COVID-19 | UVC LIGHTING LIGHT RELIEF Experts are looking to ultraviolet light to help limit the transmission of coronavirus. Andy Pearson finds out how UVC light technology works and whether it could be used in buildings T he World Health Organization (WHO) last month acknowledged emerging evidence that coronavirus might be spread by tiny particles that can remain suspended in the air for hours. By breathing, talking, coughing and sneezing, people produce aerosols that may, if the host is infected, contain pathogens. Most of the larger droplets emitted fall on surfaces within 1-2m from the source. A few larger droplets may, depending on environmental conditions, shrink by evaporation before they settle, to become an aerosol. ASHRAE, among others, says small aerosols (less than 10 m) can stay airborne and infectious for hours even days and, consequently, can travel longer distances and infect secondary hosts without contact with the primary host. The evidence has yet to be thoroughly evaluated, but if confirmed it will make management of indoor air quality even more critical for infection control and may lead to changes in government advice on measures to prevent the spread of Covid-19. The design and operation of HVAC systems can have a major impact on infectious aerosol transport. Experts recommend flushing rooms with lots of outside air to dilute airborne concentrations of the virus. But increasing the volume of outside air might not be possible in all buildings (or weathers) and for all ventilation systems, which is why some experts are looking to ultraviolet light technologies as a means of limiting the transmission of coronavirus. Ultraviolet (UV) light can inactivate a virus. There are three main types of UV radiation, classified according to their wavelength: UVA (400nm-315nm); UVB (315nm280nm); and UVC (280nm-100nm). All three are present in sunlight; the shorter the wavelength the more harmful the UV radiation. UVA makes up the bulk of UV radiation reaching the Earths surface, and can penetrate deep into the skin. UVB can also reach the Earths surface and can damage DNA in our skin. UVC radiation is the most dangerous to humans, but is filtered out of sunlight by ozone in the atmosphere. UVC is part of the ultraviolet spectrum that has the greatest germicidal effect. Its ability to kill bacteria was discovered more than a century ago. Since then, artificial sources of UVC such as mercury lamps and, more recently, UVC LEDs have been used to produce light with a wavelength of 254nm as a means of sterilising surfaces. UVC light renders micro-organisms, such as bacteria and viruses, ineffective by destroying the genetic information in their DNA, so preventing them from reproducing and, effectively, inactivating the virus. When used in this way, UVC lighting is sometimes termed ultraviolet germicidal irradiation (UVGI). UVC applications A common use of UVC lamps is inside air conditioning units. Here, the lamps disinfection properties are employed to keep the cool, moist surfaces of cooling coils and drain pans free of mould, virus and bacteria. When the virus or microbe is on the surface, the lamps are able to deliver an effective dose. UVC lamps are also used to irradiate surfaces in labs and clinical spaces, but only when the rooms are unoccupied. This reduces the likelihood of virus transfer from objects and materials. Signify, which manufactures UVC lighting, has been working with Boston University to validate the effectiveness of its UVC mercury vapour light source, with a wavelength of 254nm, to inactivate SARS-CoV-2, the virus that causes coronavirus disease 2019 (Covid-19). The researchers applied a dose of UV light with a radiant energy density of 5mJ/cm2; this resulted in a reduction of the SARS-CoV-2 virus of 99% in six seconds. Based on the data, it was determined that a dose of 22mJ/cm2 will result in a reduction of 99.9999% of surface bacteria in 25 seconds. A pre-paper by Beggs and Avital1 also found evidence suggesting that SARS-CoV-2, when suspended in air, is reasonably easy to inactivate using UV light at 254 nm. UVC is not the silver bullet, but it is good at getting to the virus in places where it is difficult to clean using conventional cleaning methods 26 August 2020 www.cibsejournal.com CIBSE August 2020 p26-28 UV-C.indd 26 24/07/2020 15:52