Open letter to the World Health Organization

November 4, 2021 – 7 Min

FAO: Dr. Tedros Adhanom Ghebreyesus, Director-General of the World Health Organization

Date: November 3, 2021

Subject: Open letter re the WHO’s current ventilation roadmap 

Dear Dr. Tedros and Team,

First, congratulations on your sole nomination to serve a second five-year term as head of the World Health Organization. Having steered the global response to the COVID-19 pandemic from the outset, there is clearly widespread confidence that you are the best person to continue at the helm.

The purpose of this letter is to implore you to update your critical guidance on ventilation. After finally accepting that COVID-19 is mainly spread via airborne particles rather than contaminated surfaces, the WHO’s April 2021 Roadmap to improve and ensure good indoor ventilation in the context of COVID-19’ rightly shifted its emphasis to infection control measures relating to clean air.

Covid-19 and its variants continue to pose a serious threat to world health and there are a number of issues in the roadmap that demand urgent attention.

10 Litres fresh air/second/person is a stretch

While the recommended air change rate of 10 Litres fresh air/second/person (L/s/person) in non-residential settings is a positive step forward, the reality is there is a huge gulf between the ideal and the real. Many buildings’ Air Handling Units provide ventilation rates nowhere close to 10 L/s/person and a substantial number of older buildings offer no ventilation at all. Similarly, many hospitals are structurally under-ventilated and cannot attain the higher 60 L/s/patient air change rate stipulated for health care settings.

On a global level, this shortfall needs to be acknowledged as the norm, rather than an exception to the rule. Where there is a need to plug the gap, portable air purifiers should not be seen as a last resort “if no other (short-term) strategy can be adopted”. This solution should be elevated as a universally practical infection control measure, such that ‘Total Ventilation = Outside Air Ventilation + Air Purification’. Within health care settings, in particular, this should take precedence over the advice to ‘consider reducing the maximum room occupancy to meet the L/s/patient standard’, which perpetuates the current backlog in many hospitals. Hospital-grade air purifiers have been proven to supplement health care HVAC systems, hence increasing operating capacity and reducing fallow time.  

I appreciate that not all air purifiers are created equal, but high quality devices exist that have solid independent validation from respected sources. Such devices are already widely used in hospitals to address fresh air supply shortages, especially in lung disease departments and infection control areas. Peer reviewed scientific research shows their effectiveness at reducing particulate matter and pathogen count in a short matter of time.

Furthermore, the premise that ‘stand-alone cleaners do not replace ventilation in any circumstance’ should also come with a caveat, as outdoor air is not always fresh. In many cities it can be badly polluted and, when dirty outside air is brought indoors through ventilation, this increases the indoor density of polluting particulate matter. In the WHO’s October 2020 video on risk mitigation, Dr. Maria Neira recognises this point, yet it is notably absent from the roadmap.

HEPA is the standard for everyone

In health care settings, the advice for bridging the ventilation gap is to ‘consider using a standalone air cleaner with HEPA filter’. In non-residential settings, on the other hand, the WHO advises ‘using a stand-alone air cleaner with MERV 14 / ISO ePM1 70-80% filter (44)’.

Shared spaces such as offices should not be using inferior grades of filtration and we would therefore urge the WHO to bring its guidance in line with the advice of other authorities like the US CDC and UK SAGE committee, which only stipulate HEPA for non-residential settings.

MERV 14 filters trap particle sizes of 0.3 to 1.0 microns with 75-84% efficacy and particles between 1.0 and 10.0 microns with 90% efficacy. The corresponding figures for HEPA filters are 99.97% and almost 100%. In fact, HEPA filtration achieves 99.99% efficacy both for the typical size that COVID-19 particles are transmitted as airborne aerosols (sub 0.1 micron) and for the typical size that COVID-19 particles are transmitted when enveloped in large respiratory fluid droplets (above 10 micron). At its least effective – 0.3 microns – HEPA is 99.97% effective.

UVC destruction should not be omitted

In addition to trapping COVID-19 particles with HEPA, it is important to destroy them. Otherwise, filters became a collection point for live viruses, which at some point (eg.during filter changes) will come into contact with people and therefore pose a threat. UVC light breaks down the DNA and RNA of pathogens like COVID-19 and thereby inactivates them. UVC should be covered as part of the solution, not as a substitute for HEPA filtration but as a complementary technology.

Although the roadmap excludes any mention of UVC, in the WHO’s March 2020 ‘Practical manual to set up and manage a Severe Acute Respiratory Infections (SARI) treatment centre and a SARI screening facility in health care facilities’, UVC is actively endorsed: 

“Microbes are uniquely vulnerable to light at wavelengths at or near 253.7 nm because the maximum absorption wavelength of a DNA molecule is 260 nm (19). Additionally, efficacy of far-UVC light inactivation has been proven on airborne viruses carried by aerosols. For example, a very low dose of 2 mJ/cm2 of 222-nm light inactivates more than 95% of airborne H1N1 virus (20), while virus-reduction factors of 3.4 or more for SARS-CoV have been achieved with the UVC-based system in platelet concentrates (21).”

The guidance should mandate that the ultraviolet lamp is enclosed, so as not to harm the eyes, and that the wavelength must be above 240 nanometers to ensure efficacy without producing any ozone.

Warning against harmful technologies

Given the WHO’s essential role in the global governance of health, there is a strong argument for extending the guidance on air purification to warn against technologies that could potentially be detrimental to human health. 

Harmful side effects are associated with the type of air purifier sometimes referred to as using ‘additive’ technologies (based on indirect chemical reaction) as opposed to ‘subtractive’ (filtration and direct inactivation).

The UK’s Scientific Advisory Group for Emergencies SAGE committee sums it up:

“Devices based on other technologies (ionisers, plasma, chemical oxidation, photocatalytic oxidation, electrostatic precipitation) have a limited evidence base that demonstrates effectiveness against SARS-CoV-2 and/or may generate undesirable secondary chemical products that could lead to health effects such as respiratory or skin irritation (medium confidence). These devices are therefore not recommended unless their safety and efficacy can be unequivocally and scientifically demonstrated by relevant test data.

The use of chemical sprays such as triethylene glycol to clean the air in an occupied space has a limited evidence base in being effective in reducing airborne virus transmission risks, and has potential health effects for those exposed over a long period of time (medium confidence).These approaches are not recommended without further evidence to support their safety and efficacy.

Spray booth type devices for decontaminating people are not recommended. They are unlikely to be effective against the virus and have serious health impact and safety concerns.”

Air purification means lower carbon emissions

The WHO asserts that climate change is the biggest health threat facing humanity and two of your initiatives under the COP26 Health Programme include ‘building climate resilient health systems’ and ‘developing low carbon sustainable health systems’.

The built environment is a massive user of energy. Here are some key stats:

  • Buildings consume roughly 40% of the energy used by the U.S. and 24% by the world.
  • HVAC systems consume two-thirds of the site energy for residential, commercial, and public buildings in the developed world.
  • Conditioning ventilated air to the appropriate thermal state can potentially amount to about half of building space-conditioning energy.
  • Furthermore, residential and commercial building sectors are responsible for 40% of  U.S. carbon dioxide (CO2) emissions.

Air purification units use very little energy, generally consuming under 250W – they cost a few pence/cents a day to run. This is a minute fraction of the daily cost of AHU/HVAC systems that heat or cool fresh air intakes.

For locations that need to increase ventilation rates to meet WHO guidelines, there is a sizable energy cost saving in using air purification instead of increasing the AHU ventilation rate. For rooms without AHUs, using an air purifier allows for windows to remain closed for longer, reducing the energy used by radiators. This is of particular benefit in large older buildings, such as schools. And of course, lower energy consumption means a significantly lower carbon footprint, an air purification advantage that supports the UN’s goal to dramatically cut worldwide emissions and limit global warming to a maximum of 2 degrees this century. This hugely important aspect should not be overlooked in the ventilation roadmap.

The suggestions made in this letter are intended to be constructive, made in good faith, with a view to building on the positive steps that the WHO has taken to date in relation to mitigating the threat of COVID-19.

I look forward to hearing from you,

Yours sincerely,

Edward Ballsdon

Managing Director


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