Updated World Health Organisation Ventilation Guidelines and Changed Emphasis on COVID19 Transmission

May 6, 2021 – 6 Min


The World Health Organisation (WHO) has updated its website, placing far greater emphasis on Covid transmission from inhaled airborne Covid19 droplets (aerosols), rather than through touching contaminated surfaces. It has also provided specific information on what constitutes good ventilation rates in the healthcare, non-residential and residential sectors. 

We welcome these WHO updates as this information provides more clarity to those putting risk mitigation strategies in place and because the new updates confirm the information that we have been providing our clients over the last 12 months.    

Looking back at previous recommendations

One of the most contentious Covid pandemic issues in 2020 was the reluctance by the WHO and US Centre for Disease Control (CDC) to recognise that Covid was an airborne transmitted disease. Despite the well-publicized open letter written by 239 scientists on 6 July 2020, the WHO and US CDC only formally recognised that the disease was airborne in late October 2020. 

This delay led to misdirected public policy, where the emphasis was very focussed on reducing transmission via fomites (i.e. surfaces, like furniture, clothes and door handles) rather than preventing contagion from the inhalation of large infected droplets and smaller aerosols. As an example, for most of 2020 the UK’s Health and Safety Executive (HSE) website dedicated pages on how to clean surfaces, with just two paragraphs on ventilation. 

When the WHO formally recognised that Covid was an airborne transmitted disease, it changed tack and immediately recommended enhanced ventilation of rooms to reduce the concentration of airborne infected particles in the air. A 5 minute video recommended to ventilate via mechanical means or opening doors and windows, taking into account room size, occupancy and the activities undertaken in a room. They summarised that 6 Air Changes per Hour (ACH) of ventilation should be achieved (i), and where that was not feasible through mechanical ventilation and opening windows, to utilise HEPA air purification units.

At the time, the 6 ACH guideline seemed vague and too inexact given that scientific research showed that different occupancy rates, room use, and room size impacted transmission rates significantly. To that end, we published a blog article that summarised ventilation and purification recommendations that were based on meetings with the United Kingdom HSE and SAGE committee, taking into account occupancy, room use and utilisation. 

Updated Guidelines

The WHO website today has a very different emphasis on how Covid spreads between people and also encompasses updated recommendations towards risk mitigation/ventilation.

Firstly, the stress is first and foremost on airborne and not fomite transmission, which has instead been downplayed. This is in line with a recent CDC research paper which suggests that surface transmission is very low, with only a 1 in 10,000 probability. This emphasis on airborne transmission is important, and is in line with the HSE website which now has 8 pages on ventilation, as well as the now more informative US CDC website.

Secondly, the WHO have updated their ventilation recommendations, providing more specific advice on ventilation for three different categories of buildings:


Ventilation rates of 60 litres per second per patient (l/s/pat) should be achieved. If an Aerosol Generating Procedure (AGP) takes place, for example in dentistry, then either 160 l/s/pat or 12 ACH is recommended, which is not too dissimilar to the UK Chief Dental Officers’ guidance. The WHO recommends mechanical ventilation (introducing fresh outside air) and not to recirculate existing air. Where mechanical ventilation is not possible, then cross ventilation (ii) is recommended, and failing that, the WHO suggests using HEPA air purifiers to clean the air. 

Non Residential

Ventilation rates of 10 litres per second per person (l/s/p) should be achieved. This is identical to the recommendations that we published in March from our communications with the SAGE/HSE and is in line with new building regulations in the UK (but much higher than, for example, the US ASHRAE building recommendations, where only 3-5 l/s/p is generally suggested). As per the healthcare sector, the WHO recommends mechanical ventilation (but not to recirculate existing air) and cross ventilation where possible, and failing these, to use air purifiers. The WHO also recommends that an air purifier should be used to close the gap between the minimum 10 l/s/p requirement and what an existing ventilation provides, something that we have been advising clients following clear evidence that HVAC systems work in tandem to provide cumulative ACH rates (i.e. HVAC Ventilation ACH + Air Purification ACH = Total Delivered ACH). 


The same 10 l/s/p ventilation rates are recommended in residential buildings where there are people in quarantine or isolation. Cross ventilation should be utilised, and failing this, the WHO recommends air purifiers. 

What is missing from the WHO guidelines:

The updated guidance does, however, raise two issues that stand out against other governmental recommendations

  1. No account is taken of a room’s occupants’ behaviour. Despite highlighting that a room’s use can change the risk of transmission, the WHO has a standard 10 l/s/p minimum recommendation across the board. Research shows that aerosol emissions during exercise can be between 10 and 100 higher than normal breathing, and thus the ventilation of a room should reflect that increase (which is why we recommend that gyms, for example, should achieve at least 30 l/s/p). 
  1. Whilst the WHO recommends HEPA filters for HVAC or portable air purifiers for the healthcare sector, for non-residential and residential sectors it suggests lower quality MERV14 filters. MERV14 filters only trap between 75% and 84% of 0.3 to 1.0 micron particles (which is Covid territory), compared to HEPA H13 filters that trap 99.97% of 0.3 micron particles (which is the most penetrating and hence, hardest to capture particle size, MPPS). This lower quality MERV14 recommendation seems odd but is likely due to the fact that it is harder to pass air through a finer HEPA filter and as a result many HVAC systems cannot handle HEPA filters. The SAGE committee and the HSE recommend portable filtration units only with HEPA filters. 


We welcome the greater emphasis by the WHO on airborne transmission over fomite transmission. This is in line with scientific research that has been available for a long time and it ties in with previous Coronavirus pandemics (SARS and MERS). This should allow people to prioritise truly effective risk mitigation strategies and motivate the right behaviours to limit transmission, e.g. ventilation, air purification, masks and safe distancing instead of overzealous surface cleaning. 

Furthermore, it is useful for the WHO to have defined what constitutes good ventilation inside buildings, moving from an oversimplified 6 ACH recommendation to one that takes into account occupancy and the type of building (healthcare vs non-residential). The ventilation guidelines are in line with the minimum recommended by the SAGE and HSE that we highlighted a couple of months ago, which again will help people understand what constitutes a good risk mitigation strategy. That said, there is a strong case to also incorporate the room occupants’ activity into the equation as well as to use HEPA 13/14 filters rather than the lower quality MERV14 filtration. 

5 May 2021



  1. Air Change per Hour. Ventilation airflow rate (m³/hr) divided by room volume. It indicates how much outdoor air is introduced into a room in 1 hour compared to the air volume of that room, or how much air is purified in 1 hour compared to the air volume of the room. For example, if an air purifier filters 300 m³ of air in one hour in a 100 m³ room, then 3 ACH will be achieved. 
  1. Cross ventilation occurs where there are ventilation openings on both sides of the space. Air flows in one side of the building/room and out the other side through, for example, a window or door. Cross ventilation is usually wind driven.

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