The Europen Centre for Disease Prevention and Control has issued a guidance note on the importance of ventilation in reduced COVID transmission. “Poor ventilation in confined indoor spaces is associated with increased transmission of respiratory infections. There have been numerous COVID-19 transmission events associated with closed spaces, including some from pre-symptomatic cases.
The role of ventilation in preventing COVID-19 transmission is not well-defined (i.e. by preventing dispersal of infectious particles to minimise the risk of transmission, or preventing transfer of an infectious dose to susceptible individuals). COVID-19 is thought to be primaril y transmitted via large respiratory droplets, however, an increasing number of outbreak reports implicate the role of aerosols in COVID-19 outbreaks. Aerosols consist of small droplets an d droplet nuclei which remain in the air for longer than large droplets.
Studies indicate that SARS-CoV-2 particles can remain infectious on various materials, as well as in aerosols in indoor environments, with the duration of infectivity depending on temperature and humidity. So far, transmission through fomites has not been documented, but it is considered possible.
Several outbreak investigation reports have shown that COVID-19 transmission can be particularly effective in crowded, confined indoor spaces such as workplaces (offices, factories) and during indoor events – e.g. churches, restaurants, gatherings at ski resorts, parties, shopping centres, worker dormitories, dance classes, cruise ships and vehicles. There are also indications that transmission can be linked to specific activities, such as singing in a choir or during religious services that may be characterised by increased production of respiratory droplets through loud speech and singing.
In a study of 318 outbreaks in China, transmission in all cases except one occurred in indoor spaces. The only case of outdoor transmission identified in this study involved two people. However, outdoor events have also been implicated in the spread of COVID-19, typically those associated with crowds, such as carnival celebrations and football matches, highlighting the risk of crowding even at outdoor events. However, exposure in crowded indoor spaces is also very common during such events.