Ultraviolet radiation has been used to prevent the airborne spread of Mycobacterium tuberculosis (M.tb), or TB, as it is more commonly known.
South African hospitals are still being designed the way they were two decades ago and the TB pandemic of today is not being addressed in the design of new facilities. To investigate the relationship between design and disease spread, a new facility (known colloquially as AIR, and under the joint control of the MRC and CSIR with overseas consultants) has been constructed at Witbank and guinea pigs are used as human surrogates (they closely resemble humans in their response to TB). This year one of the topics under research will be the effectiveness of UV radiation in preventing the spread of the disease, particularly the MDR (multi drug resistant) and XDR variants. It is hoped that the output of this study will allow the design of more effective TB wards where infection of others (both patients and medical staff) is minimised.
The system
UVGI can be used in hospital wards, waiting areas and other environments in one of two ways. It can be fitted into an air extraction system which already has very effective filtering or it can be used in light fittings to irradiate the upper part of a room. The filtration method only works where the building has been designed for this task and this is not usually the case for makeshift TB wards established as the pandemic spreads. The UV lamps are safe as they are placed out of sight in the ducts and calculations can be made on the UV irradiance required to kill 90% or more of the TB bacteria. The duct must also contain a high efficiency filtration unit capable of removing the rest. The clean air can be ducted to the outside or returned to its site of origin to remove any residual bacteria on a second pass. The upper air light fittings are a cost-effective way of controlling the spread of bacteria in an already constructed ward or waiting room.
A comprehensive study was carried out in the USA in 2002 using modern UVGI louvred fittings that created a band of UV irradiation in the upper layer of the room with an average depth of some 300 mm. The recommendation of the study was that the irradiance should be in the region of 5 W/m² in order to reduce airborne concentrations within the room by 90%. The effectiveness of the system was evaluated using a heated mannequin to simulate the human body with safe test TB surrogates being used. To be effective the heated air expelled by the patient coughing must rise through the UV irradiated layer. The level of irradiance required to kill the bacteria varies (various research results show a variance in kill factor which varies by an issue of five) and Professor Leuschner of the University of Pretoria uses the more generally accepted figure of 10,8 W/m² so as to achieve a survival fraction of 10% in 10 seconds exposure to the radiation. The lamps used are louvred (or contain baffles) so as to direct the UV radiation up so that patients are not exposed either directly or indirectly and cannot view the UV lamps directly. Leuschner, who is involved in testing of the fittings for local companies, uses a figure of 600 mW/m² as being a safe level of exposure to 254 nm radiation (20 mW/m² has been used by others as safe at the 1,5 m level).
UV lamps are not like normal fluorescent bulbs and output decreases with time, the lamps requiring renewal long before they stop working completely. The installation must be monitored regularly over time using a calibrated UV meter in order to ensure effectiveness and safety.
Despite all the questions surrounding upper air disinfection, hospitals within South Africa are installing them on the presumption that some form of prevention of the spread of TB is better than none. Some light fitting manufacturers are fitting suction fans at the end of the lamp housing in order to improve upper air mixing. Specifically, guidelines are necessary for the light fittings, the position and number of lamps and ensuring that the air flow in the room is adequate to ensure the necessary upper air interaction with the active UV radiation. Other considerations include humidity, as previous studies have indicated decreased effectiveness under higher humidity levels. Above all else the lamps must be regularly monitored for UV irradiance using a UV meter and readings must be taken to ensure that patients are not directly exposed from any normal position in the room. Perhaps part of the system monitoring could be automated with a control panel indicating which lamps are operating below effective levels.
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