Christine Peters: Covid-19 is airborne, so what needs to happen next?

It would be hard to have missed the clashing of the views in infectious diseases, aerosol science, and evidence based medicine over “airborne” transmission of SARS-CoV-2 over the past few months. The arguments have flamed across medical and science journals, as well as social and mainstream media, igniting worker protests regarding provision of personal protective equipment (PPE), and vague enthusiasm regarding ventilation in public messaging. [1,2] In the heat of the fray it has been easy to forget the crucial practical implications for healthcare and the community.

A strong (if not universal) consensus has emerged that the evidence for airborne transmission is robust and convincing enough to warrant action. So, what needs to happen next? 

Put simply the risk is in the sharing of air—the inhaling of a virus that is exhaled in airborne particles from an infectious person. There are many parameters to that risk, but protection boils down to two main additional interventions on top of droplet/fomite precautions. [3]     

Firstly, ventilation. Ventilation works by reducing the numbers of aerosols that are suspended in the air that may be inhaled. It does not work on large droplets that drop—gravity attends to those. The need for adequate ventilation is supported in government guidelines (though it is not consistent with a droplet purist approach to transmission). While ventilation was highlighted by the SAGE environmental and monitoring group (EMG) papers since last June, prominent public messaging only caught up a few months ago, with the welcome addition of “fresh air” to the “Hand-Face-Space” tag line.

Healthcare premises have lagged behind in targeting ventilation as a priority. How many wards admitting covid-19 patients were assessed thoroughly with regard to ventilation last summer? How about ambulances in which staff have hours of exposure in a small, poorly ventilated space? Or wards in which windows are screwed shut and there is no mechanical ventilation?

You can’t reasonably ask NHS hospital estates to fix the ventilation “by next Tuesday, thanks.” However the first step is to acknowledge the need for ventilation. Improving ventilation is a colossal task when considering the shortfall in estates investment over recent decades, and even in new build projects where ventilation design has prioritised energy saving over infection control. We now need a straight forward commitment to provide ring fenced money to optimise ventilation in healthcare settings based on a multi-disciplinary approach to design.

It is now essential to obtain baseline assessments including maintenance records, design parameters, and functional validation. Looking at tables in standards documents is not a reliable way of determining what the ventilation is like in reality. CO2 monitoring will help give a rapid indication of the efficiency and is useful for monitoring ventilation, but is only one of a number of tools available. [4] The highest risk areas such as positive patient rooms, and admission units need to be prioritised and a timeline constructed, with input from ventilation engineers and architects who can advise on problem solving. A raft of options are available and local HEPA filtration for air cleaning is a possible solution in areas in urgent need of improvement. 

There is a current and immediate need for centrally driven minimum ventilation standards for settings where workers are dealing with infectious patients, coupled with a local responsibility to deliver these standards within a time scale. Simply producing guidelines or emails that mention “check ventilation” may reduce litigation and so-called organisational risk, but it’s only through implementation that infection risk to staff and patients will actually be impacted. 

Secondly, we need to re consider personal protective equipment (PPE). There is surprisingly little controversy regarding the superiority of fit tested FFP3 (also termed RPE—Respiratory Protective Equipment) in preventing inhalation of infectious aerosol over FRSM. [6] Hence their current recommendation in so called Aerosol Generating Procedures (AGP) settings. No industry that cares about aerosol risk (e.g. asbestos removal) would dream of supplying ear looped FRSM that gape at the cheeks and regularly slip off people’s noses. SAGE say:

“The physics of aerosol behavior means that if small particles can be generated in sufficient quantity to cause infection at longer distances, they will be also present at close proximity to the infectious person and at much greater concentration. Modelling studies suggest that in close proximity inhalation (of a wide range of aerosol sizes) may be a more likely exposure route than deposition of large droplets on the mucous membrane” [5]

When one accepts this, it is logical to insist on RPE when close to an infectious patient. For this short range aerosol transmission, ventilation has minimal impact in reducing the concentration of the infectious aerosols so RPE remains a critical mitigation measure even when ventilation is optimized. When ventilation is not optimal, RPE becomes even more important for protection. Think about the carer or paramedic entering an area of unknown aerosol risk—multiple occupancy, poorly ventilated spaces for example—think about the living spaces of chain smokers for a visualisation of the invisible hazard posed by coughing (and therefore aerosol producing) acute covid-19 patients. 

A number of operational challenges have been emphasised as a reason for not changing guidance. FFP3 level protection for large numbers of staff is certainly hard to implement overnight. But we have had more than a year to resolve this. Happily some trusts in England and Wales have already gone ahead and overcome those challenges with regard to rapidly fit testing. Some have also provided innovative masks using designs created by frontline workers working with industry, and decontamination has been used effectively and robustly, reducing waste. These sterling, safety focussed efforts need to be emulated across the country. 

It’s past time to act on the opportunity that covid-19 airborne transmission presents for additional targeted measures to reduce risk of transmission in healthcare. In doing so we will also ensure renewed confidence of healthcare workers and others that their safety is a genuine priority, reduce nosocomial risks for patients, and support the global efforts to reduce the dreadful toll of the pandemic. 

Christine Peters, consultant clinical microbiologist, NHS Greater Glasgow and Clyde, Co-Founder Fresh Air NHS.

Competing interests: none declared.


  1. Greenhalgh T, Jimenez J, Prather K, Tufekci Z, Fisman D, Schooley R. Ten scientific reasons in support of airborne transmission of SARS-CoV-2. 2021.
  2. Molteni M. The 60-Year-Old Scientific Screwup That Helped Covid Kill [Internet]. Wired. 2021 [cited 10 June 2021]. Available from:
  3. Addleman S, Leung V, Asadi L, Sharkawy A, McDonald J. Mitigating airborne transmission of SARS-CoV-2. 2021.
  4. Bazant M, Kodio O, Cohen A, Khan K, Gu Z, Bush J. Monitoring carbon dioxide to quantify the risk of indoor airborne transmission of COVID-19. 2021.
  5.  [Internet]. 2021 [cited 10 June 2021]. Available from: 
  6. [Internet]. 2021 [cited 10 June 2021]. Available from: