Evaluation of a Methodology for Quantifying the Effect of Room Air Ultraviolet Germicidal Irradiation on Airborne Bacteria

As a result of the recent resurgence in tuberculosis (TB) and the increasing incidence of multidrug-resistant TB, there has been renewed interest in engineering controls to reduce the spread of TB and other airborne infectious diseases in high-risk settings. Techniques such as the use of lamps that produce ultraviolet germicidal radiation may reduce exposure to infectious agents by inactivating or killing microorganisms while they are airborne. We designed and evaluated a test method to quantitatively estimate the efficacy of germicidal lamps, in conjunction with dilution ventilation, for reducing the concentration of viable airborne bacteria. Bacterial particles were generated in a 36m³ room and collected with midget impingers at 5-7 locations. The effectiveness of the control technique was determined by comparing concentrations of culturable airborne bacteria with and without the control in operation. Results for a single, 15 W germicidal lamp showed reductions of 50% for Bacillus subtilis (B. subtilis) and Micrococcus luteus (M. luteus); tests with Escherichia coli (E. coli) showed nearly 100% reduction (E. coli were isolated only from the sampler nearest the aerosol source when the lamp was operating). The addition of louvers to a lamp greatly reduced its efficacy. Decay experiments showed that roughly 4-6 equivalent air changes per hour were achieved for B. subtilis with one or two lamps operating. These preliminary experiments demonstrated that this methodology was well suited for these evaluations and identified factors that could be modified to refine the study design for future work.