| Ultraviolet germicidal irradiation (UVGI) of upper room air is used as an engineering control method to prevent the spread of airborne infectious disease. Under full-scale conditions, a test room (90 m3), fitted with a modern UVGI system (average upper zone UV irradiance was 43 μW cm−2) was used to conduct bioaerosol inactivation experiments. Microorganism cells were aerosolized into the room such that their numbers and physiologic state were comparable both with and without the UVGI and ventilation system operating. Collected bacteria were quantified using direct microscopy and standard culturing assays.; Under ideal conditions (24°C, 50% relative humidity and complete air mixing), UVGI reduced the room-averaged concentration of culturable airborne bacteria between 46 and 80% for B. subtilis spores, 83–98% for M. parafortuitum cells, and 96–97% for M. Bovis BCG cells. Increasing the ventilation rate from zero to six air changes per hour decreased the relative inactivation efficacy of the UVGI system between 6% and 25%. Mycobacteria were more sensitive to UVGI compared to spores. The UVGI system provided 17 equivalent air changes per hour or 1.03 × 10−3 cm2/μW.s of Z value (equivalent air exchange rate normalized against UV irradiance) for Mycobacterium parafortuitum.; UVGI efficacy can be affected by environmental factors such as relative humidity (RH), and air mixing conditions. The impacts of these factors were determined by measuring UVGI efficacy under various environmental conditions. High RH (75–100%) decreased the UVGI effectiveness by 30–45% and the UVGI equivalent air-exchange rate by half. UVGI effectiveness was highest when RH was 50%. The UVGI efficacy decreased by as much as 80% with incomplete air mixing induced by wintertime ventilation conditions.; To predict the UVGI efficacy, a CFD model was developed and multi-zone models were applied and validated against experimental results within a reasonable range of accuracy. A field study was undertaken to investigate UVGI systems in three hospitals with UVGI systems in Denver. The CFD model suggested that some of these systems performed well, while others did not. It was concluded that UVGI system is not as useful in an efficiently ventilated room as a nonventilated room and one UVGI system in these hospitals was installed in a high-ventilated room improperly. |
