Last edited 13 Apr 2021

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Building ventilation and COVID-19 transmission risk



[edit] Introduction

Recycle, refit, refurb, reuse and upcycle – often these are words seen around trendy topics like interiors, furniture and sustainability. Indeed, this has been a growing theme globally – reducing waste, making good of what we have got and applying some reimagination and ingenuity.

[edit] Safety indoors

Social distancing and mask wearing can only go so far in preventing transmission of COVID-19 indoors. They are not barriers to contaminated aerosols, which can build up in poorly ventilated buildings. Specialist building service engineers, Milieu, have brought together the latest research on COVID-19 transmission and share their expertise on how to use ventilation to reduce the risk of COVID transmission indoors.

It is generally agreed by the scientific and medical community that the main route of coronavirus transmission is by respiratory droplets (larger droplets and particles that are exhaled when people cough, sneeze, sing, talk or breathe), which are passed on through close contact with an infected individual. Transmission by these larger respiratory droplets is usually within six feet, hence the two metre social distancing rule.

It was discovered that COVID-19 can also be spread through exposure to virus-containing respiratory droplets that remain suspended in the air over longer distances. These smaller droplets are known as aerosols.

Outdoors, fresh air will dilute and disperse any virally charged particles; it is indoors where this mode of transmission puts us all at higher risk of contracting COVID-19. Therefore, unless clever ventilation systems are incorporated into buildings, transmission risk is even higher.

[edit] Healthier airflow

Aerosols tend to follow airflow. Opening windows and doors allows fresh air to enter indoor spaces, increases airflow and reduces the concentration of virally charged particles.

In rooms where this is not possible, alternate methods of ventilation such as air conditioning (AC) need to be used. However, AC systems that recycle air, without an adequate supply of fresh air, could be responsible for recirculating and spreading airborne viral particles.

In traditional AC systems, where units are placed on the wall, airflow is horizontal, potentially delivering infectious respiratory droplets into the pathways of others.

In an underfloor ventilation system installed by Milieu at Pennybank, Clerkenwell, the airflow is vertical, streaming the air up past occupants to be expelled through cleverly concealed vents. This not only reduces the concentration of any virally charged particles, but it also reduces mixing of air between people sharing a working space.

[edit] Ventilation effectiveness

Milieu has developed a Ventilation Effectiveness Toolkit ( that assesses ventilation using computational modelling. This method analyses volume, occupancy, vocal activity and existing ventilation rates (natural and mechanical) of a building.

In response to experiences associated with COVID-19, we can take steps to reduce its transmission and reduce the infection rate of any future strains of SARs or other viruses, such as flu and the common cold, all of which impact our health and productivity. An effective way of doing this is by improving the ventilation of our buildings.

This article originally appeared under the headline, 'Reduce the risk of COVID transmission: ventilate your buildings' in the Architectural Technology Journal (at) issue 137 published by CIAT in spring 2021. It was written by Milieu Engineering Consultancy.


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