COVID-19

=== Indoor ventilation and avoiding crowded indoor spaces ===
The CDC states that avoiding crowded indoor spaces reduces the risk of COVID-19 infection.<ref name="CDC-2020b">{{#invoke:Cite web||last=CDC|date=11 February 2020|title=Scientific Brief: SARS-CoV-2 Transmission|url=https://www.cdc.gov/coronavirus/2019-ncov/science/science-briefs/sars-cov-2-transmission.html|access-date=10 May 2021|website=U.S. [[Centers for Disease Control and Prevention]] (CDC) }}</ref> When indoors, increasing the rate of air change, decreasing recirculation of air and increasing the use of outdoor air can reduce transmission.<ref name="CDC-2020b" /><ref name="ecdc_transmission2">{{#invoke:Cite web||date=7 September 2020|title=Transmission of COVID-19 |url=https://www.ecdc.europa.eu/en/covid-19/latest-evidence/transmission|access-date=14 October 2020|publisher=[[European Centre for Disease Prevention and Control]]}}</ref> The WHO recommends [[Ventilation (architecture)|ventilation]] and [[air filtration]] in public spaces to help clear out infectious aerosols.<ref name="CDCasof07092020" /><ref name="KoFZO">{{#invoke:cite AV media ||date=30 October 2020 |url=https://www.youtube.com/watch?v=XJC1f7F4qtc |title=WHO's Science in 5 on COVID-19 – Ventilation – 30 October 2020 |publisher=[[World Health Organization (WHO)]] |via=[[YouTube]] |access-date=8 December 2022 |archive-url=https://web.archive.org/web/20221025043909/http://www.youtube.com/watch?v=XJC1f7F4qtc |archive-date=25 October 2022 |url-status=live}}</ref><ref name="Lancetdroplet05272020">{{#invoke:cite journal||vauthors=Somsen GA, van Rijn C, Kooij S, Bem RA, Bonn D|date=July 2020|title=Small droplet aerosols in poorly ventilated spaces and SARS-CoV-2 transmission|journal=The Lancet. Respiratory Medicine|publisher=Elsesier|volume=8|issue=7|pages=658–659 |doi=10.1016/S2213-2600(20)30245-9|pmc=7255254|pmid=32473123}}</ref>

Exhaled respiratory particles can build-up within enclosed spaces with inadequate [[Ventilation (architecture)|ventilation]]. The risk of COVID‑19 infection increases especially in spaces where people engage in physical exertion or raise their voice (e.g., exercising, shouting, singing) as this increases exhalation of respiratory droplets. Prolonged exposure to these conditions, typically more than 15 minutes, leads to higher risk of infection.<ref name="CDC-2020b" />

[[Displacement ventilation]] with large natural inlets can move stale air directly to the exhaust in [[laminar flow]] while significantly reducing the concentration of droplets and particles. [[Passive ventilation]] reduces energy consumption and maintenance costs but may lack [[Demand controlled ventilation|controllability]] and [[Heat recovery ventilation|heat recovery]]. Displacement ventilation can also be achieved mechanically with higher energy and maintenance costs. The use of large ducts and openings helps to prevent mixing in closed environments. Recirculation and mixing should be avoided because recirculation prevents dilution of harmful particles and redistributes possibly contaminated air, and mixing increases the concentration and range of infectious particles and keeps larger particles in the air.<ref>{{#invoke:cite journal ||vauthors=Lipinski T, Ahmad D, Serey N, Jouhara H |date=1 November 2020 |title=Review of ventilation strategies to reduce the risk of disease transmission in high occupancy buildings |url=https://www.sciencedirect.com/science/article/pii/S266620272030032X |journal=International Journal of Thermofluids |volume=7–8 |pages=100045 |doi=10.1016/j.ijft.2020.100045 |s2cid=221642242 |issn=2666-2027}}</ref>