Potential application of Air Cleaning Devices to Manage Transmission of COVID-19 SAGE-EMG

November 23, 2020 – 8 Min

The Scientific Advisory Group for Emergencies (SAGE) has been providing British Government ministers and officials with free and frank advice throughout the Covid19 pandemic, based on external scientific evidence and a wide source of essential information. The Environmental and Modelling group (EMG) for SAGE recently released an important research paper on the subject of Air Purification covering:

  • Different technologies available to clean the air
  • The Effectiveness of the devices
  • Practical Application
  • Chemical/Spray/Misting approaches

Below is a summary of the different issues with key important excerpts (in italics) from the research paper that we think will be of interest to existing and potential clients. There is also a brief Rensair comment in blue highlighting how the hospital-grade Rensair air purification unit fits into the research and recommendations of this report. Hope you find this interesting and useful. Regards Edward



  • Air cleaning devices where the primary principle of operation is based on fibrous filtration (i.e. HEPA13 filters) or germicidal UV (UVC) are likely to be beneficial if deployed correctly
  • These devices are recommended for settings where the ventilation is poor, and it is not possible to improve it by other means. 
  • The efficacy and safety of such devices should be evidenced by relevant test data.
  • 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.
  • Effectiveness of air cleaning devices depends on multiple parameters including the underlying technology, the design of the device, the in-room location of the device, the environment that it 2 is used in and the maintenance of the device. The performance of most devices is based on data measured in idealised controlled environments and is likely to be different and often lower in a real-world setting (high confidence). Caution should be used when considering idealised performance data stated by a manufacturer.
  • There may be unintended consequences from the application of air cleaning devices including emissions that could cause health effects, noise, changes in temperature and drafts. Further, it is clear there is a requirement for regular maintenance and consumable requirements for some devices.
Part 1: Different technologies available to clean the air

Annex 1 has a very useful summary of Air Cleaning Technologies and their potential application.

Currently there is little data available on the health risks of devices containing multiple technologies (ASHRAE 2015). However, if a device incorporates a technology that is known to pose a health risk that cannot be easily mitigated, the device should not be used regardless of the other technologies it incorporates unless its safety and efficacy can be unequivocally and scientifically demonstrated by relevant test data.

While there is no direct evidence that use of high efficiency particulate filters can reduce the transmission of SARS-CoV-2, it is widely accepted that the virus is contained within exhaled droplets and aerosols, with those in the 1-100 micrometer range likely to pose the highest risks. 

Devices incorporating HEPA filters or other high grades of filter (e.g. MERV 13 or higher) are therefore likely to be effective at removing a substantial proportion of airborne virus. HEPA filters are generally rated on their efficiency at the most penetrating particle size which is between 0.1 and 0.3 micrometers.

The evidence suggests that devices based on high efficiency filtration and germicidal UVC technologies are likely to be effective against the SARS-CoV-2 virus. Well designed and appropriately installed devices based on these technologies are appropriate to use to supplement ventilation in some situations.

Technologies based on UVA/UVB, ionisation, plasma, electrostatic precipitation and oxidation methods have limited evidence of efficacy against the virus and/or significant concerns over toxicological risks during application. As such SAGE EMG does not recommend using these devices in occupied rooms against COVID-19 without further independent evidence to demonstrate their viability and safety in realistic settings

Rensair not only utilises HEPA13 filters, but also UVC light to shine on the filters to eradicate trapped airborne pollutants.

Part 2: The Effectiveness of the devices

The majority of systems on the market quote values of 99% single-pass effectiveness or greater. Testing of these devices shows that while this is correct for some, others may have much lower single pass effectiveness due to poor manufacturing or issues such as poorly fitting filters

Computational modelling studies show that even something as simple as positioning the device on the opposite wall of the same room could reduce the UV dose received by the room air by up to 1.6 times due to the interaction between the UV irradiance field and the room airflow (Noakes et al 2006).

In a yet-to-be-peer-reviewed study, Curtius et al (2020) made experimental measurements of particle concentrations within classrooms with and without air cleaners. They observed a greater than 90% reduction in aerosol concentration and estimated a six fold reduction in risk from the use of air cleaners.

Potential application of air cleaning devices in different scenarios (from Table 3) 


  • Small office/meeting room with less than 10 people
  • Large office/education environment with 20-30 people
  • Performance venue with 200+ people
  • Hospitality setting (bar or restaurant)
  • Small business/smaller retail premises
  • General hospital or care environment
  • Dentistry or healthcare aerosol generating procedure

Not Recommended:

  • Home
  • Large retail premises
  • Large manufacturing environment
  • Public transport

On its website, Rensair provides tests carried out by well-respected laboratories to demonstrate the efficacy of Rensair units. They demonstrate that not only do Rensair units provide extremely high levels of air purification, but due to the exceptional design, they are also effective independent of where placed in a room. 

Part 3: Practical Application

Many air cleaning devices can have negative health impacts which most commonly relate to the significant effect on the indoor air chemistry that results when chemicals are emitted directly or formed from the resulting chemistry following their use (Table 4 provides the potential health hazards associated with air cleaning devices).

The health hazard posed by an air cleaner depends strongly on the technology. Some devices such as chemical oxidation devices deliberately produce compounds, such as ozone, to inactivate microorganisms. If the concentration is sufficient for the device to effectively disinfect the room, it is likely that the concentration will also exceed safe limits for human health

Devices based on UVC at 254nm wavelength pose a particular risk to skin and eyes if designed and installed poorly. Some poorly designed UVC devices can also produce ozone. Both chemical by-products and UV light can potentially cause damage to materials and fixtures in a building.

Noise: Above 80 dB(A) daily exposure, for people at work, there ought to be noise risk assessment under the Control of Noise at Work Regulations (HSE, 2005). The threshold level of 75dBA equates to ‘no risk’ in terms of a 24-hour daily noise exposure.

Dust and particle build up reduces the CADR (Clean Air Delivery Rate) of air cleaners due to both reduced filter performance and increased filter bypass. The typical operating life for UVC lamps is 9000h.

Within a class of devices, some are high quality and likely to be effective while some use poor components and poor manufacturing and are ineffective. There are examples of devices which do not meet safety limits on emissions despite claims by the manufacturer.

The design of Rensair was optimised to make it efficient for use in hospitals. It incorporates many features to ensure that the HEPA13 filter remains efficient with low noise levels over a long period of time, provides an impressive coverage of air purification, and that the maintenance is a simple, straightforward and safe operation. 

Part 4: Chemical/Spray/Misting approaches

Health and safety concerns – whole-body walk-through treatment. Major concerns exist over the impact of such treatments on people’s health and well-being. In recent months scientists and medical authorities internationally have voiced their concerns about the harm that whole body spray systems may do and their ineffectiveness.

Table 5 depicts the health risks of dermal and inhalation exposure from common sprayed disinfectants. 

There is currently no strong evidence that using a continuous spray chemical in the air will be an effective control against SARS-CoV-2 transmission. Use of spray chemicals for air cleaning is not recommended by SAGE EMG; the improvement of ventilation systems or application of filter-based or UVC air cleaning devices is a more appropriate solution.


In selecting devices, it is important to consider all aspects, not just the potential ability to remove or kill the virus.

Within a class of devices, some are high quality and likely to be effective while some use poor components and poor manufacturing and are ineffective. There are examples of devices which do not meet safety limits on emissions despite claims by the manufacturer.

To use air cleaning devices effectively, urgent action is therefore needed to support industry and consumers in ensuring they are selecting and using devices safely and effectively. This includes:

  • Further research on the efficacy of devices including evidence of the technology against SARS-CoV-2 virus (or a suitable viral surrogate) and other pathogens, performance of devices in real-world settings, and behavioural responses to the use of such devices.
  • Appropriate advice to support the manufacturers of devices and impartial guidance for consumers to allow them to identify appropriate technologies and high quality products and cut through the marketing information on manufacturers websites.
  • Guidance and training for facilities managers and building services practitioners on the selection, design, installation and maintenance of air cleaning devices
  • Standards for device testing and approved facilities where industry can access independent and verifiable testing

Rensair welcomes the 4 points raised by SAGE. In the course of working with clients, Rensair provides its clients with

  • Reports on the independent testing of its units, available on its website
  • A full customer service, highlighting key scientific research and guidelines, as well as a full explanation of how its units work
  • Detailed plans for air purification, including how many units are needed to achieve the necessary room Air Changes per Hour requirements, where to install units, as well as how to operate and maintain them once implemented.

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