The objective of this research was to study the factors that relate to the effectiveness of upper-room ultraviolet germicidal irradiation for inactivating airborne microorganisms. The work was conducted in a room-sized chamber designed and furnished for investigations of this nature. Nebulized Serratia marcescens, Bacillus subtilis spores, and vaccinia virus were used as test aerosols. Most data were collected from steady-state experiments comparing the number of viable organisms in the chamber air remaining with UV lamps turned on to the number with UV lamps turned off, but some decay experiments were conducted to compare the two methods. UV power level had a strong influence but was fully effective only in the presence of air mixing that produced vigorous vertical air currents. A conclusion of the study is that an upper-room ultraviolet installation is a complex system that requires careful integration of UV luminaires, UV power, and room ventilation arrangements.

Seven years of experience operating upper-room ultraviolet irradiation systems (UVGI) in 14 homeless shelters in six cities has taught the Tuberculosis Ultraviolet Shelter Study (TUSS) many valuable lessons about maintaining uninterrupted UV services and acceptable intervals between UV lamp changes and cleaning activities.One year (9,000 hours) of continuous illumination resulted in a reduction in UVC output of the two types of low pressure mercury lamps by 17 percent for the quartz, instant start tubular lamp and 25 percent for the preheat compact lamps. The reduction over time was due to metal accumulations on the inside of the glass plus increasing opaqueness (solarization) due to UV irradiation. Cleaning frequency was found to be highly site-specific. Lamp failure was infrequent and within the manufacturers' stated life period.