Engineering Novel interventions

Sunlight and UVC decontamination

Whilst the US president, Donald Trump’s suggestion that light could be brought “inside the body” was unhelpful, he stimulated renewed attention to the use of light to control SARS-CoV-2 outside the body. This continues a well established line of research, beginning with Downes and Blunt, who in 1877 found that sunlight limited microbial growth. In this post we will show how sunlight and UVC decontamination could be used to slow the spread of COVID-19 in hospital settings in 2021 and beyond.

Sunlight and UVC

Light is measured by it’s frequency (Hz), wavelength (nm) and energy (eV). The electromagnetic spectrum describes the full range of light, from gamma rays to radio waves.

Electromagnetic spectrum (Credit: NASA’s Imagine the Universe)

Sunlight includes visible and ultraviolet light (UVA and UVB). UVC is blocked by Earth’s atmosphere and therefore does not form any part of sunlight reaching us. This is lucky because it would cause severe DNA damage if we were exposed to it. Inside buildings, UVC irradiation of air and surfaces can be achieved through specialised equipment in the absence of anyone that could be exposed to it.

How does sunlight and UVC affect SARS-CoV-2?

Dabish and his colleagues, found that a combination of higher sunlight, temperature and relative humidity sped up the decay rate of the Corona virus (2020). Based on this research a calculator was then developed by the Department of Homeland Security to show how these conditions change the time taken for 50, 90 and 99% of the virus to be inactivated. For example, during a clear mid-summer day in Edinburgh (UV index 5, temperature 20°C and 70% relative humidity), 99% of the virus will be inactivated in 28 minutes. If it was extremely overcast (UV index 0), but otherwise similar weather conditions to above, it would take 6 times longer for the virus to be inactivated. The time for the virus to be inactivated using UVC is much faster. Kitagawa et al., found that UVC (222 nm in wavelength) at 0.1 mW/cm2  resulted in a 99.7% reduction after 30 seconds (2020).

Where do sunlight and UVC fit within the engineering controls being considered by SAGE?

The UK Government advisory committee, the SAGE – Environmental and Modelling group, reviewed the evidence of sunlight and UVC decontamination in their “Transmission of SARS-CoV-2 and Mitigating Measures” as part of their report on the 4th of June, 2020. Evidence was rated on the control efficacy and real-world effectiveness, and their confidence in it. We digitised the data and visualised the results in interactive bubble plots. Bubble size relates to how many of the 14 raters considered the control efficacy/ effectiveness/ confidence to be high or very high. The figure below shows the engineering controls that were considered and how they compare in an overall rating that combines the scores above.

Sunlight and UVC decontamination both show potential as COVID-19 controls – being in the middle of the 10 measures assessed. Some of the most suitable engineering controls are now being routinely applied, including hand wash stations, screens/partitions and provision of fresh air. It makes sense to look down the list to additional interventions, like sunlight and UVC decontamination, that score higher on efficacy, but lower on effectiveness and confidence (i.e. there is a lack of research on their implementation).

Using sunlight to prevent infection in hospital care settings

World Health Organisation guidelines mention the use of sunlight to reduce hospital acquired infections, especially for pathogens that are airborne. Given that COVID-19 is shown to be predominantly transmitted by aerosol in hospital settings (57% in a recent study by Dr Rachel Jones) then it is worthwhile to consider controls that have a focus on this route. The guidance states that in addition to negative air pressure and ventilation of 6-12 air-changes per hour, patients should be in single rooms with sunlight. Glass completely blocks UVB so ideally the window should be open. However, opening a window more than 10 cm is not allowed in British hospitals (Department of Health, 2013). There may be opportunities develop acrylic sheets to be inserted in open windows to allow UVB to pass through.

Using UVC decontamination to prevent infection in hospital care settings

UVC decontamination is seen as a good alternative to ventilation, when the latter cannot be improved (SAGE-EMG, 2020). UVC decontamination can take 20-45 minutes (depending on room size), typically in an unoccupied room. The equipment should only be used by trained staff with risk assessments and controls in place. The effectiveness is dependent on the distance from the equipment to the contaminated air/surfaces and the impact of objects that cause shadowed areas. A promising avenue of research is far UVC (207 to 222 nm), which does not seem to harm mammalian skin (Welch et al., 2018), but further research is needed to prove the safety of using this light in occupied rooms.

Some products on the market have attempted to address the limitations mentioned above. One product is the UVD Robot (image under the blog post title), which takes 10 minutes to decontaminate a typical patient room and is completely autonomous, removing safety concerns for operators and human error in application. Another product is Sodeco’s UVC air purifier, a low noise unit that can be used in occupied rooms and provides the equivalent of 6.5 air changes per hour in a 500 m3 room.

Testing the effectiveness of sunlight and UVC decontamination interventions

We have adapted a model for understanding COVID-19 transmission in hospital care settings to test the effectiveness of sunlight and UVC decontamination. We will determine the probability of infection and the dominant route of transmission (i.e. through contact with surfaces, inhaling contaminated aerosol or intercepting cough droplets) before and after applying the interventions mentioned above. The results will be shared in a peer-reviewed publication and on this blog, so stay tuned!

Mark Cherrie


Virus protection from face shields

Face shields are widely used in healthcare to protect workers from virus contamination in droplets (i.e. large visible drops of mucous) and aerosols (i.e. very small particles that can be inhaled) from patient coughs, but how effective are they?

Research by the US National Institute for Occupational Safety and Health (NIOSH) helps us understand what protection they can provide. The researchers simulated a coughing patient using machinery that generates a droplet spray and placed a breathing mannequin in front of it. They varied the distance between the coughing and the breathing headforms and had either large (8.5 μm) or small diameter (3.4 μm) aerosol produced with each cough. The experiment did not measure the protection from droplets splattering into the face, which we can assume was 100% In the experiments the visor was very good at protecting against the initial larger aerosol exposure, where more than 95% was diverted away by the visor. The face shield also reduced the surface contamination of a respirator worn behind the visor by a similar amount. However, it was less effective for the smaller diameter aerosol where it blocked around 70% of the initial cough aerosol and 76% of the respirator surface contamination.

The visor was much less effective at protecting the worker in the period after the cough, and in the 30 minutes following a cough the aerosol dispersed through the room and the face shield only reduced aerosol inhalation by 23%. This underlines the need to wear the visor in combination with an effective respirator to achieve effective protection. The authors concluded that the:

‘…results show that health care workers can inhale infectious airborne particles while treating a coughing patient. Face shields can substantially reduce the short-term exposure of health care workers to large infectious aerosol particles, but smaller particles can remain airborne longer and flow around the face shield more easily to be inhaled. Thus, face shields provide a useful adjunct to respiratory protection for workers caring for patients with respiratory infections. However, they cannot be used as a substitute for respiratory protection when it is needed.


Lindsley WG, Noti JD, Blachere FM, Szalajda JV, Beezhold DH. (2014) Efficacy of face shields against cough aerosol droplets from a cough simulator. Journal of Occupational and Environmental Hygiene; 11: 509-18.


Ventilated headboards

Hospital patients with Covid-19 can contaminate the room air and surfaces with the virus. Healthcare workers have had to rely on personal protective equipment (PPE) to try to protect themselves from being infected. One alternative way to reduce the risks for workers is to use a ventilated headboard on the bed.

Ventilated headboard
The prototype ventilated headboard

These headboards are designed to extract air contaminated with Corona virus from behind the patients head. The air is passed through a high efficiency filter before being discharged. The original design was published by the US National Institute for Occupational Safety and Health nearly ten years ago, but the system has still not been commercialised. However, the researchers provided a set of DIY instruction to build the system.

The researchers list a number of advantages for their design, including:

  • Proven design that successfully captured and removed over 99% of airborne infectious-sized aerosol in a laboratory test
  • Cost-effective system, where the cost per isolated patient is much less than traditional airborne infection isolation rooms
  • Healthcare workers operate outside the “hot zone” of infectious aerosol
  • Easy patient access
  • Scalable from one to many units
  • Highly adaptable to fit most sizes of hospital beds
  • Quick and easy installation
  • Easily dismantled for storage

Watch a short video about the system on YouTube.

While the researchers demonstrated the system has a very high effectiveness we believe that it is unlikely to achieve such performance in real healthcare situations. It seems more realistic to assume that a ventilated headboard could reduce SARS-CoV-2 aerosol emissions from an infected patient in hospital by around 90%


Dungi S, Ghia U, Mead K, Gressel M. (2015) Effectiveness of a Local Ventilation/Filtration Intervention for Health-Care Worker Exposure Reduction to Airborne Infection in a Hospital Room. Published by ASHRAE.