released contaminated aerosols was well understood in order to apply the Wells Riley-model. A lab- and field study were conducted to test if the model assumptions were met. Also, the possibility to measure aerosols in the field was investigated. Based on these outcomes, a prototype consisting of a ventilation system, air cleaning system and cooling system combined with a smart control algorithm, was designed and installed in a room for group classes. The performance of the systems was tested during exercise classes of yoga, zumba and indoor cycling (spinning)., CLIMA 2022 conference, 2022: CLIMA 2022 The 14th REHVA HVAC World Congress

The following scientists reviewed the document: Jonathan Abbatt, John Adgate, Alireza Afshari, KangHo Ahn, Francis Allard, Joseph Allen, Celia Alves, Meinrat O. Andreae, Isabella Annesi-Maesano, Ahmet Arısoy, Andrew P. Ault, Gwi-Nam Bae, Gabriel Bekö, Scott C. Bell, Allan Bertram, Mahmood Bhutta, Seweryn Bialasiewicz, Merete Bilde, Tami Bond, Joseph Brain, Marianna Brodach, David M. Broday, Guangyu Cao, Christopher D. Cappa, Annmarie Carlton, Paul K. S. Chan, Christopher Chao, Kuan-Fu Chen, Qi Chen, Qingyan Chen, David Cheong, Per Axcel Clausen, Ross Crawford, Derek Clements-Croome, Geo Clausen, Ian Clifton, Richard L. Corsi, Benjamin J. Cowling, Francesca Romana d'Ambrosio, Ghassan Dbaibo, Richard de Dear, Gianluigi de Gennaro, Peter DeCarlo, Philip Demokritou, Hugo Destaillats, Joanna Domagala-Kulawik, Neil M. Donahue, Caroline Duchaine, Marzenna R. Dudzinska, Dominic E. Dwyer, Greg Evans, Delphine K. Farmer, Kevin P. Fennelly, Richard Flagan, Janine Fröhlich-Nowoisky, Manuel Gameiro da Silva, Christian George, Marianne Glasius, Allen H. Goldstein, João Gomes, Michael Gormley, Rafal Górny, David Grimsrud, Keith Grimwood, Charles N. Haas, Fariborz Haghighat, Michael Hannigan, Roy Harrison, Ulla HaverinenShaughnessy, Philippa Howden-Chapman, Per Heiselberg, Daven K. Henze, Jean-Michel Heraud, Hartmut Herrmann, Philip K. Hopke, Ray Horstman, Wei Huang, Alex Huffman, David S. Hui, Tareq Hussein, Gabriel Isaacman-VanWertz, Jouni J.K. Jaakkola, Matti Jantunen, Lance Jennings, Dennis Johansson, Jan Kaczmarczyk, George Kallos, David Katoshevski, Frank Kelly, Søren Kjærgaard, Luke D. Knibbs, Henrik N. Knudsen, GwangPyo Ko, Evelyn S.C. Koay, Jen Kok, Nino Kuenzli, Markku Kulmala, Kazukiyo Kumagai, Prashant Kumar, Kazumichi Kuroda, Kiyoung Lee, Nelson Lee, Barry Lefer, Vincent Lemort, Xianting Li, Dusan Licina, Chao-Hsin Lin, Junjie Liu, Kam Lun E. Hon, John C. Little, Li Liu, Janet M. Macher, Ebba Malmqvist, Corinne Mandin, Ivo Martinac, Dainius Martuzevičius, Mark J. Mendell, David Miller, Claudia Mohr, Luisa T. Molina, Glenn Morrison, Roya Mortazavi, Edward Nardell, Athanasios Nenes, Mark Nicas, Zhi Ning, Jianlei Niu, Hidekazu Nishimura, Colin O'Dowd, Bjarne W. Olesen, Paula J. Olsiewski, Spyros Pandis, Daniel Peckham, Tuukka Petäjä, Zbigniew Popiolek, Ulrich Pöschl, Wayne R. Ott, Kimberly Prather, Andre S. H. Prevot, Hua Qian, Shanna Ratnesar-Shumate, James L. Repace, Tiina Reponen, Ilona Riipinen, Susan Roaf, Allen L. Robinson, Yinon Rudich, Manuel Ruiz de Adana, Masayuki Saijo, Reiko Saito, Paulo Saldiva, Tunga Salthammer, Joshua L. Santarpia, John H. Seinfeld, Gary S. Settles, Siegfried Schobesberger, Paul T. J. Scheepers, Max H. Sherman, Alan Shihadeh, Manabu Shiraiwa, Jeffrey Siegel, Torben Sigsgaard, Brett C. Singer, James N. Smith, Armin Sorooshian, Jerzy Sowa, Brent Stephens, Huey-Jen Jenny Su, Jordi Sunyer, Jason D. Surratt, Kazuo Takahashi, Nobuyuki Takegawa, Jørn Toftum, Margaret A. Tolbert, Euan Tovey, Barbara J. Turpin, Annele Virtanen, John Volckens, Claire Wainwright, Lance A. Wallace, Boguang Wang, Chia C. Wang, Michael Waring, John Wenger, Charles J. Weschler, Brent Williams, Mary E. Wilson, Armin Wisthaler, Kazimierz Wojtas, Douglas R. Worsnop, Ying Xu, Naomichi Yamamoto, Xudong Yang, Hui-Ling Yen, Hiroshi Yoshino, Hassan Zaraket, Zhiqiang (John) Zhai, Junfeng (Jim) Zhang, Qi Zhang, Jensen Zhang, Yinping Zhang, Bin Zhao, Tong Zhu., Understanding the transmission of respiratory infections indoors requires expertise in many distinctly different areas of science and engineering, including virology, aerosol physics, flow dynamics, exposure and epidemiology, medicine, and building engineering, to name the most significant. No one person has expertise in all these areas. However, collectively, the community of the signatories to the Comment understands the characteristics and mechanisms behind the generation of respiratory microdroplets, survival of viruses in the microdroplets, transport of the microdroplets and human exposure to them, and the airflow patterns that carry microdroplets in buildings. We have dedicated our careers working in this multidisciplinary field, and our statement stems from our collective expertise spanning the entire field.