#15,553
Modifying or controlling indoor environmental factors (temperature, humidity, air exchanges per hour) has long been looked at as a way to reduce the risk of transmission of airborne viruses (see here, here, and here).
In 2018's bioRxIv: Humidity As A Non-pharmaceutical Intervention For Influenza A, researchers reported finding: An increase in average AH from 6.33 mb in control rooms to 9.89 mb in humidified rooms (RH ~42-45 %) was associated with a significant decrease in influenza A virus presence in fomite and air samples in humidified rooms compared to control rooms.
In last year's The WHO NPI Guidance : Environmental Measures, however, the World Health Organization's recommendations for adjusting indoor environments during a pandemic only gave a weak endorsement to increased ventilation, and recommended against modifying indoor humidities.
In conclusion, the available evidence indicates that:
- Transmission of COVID-19 commonly occurs in closed indoor spaces.
- There is currently no evidence of human infection with SARS-CoV-2 caused by infectious aerosols distributed through the ventilation system ducts of HVACs. The risk is rated as very low.
- Well-maintained HVAC systems, including air-conditioning units, securely filter large droplets containing SARS-CoV-2. It is possible for COVID-19 aerosols (small droplets and droplet nuclei) to spread through HVAC systems within a building or vehicle and stand-alone air-conditioning units if air is recirculated.
- Air flow generated by air-conditioning units may facilitate the spread of droplets excreted by infected people longer distances within indoor spaces.
- HVAC systems may have a complementary role in decreasing transmission in indoor spaces by increasing the rate of air change, decreasing recirculation of air and increasing the use of outdoor air.
Since then, a good deal more has been learned about the indoor transmission of SARS-CoV-2, and the potential role of HVAC systems in spreading the virus. As a result, the ECDC has released their 1st update (14 page PDF) to their initial technical report, which is summarized below.
Technical report11 Nov 2020
This document aims to provide guidance for public health authorities in EU/EEA countries and the UK on the ventilation of indoor spaces in the context of COVID-19.
Executive summary
Key messages
- It is now well-established that COVID-19 transmission commonly occurs in closed spaces;
- If well-maintained and adapted for use in the COVID-19 pandemic, heating, ventilation and air-conditioning (HVAC) systems may have a complementary role in decreasing potential airborne transmission of SARS-CoV-2;
- Four bundles of non-pharmaceutical interventions (NPIs) should be considered to reduce potential airborne transmission of SARS-CoV-2 in closed spaces: the control of COVID-19 sources in closed spaces; engineering controls in mechanically ventilated (by HVAC systems) and naturally ventilated closed spaces; administrative controls; and personal protective behaviour.
Changes to the current update
The first update of the ECDC ventilation guidance document contains:
- key new findings that emphasise four bundles of NPIs to reduce the risk of SARS-CoV-2 transmission in closed spaces;
- updated references on the evidence of transmission in closed spaces;
- recommendations based on the new evidence and on national and international guidance; and
- an overview of national guidance ventilation documents in the context of COVID-19 based on an inquiry sent to ECDC’s National Focal Points (NFPs) for Preparedness and Response and NFPs for Influenza and other respiratory diseases.
Download
Of note, while citing difficulties in controlling indoor humidity for a lot of venues, this update suggests humidity control might be an option for reducing the spread of SARS-CoV-2 (see below):
A relative humidity of 40–60% may help to limit the spread and survival of SARS-CoV-2 within a closed space [24,33]. Humidity levels in this range could therefore be considered for HVAC systems. However, even new buildings with state of the art HVAC systems cannot usually exceed more than 40% relative humidity, especially in winter, and older systems often cannot exceed much lower relative humidity levels because of the risk of damaging the HVAC system as well as room structures due to the risks of condensation and mould development [2,33].
While this many not be a practical intervention for a lot of commercial buildings, it will be interesting to see what impact - if any - humidity control will have during this pandemic in those sites that attempt it.
