Fundamental factors affecting upper-room ultraviolet germicidal irradiation - part II. Predicting effectiveness.

Compared with increasing outdoor air ventilation rate, upper-room ultraviolet germicidal irradiation (UVGI) is an attractive technology for lowering the indoor concentration of airborne microorganisms and thereby reducing the risk of airborne transmission of disease. With relatively modest vertical air circulation, most of the air in a room can be irradiated in a relatively brief time period without noise or significant power consumption. The hypothesis tested in this study is that the efficacy of upper-room UVGI to inactivate or kill airborne infectious microorganisms can be determined from an index of UVGI effectiveness, a dimensionless parameter designed to characterize adequacy of vertical air circulation, amount of UVGI provided, and their interaction. This index of effectiveness, which is determined independently of microbiological testing, was found to correlate well with experimental measurements made in a room-size chamber. A comparison of two other dimensionless parameters - the irradiation number and mixing number, from which effectiveness index is calculated - provides insight into whether the quantity of UV provided to the upper room or the intensity of the vertical air circulation is the controlling factor for effective application of upper-room UVGI. The irradiation number is calculated from the UV power output of the fixture(s), a parameter that is fixture specific and much easier to measure than mean fluence rate. An equation was also developed that relates UV fixture power output to mean fluence rate in either the irradiated zone or the entire room. In addition, reductions in viable microorganism concentration due to UVGI predicted from a two-box model are compared with experimental measurements.