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Volume 27, Number 9—September 2021

SARS-CoV-2 Superspread in Fitness Center, Hong Kong, China, March 2021

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To the Editors: I read with interest the article by Chu et al. (1), which concluded that poor ventilation might have contributed to a severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) superspreading event at a fitness center in Hong Kong, China. As an example of SARS-CoV-2 not spreading in a converse environment, I report the absence of apparent transmission at a gym in Montgomery County, Virginia, USA, that emphasized ventilation as part of its coronavirus disease (COVID-19) precautions upon reopening in June 2020. The gym increased ventilation by opening 10 exterior doors and keeping them open even during cold or inclement weather. The gym also limited class sizes, stressed hygiene, and required ≥10 feet of distancing. Masks were not worn.

With the doors closed, the air change rate was estimated to be 0.07 air changes/hour, corresponding to a ventilation rate of 7.6 L/second/person (L/s/p) on the basis of an occupancy of 10 persons, below the 10 L/s/p minimum recommended by ASHRAE (American Society of Heating and Air-Conditioning Engineers) for health clubs (2). With the doors open, these values were estimated to be 2.4 air changes/hour and 240 L/s/p.

On September 24, 2020, an instructor at the gym developed upper respiratory symptoms and lost his sense of smell and taste. He was tested for SARS-CoV-2 infection and received a positive result on September 28, 2020. That day, the gym owner contacted 50 persons who had attended ≥1 of the instructor’s classes during September 21–25, 2020 to notify them of potential exposure. During subsequent follow-up, none of these 50 persons reported any COVID-19 symptoms, and 5 people who got tested received negative results . It is likely that increasing ventilation greatly mitigated the risk of transmission (3). Subsequently, the gym acquired a CO2 sensor and kept the CO2 level, an indicator of respiratory emissions, well below 600 ppm (4) by adjusting the number of open doors.



Thanks to Kyle T. Bernstein, John T. Brooks, and Velvet Minnick for their assistance with this investigation.

This work was supported by the Virginia Tech Center for Emerging, Zoonotic, and Arthropod-borne Pathogens.


Linsey C. MarrComments to Author 
Author affiliation: Virginia Polytechnic Institute and State University, Blacksburg, Virginia, USA



  1. Chu  DKW, Gu  H, Chang  LDJ, Cheuk  SSY, Gurung  S, Krishnan  P, et al. SARS-CoV-2 superspread in fitness center, Hong Kong, China, March 2021. Emerg Infect Dis. 2021;27: Epub ahead of print. DOIPubMedGoogle Scholar
  2. American Society of Heating, Refrigerating and Air-Conditioning Engineers. Ventilation for acceptable indoor air quality. ANSI/ASHRAE Standard 62.1–2019. 2019 [cited 2021 Jan 7].
  3. Yang  W, Marr  LC. Dynamics of airborne influenza A viruses indoors and dependence on humidity. PLoS One. 2011;6:e21481. DOIPubMedGoogle Scholar
  4. Peng  Z, Jimenez  JL. Exhaled CO2 as a COVID-19 infection risk proxy for different indoor environments and activities. Environ Sci Technol Lett. 2021;8:3927. DOIGoogle Scholar


Cite This Article

DOI: 10.3201/eid2709.211177

Original Publication Date: June 30, 2021

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Table of Contents – Volume 27, Number 9—September 2021

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Please use the form below to submit correspondence to the authors or contact them at the following address:

Linsey C. Marr, Civil and Environmental Engineering, Virginia Tech, 1145 Perry St, Durham 411, Blacksburg, VA 24061, USA

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Page created: June 09, 2021
Page updated: August 18, 2021
Page reviewed: August 18, 2021
The conclusions, findings, and opinions expressed by authors contributing to this journal do not necessarily reflect the official position of the U.S. Department of Health and Human Services, the Public Health Service, the Centers for Disease Control and Prevention, or the authors' affiliated institutions. Use of trade names is for identification only and does not imply endorsement by any of the groups named above.