Institutional Tuberculosis Transmission. Controlled Trial of Upper Room Ultraviolet Air Disinfection: A Basis for New Dosing Guidelines

Abstract

Rationale: Transmission is driving the global tuberculosis epidemic, especially in congregate settings. Worldwide, natural ventilation is the most common means of air disinfection, but it is inherently unreliable and of limited use in cold climates. Upper room germicidal ultraviolet (UV) air disinfection with air mixing has been shown to be highly effective, but improved evidence-based dosing guidelines are needed.

Objectives: To test the efficacy of upper room germicidal air disinfection with air mixing to reduce tuberculosis transmission under real hospital conditions, and to define the application parameters responsible as a basis for proposed new dosing guidelines.

Methods: Over an exposure period of 7 months, 90 guinea pigs breathed only untreated exhaust ward air, and another 90 guinea pigs breathed only air from the same six-bed tuberculosis ward on alternate days when upper room germicidal air disinfection was turned on throughout the ward.

Measurements and main results: The tuberculin skin test conversion rates (>6 mm) of the two chambers were compared. The hazard ratio for guinea pigs in the control chamber converting their skin test to positive was 4.9 (95% confidence interval, 2.8-8.6), with an efficacy of approximately 80%.

Conclusions: Upper room germicidal UV air disinfection with air mixing was highly effective in reducing tuberculosis transmission under hospital conditions. These data support using either a total fixture output (rather than electrical or UV lamp wattage) of 15-20 mW/m(3) total room volume, or an average whole-room UV irradiance (fluence rate) of 5-7 μW/cm(2), calculated by a lighting computer-assisted design program modified for UV use.

Keywords: air disinfection; infection control; tuberculosis prevention; tuberculosis transmission; ultraviolet irradiation.

Figure 1.

Layout of the airborne infections research facility showing three two-bed patient rooms, corridor, and patient day room.

Figure 2.

(A) Ultraviolet germicidal irradiation wall fixture, (B) room ventilation arrangement, and (C) ceiling paddle fan model in use during experiments. Note that the photograph in A was taken wearing ultraviolet protective goggles because even brief direct exposure at close range to upper room ultraviolet germicidal irradiation lamps can cause corneal inflammation. The arrows in B indicate air movement.

Figure 3.

A computer-assisted design–generated illustration estimating the ultraviolet (UV) germicidal irradiation distribution in one of the patient rooms in the airborne infections research facility. The fixtures are represented by the blue ovoid structures on the near and far upper walls. The upper tricolor distribution represents three levels of ultraviolet germicidal irradiation flux in the “kill zone,” whereas the lower “safety” distribution indicates peak UV fluence rates 1.7 m (5.5 ft) from the floor.

Figure 4.

Kaplan-Meier risk estimates of combined tuberculin skin test conversions (y-axis) of exposed guinea pigs in control and intervention chambers by month (x-axis) of exposure. P < 0.0005; combined hazard ratio, 4.9 (95% CI, 2.8–8.6). CI = confidence interval.