A Multi-Zone Model Evaluation of the Efficacy of Upper-Room Air Ultraviolet Germicidal Irradiation

Abstract

Engineering controls can be used to reduce the spread of airborne infectious disease, particularly tuberculosis (TB), in high-risk settings. This article evaluates published data on the efficacy of upper-room air ultraviolet germicidal irradiation (UVGI). A three-zone representation of a TB patient room equipped with a germicidal UV lamp is developed. The lamp irradiates the upper-room zone and inactivates airborne mycobacteria; the unirradiated lower-room zone also contains a near-field zone surrounding the TB patient. Infectious particles are generated in the near-field zone and transported throughout the room by air flow between zones. Each zone is independently well-mixed; the whole room, however, is not well-mixed. The three-zone model is applied to a previously published study of UVGI against airborne mycobacteria in a test room. Based on the estimated slopes of the semi-log concentration decay curves for viable mycobacteria, and on the assumption that the test room was essentially well-mixed, the published study reported that UVGI provided 10 to 25 equivalent air changes per hour. However, when the same decay curve slopes are interpreted in the context of the three-zone model, UVGI is seen to be far less effective in reducing exposure intensity near the TB patient. Near-field exposure intensity is relevant because health care workers are usually in close proximity to the TB patients they attend. In general, the interpretation of concentration decay data depends on the specific model of room air mixing that is assumed appropriate. It is recommended that tests of the efficacy of UVGI and other control devices against airborne microorganisms be based on steady-state concentration measurements rather than concentration decay measurements, because the former measurements do not require inferences based on a particular model.

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

“…For example, the probability of inhaling an infectious agent is greater for persons occupying those areas of a room in which the concentration of airborne microorganisms is higher (Nicas 1996;Nicas and Miller 1999;Miller-Leiden et al 1996). Our measurements suggest that the occurrence of elevated near-eld concentrations depends on the airborne bacteria under observation.…”

“…In the upper zone of our chamber, we measured an average irradiance of 25-52 l W cm ¡ 2 (Table 2). The UV irradiance for the earlier study was estimated to average 30 l W cm ¡ 2 in the upper zone of the test room (Nicas and Miller 1999). Thus, the UV irradiance in our chamber was probably comparable within a factor of 2 to that of Riley et al Further work is underway to measure the ef cacy of UVGI applying the decay methodology to different microorganisms and varied UV irradiance levels.…”

“…Aerosolized bacteria may not have been perfectly mixed with the indoor air, especially during the initial decay period (Drescher et al 1995;Mage and Ott 1996). If the airborne bacteria are not well mixed, the determination of ACH O , ACH V , and ACH UV would be subject to error (Nicas and Miller 1999). An advantage to the decay method is that only relative aerosol concentrations are needed and determination of initial concentrations is not required.…”

“…Riley et al (1976) achieved relatively large rate constants (10-25 h ¡ 1 ) for inactivation of Mycobacterium bovis (M. bovis) BCG, a surrogate for M. tuberculosis, depending on the total power of the germicidal lamps. In the latter study, room air mixing was not characterized and the rate constant was estimated from the slope of a concentration decay curve which may have led to an overestimation of UVGI ef ciency if the room was not well mixed (Nicas 1996;Nicas and Miller 1999).…”

Control Effectiveness of Ultraviolet Germicidal Irradiation on Bioaerosols

“…The nish on walls, ceilings, and other items may in uence the irradiance throughout a room. Furthermore, it was found that the degree of room air mixing was related to UVGI ef ciency (Nicas 1996;Nicas and Miller 1999). In regard to the effects of relative humidity (RH), it was reported that there was a declined inactivation of airborne organisms by UV at RH in excess of 60 to 70%.…”