Efficacy assessment of ultraviolet germicidal irradiation (UVGI) devices for inactivating airborne Mycobacterium tuberculosis

Abstract

INTRODUCTION : Airborne transmission of Mycobacterium tuberculosis (TB) and other infectious agents within indoor environments has been a recognised hazard for decades. The increasing incidence of airborne diseases and drug resistance has renewed interest in ultraviolet germicidal irradiation (UVGI) to reduce transmission. The aim of this study was to determine the efficacy of UVGI devices, available in South Africa, for inactivating airborne TB bacteria. METHODOLOGY : Thirteen UVGI devices from major South African suppliers were challenged with M. tuberculosis H37Ra bacilli (~1 x 106 vegetative cells/ml) when OFF and when ON, for one hour. Air samples (n = 130) were collected using PTFE filters. Sample extracts were analysed using quantitative real time polymerase chain reaction (qPCR), targeting the 16 subunit ribosomal ribonucleic acid (16S rRNA) gene. The DNA extraction efficiency was also determined, using the Quanti-iT PicoGreen assay. Irradiance measurements, including ultraviolet-C (UVC) output and maintained UVC flux of the devices, were recorded using an integrating sphere. The data were analysed using descriptive and inferential statistics. RESULTS : There was no difference between the mean concentration of the DNA extracted from the aqueous and air samples (p = 0.3494). An accumulation of TB DNA copies/m3 with increasing time, when the devices were OFF, was observed as expected. Forty-six percent (6 of 13) of UVGI devices tested yielded 100% effectiveness in a controlled laboratory setting; 5 of 6 had built-in fans which may have contributed to their efficacy. The effectiveness of the remaining devices ranged from 43.7% to 95.1%. CONCLUSION : The efficacy of UVGI devices available in South Africa is highly variable, with minimum UVC output. The reduced levels of effectiveness of some devices might be due to the design of the devices, which needs to be reassessed by manufacturers. The effectiveness of UVGI devices and quantification of microbial survival rate can be assessed robustly using qPCR.