June 4, 2003—With funding from the National Institute for Occupational Safety and Health (NIOSH), researchers at the University of Colorado have conducted a six-year study on protecting employees in health-care and other industries from job-related tuberculosis (TB) infection.
Recreating a hospital setting with a mannequin as a patient, the NIOSH-funded study investigated the effectiveness of ultraviolet light as a key component of controls used to reduce health-care employees’ exposure to TB. The findings are also applicable to other workplaces such as correctional facilities, homeless shelters, residential care facilities, and nursing homes.
TB usually is spread from person to person as a direct result of breathing air contaminated with airborne TB bacteria released by an infectious person, usually through coughing or sneezing. Once airborne in a room, TB bacteria may remain airborne for hours due to their small size. Although TB cases have declined in recent years after resurgence in the late 1980s, the disease still poses an occupational risk to healthcare personnel.
To reduce the potential for TB transmission in healthcare facilities, CDC guidelines recommend that a number of controls be used. Currently, many facilities use ultraviolet germicidal irradiation, referred to as UVGI, as an auxiliary control measure when their ventilation systems in hospital rooms are unable to provide air exchange rates recommended by CDC. In addition, UVGI is used for air disinfection in other areas such as waiting rooms. The UVGI lamps are suspended from or located near the ceiling, or are placed in ventilation ducts.
The study investigated the effectiveness of UVGI to kill or inactivate two different TB-like bacteria that were released into the room air. The study looked at environmental factors such as humidity, ventilation, and air-mixing effects, as well as the actual UV levels coming from the UVGI lamps, and whether the location of the lamps within the room affect the UVGI effectiveness.
Findings from the studies included:
- Increasing the irradiance level of the UVGI lamps increased the effectiveness of inactivating the TB-like bacteria. The relationship was linear up to a certain level. Further increasing the irradiance above this high level resulted in little increase in the inactivation of the airborne TB-like bacteria.
- High relative humidity above 75 percent lowered the effectiveness of UVGI to inactivate the TB-like bacteria.
- Mostly, ventilation and UVGI worked together to remove or inactivate the airborne TB-like bacteria at a greater rate than either system working alone. Low to moderate levels of ventilation in the room did not negatively affect UVGI effectiveness.
- The study clearly demonstrated that the air in a room must be mixed for UVGI to effectively inactive the TB-like bacteria. When warm air entered the room via a duct close to the ceiling (which may occur in the winter when the heating system is turned on), the warm air simply “rested” on the much cooler air below and the efficacy of the UVGI system was dramatically diminished. No mixing fans were on during this experiment but moderate ventilation was present.
NIOSH is assessing the findings to help develop up-to-date recommendations on the ability of UVGI systems to help protect health-care employees from TB. The findings are being evaluated for inclusion in subsequent updates to the CDC guidelines on controlling TB transmission. They are also being used to develop a comprehensive NIOSH technical report on the application of UVGI systems. For more information on occupational risk for TB, respiratory protection, and links to other Web sites, visit the NIOSH/Tuberculosis site.
