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.
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!