COVID-19 Transmission and Strategies to Limit Infection

February 5, 2021 – 7 Min


SARS CoV-2 (Covid19) is a strain of the Coronavirus family, a group of related RNA viruses that cause diseases in mammals and birds. The earliest reports of animal coronavirus infection occurred in the late 1920s, whilst human coronaviruses were discovered in the 1960s. Both SARS and MERS were coronavirus pandemics. Given the long history, there is plenty of published and peer reviewed research on Coronavirus.


Two principal methods of Covid19 transmission are widely discussed: 


Fomites are objects or materials which are likely to carry infection, such as clothes, utensils, and furniture. It has been generally assumed that Covid19 could be caught by humans who touch surfaces that have been infected by the virus. This has resulted in widespread policies to clean and disinfect all types of surfaces (e.g., clothes, furniture, floors) which might have been infected by the Covid19 virus. 


A Covid19 infected person emits Covid19 infected droplets when talking, sneezing, coughing, talking loudly/singing. These emissions are a combination of 

i) large droplets, which fall quickly to a surface, and

ii) very small droplets, called aerosols, which can remain suspended in the air for hours. 

A healthy human can catch Covid19 by inhaling Covid19 infected large droplets and small airborne aerosols directly into the lungs. 



a) Transmission Risk

Although the World Health Organisation (WHO) has been warning about the risks of catching Covid19 from fomites since the early days of the Covid19 pandemic, there is now huge uncertainty about  Covid19 being actually transmitted in this way. Various scientific research papers have raised doubts about how long the virus can live on a surface and whether it can indeed then infect other humans. The WHO now notes that “Despite consistent evidence as to SARS-CoV-2 contamination of surfaces and the survival of the virus on certain surfaces, there are no specific reports which have directly demonstrated fomite transmission”. The US CDC states that “COVID-19 spreads less commonly through contact with contaminated surfaces”. This all suggests that the risk of Covid19 transmission through fomites is low.

b) Recommendations

To counter possible fomite Covid19 transmission, the WHO recommends the cleaning and sterilisation of surfaces. Governmental advice has been in line with the WHO and there are detailed recommendations on Health and Safety websites (e.g. UK UK HSE). 

c) Transmission risk mitigating strategies

The clearest strategies against fomite transmission include hand washing and surface cleaning, the latter via the use of disinfectant liquid chemicals or gases and/or UVC light. Disinfectant chemicals are both applied directly to surfaces or via “foggers” where they are sprayed into the air in empty rooms to then settle on surfaces. Alternatively, high powered UVC lights are utilised in empty rooms, with the illumination sanitising surfaces. 

d) Positives and Negatives of Fomite strategies

The big positive is that surfaces are effectively cleaned. However, this comes with important negatives, the first being whether there is an actual necessity to clean fomites given that the WHO, US CDC and research all suggest this is NOT a mode of Covid19 transmission. Furthermore, gases, chemicals and direct UVC light can be dangerous to humans. In many cases, many hours of ventilation are required after rooms have been sprayed with liquid or gases due to the dangerous chemicals utilised (which is why these operations are carried out in empty rooms and by specialist staff). Furthermore, some chemicals and gases, as well as UVC light, can damage soft rubbers and plastics present in the room (e.g., exposed electrical cables and wiring).

e) Conclusion

For sure cleanliness is always recommended. However, given the mounting research that points towards a lack of Fomite Covid19 transmission, there are some serious questions to be answered about the level of surface cleaning required as well as some of the dangerous chemicals and gasses that are utilised.


a) Transmission Risk

The WHO and governments have widely recognised the transmission risks from inhaling large droplets from Covid19 infected persons. 

b) Recommendations and risk mitigation strategies 

To prevent contamination through inhaling large infected droplets, the WHO and governments recommend the wearing of masks and for people to keep a “safe distance” apart from each other. The wearing of masks is to prevent infected people exhaling the large droplets as much as for healthy people to not inhale infected droplets. The “safe distance” policy is to ensure that people are not in a range of any falling large infected droplets being emitted by an infected person. 

d) Positives and Negatives of Large Droplet strategies, and Conclusion

The recommendations are relatively cheap, simple and effective when there is compliance to 1) use proper masks in a correct fashion and 2) social distancing is maintained. 


a) Transmission Risk

Until relatively recently (October 2020), the WHO and governments placed very little importance on Covid19 infection from inhaling Covid19 infected aerosols which could remain airborne for hours and become concentrated in poorly ventilated spaces. However, after interventions by the scientific community (most publicly in an open letter by 239 scientists), and the publication of various research papers and case studies of potential airborne transmission events, the WHO recognised the risk of Covid19 airborne aerosol transmission. Governments and their health agencies followed suit. (e.g. US CDC and UK through the UK SAGE committee). 

b) Recommendations to mitigate against aerosol transmission risks

Until recently, few recommendations were made to mitigate airborne covid19 aerosol transmission risks. However, the WHO and governments (eg UK SAGE committee) now recommend enhanced ventilation, bringing outside air indoors, and where that is not possible, to use effective air purification units. 

c) Transmission risk mitigating strategies

The WHO recommends ventilating rooms with outside air to achieve at least 6 Room Air Changes per Hour (ACH). 6 ACH means introducing the equivalent of six times the volume of the room in fresh air every hour (for example, a room of 100m3 would require 600m3 of outside air to be introduced into that room every hour). 

If that standard of ventilation cannot be achieved, the WHO recommends the use of HEPA air filtration machines to remove airborne particles (from the air. The UK Government recommends something similar – the use of HEPA filters and/or the use of UVC light. Both the UK SAGE and US Environmental Protection Agency disapprove of other technologies for air purification purposes, including plasma, ionizers and chemical “foggers”, as well as ozone generators (which are extremely dangerous and ineffective in purifying air).

d) Positives and Negatives of airborne aerosol strategies

Testing and air flow modelling shows how good ventilation keeps a low concentration of aerosols in a room. However not all buildings can achieve the WHO recommended 6 ACH levels, and this policy becomes difficult to achieve in buildings with internally circulated HVAC systems (rather than bringing in outside air) or those with no windows or during the colder winter months.

HEPA filtration units are already widely used in hospitals and are recommended by health agencies for Covid19 and other airborne diseases (e.g., SARS and tuberculosis). This is due to their ability to filter extremely fine particles including viruses (and the coronavirus family) and bacteria. Because people can be present in a room as portable hospital-grade air purifiers operate, units can provide a continuous air cleaning operation, thus mitigating the risk of transmission should an infected person enter a room and emit Covid19 aerosols. 

On the negative front, some poorly designed HEPA air purifiers will not pass much air through the very fine HEPA filter, whilst others will only clean the air close to the filter, and not the room as a whole. Furthermore, as trapped viruses and bacteria can continue to live on a filter for numerous days, the replacement of the HEPA filter in an air purification device is potentially a hazardous job to complete. 

e) Conclusion

Ventilation is the first step to mitigate against the risks of Covid19 airborne aerosol transmission, and if that is not possible, then the use of portable air purifiers is recommended to constantly remove Covid19 airborne aerosols from the air. The UK Government recommends to only use portable air purifiers that:

i) have been independently tested (i.e., not by the manufacturer, but by an independent scientific laboratory, with published freely available results), 

ii) use well tried and tested technology (HEPA and UVC and not ionisers, plasmas, gases etc) and 

iii) are well designed and manufactured. 


In the absence of good ventilation, Rensair hospital-grade air purification units offer a good solution to mitigate against the risk of Covid19 airborne aerosols. They should be used in conjunction with government directives to mitigate against the risk of Covid19 transmission from large droplets (masks and “safe distancing”).

Rensair units meet all the requirements set out by the UK SAGE committee. They were designed with and used by Scandinavian hospitals for more than 10 years, meeting all their stringent requirements. Independent tests published on the Rensair website highlight their efficacy in both the high air volume that passes and gets purified through the HEPA filter as well as the 99.97% eradication of viruses, bacteria, pollen, yeasts, moulds and other airborne particles. The tests confirm the effectiveness of Rensair units in cleaning air around the room, not just around the unit itself. Unlike other air purifiers, Rensair utilises UVC light to permanently illuminate the HEPA filter, rapidly killing trapped viruses and bacteria on the filter which renders the filter void of any live particles which allows for safe maintenance of the units.

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