Ventilation is a key control for limiting transmission of COVID-19

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Since March 2021 when the World Health Organisation (WHO)  finally accepted that COVID-19 is an airborne transmitted virus, a stronger focus on ventilation has led to a greater emphasis on air quality, circulation and replacement. While the virus is transported across room spaces in small particles, it can build-up in density in poorly ventilated spaces. But what is the best protection against virus laden aerosols ? How frequently should air in a room space be replaced ?

The basic measure for what constitutes good air is the number of replacements made in an hour and the level of carbon dioxide present (that represents the concentration of exhaled air in a room that comes from other people - and commensurately, the risk of COVID-19).

  

New Scientist (August 2021) published some relevant data from various selected environments around London with surprising results as the chosen locations have an wider application than only that large city in the UK. While the key measure for air quality is the amount of CO2  present representing the air breathed out by other people, it is also dependent on there being no other source such as a natural gas cooker or heater. Outdoor air has a concentration of around 410 parts per million (ppm) and below 800 ppm is generally regarded as well ventilated and above 1500 ppm is regarded as critical to exit.

Space, details and highest reading

• Outdoors:  413 ppm

• Private car, two people, windows closed: 1,740 ppm
• Private car, two people, windows open:  413 ppm
• Private car, two people, non recirculating ventilation on: 413 ppm  
• Private car, two people recirculating air con on: 1,589 ppm 

• Bus, lower deck, full doors opening at stops: 724 ppm
• London train tube: platform between trains: 783 ppm
• London train tube full carriage standing room only: 1,076 ppm

• Supermarket, large, empty: 413 ppm
• Supermarket, small, busy: 1,100 ppm

• Restaurant, busy by open door: 739 ppm
• Pub, main bar, full, doors and windows open: 420 ppm

Indoor air quality standards essentially fail to protect from airborne pathogens such as viruses and bacteria leading to a urgent needs for new building and ventilation designs to mitigate airborne risks.