UV system helps protect workers against TB
Lab used to test control of airborne disease
The test lab in the engineering department at the University of Colorado in Boulder looks very much like a hospital room, complete with a patient hooked up to a hanging IV drip. However, the patient is a mannequin being used to study the effectiveness of ultraviolet light to reduce health care employees’ exposure to tuberculosis.
The six year study was funded by the National Institute for Occupational Safety and Health (NIOSH).
"The CDC issued new guidelines for preventing TB in health care facilities in 1994," recalls Shelley Miller, PhD, assistant professor in the mechanical engineering department and lead researcher on the NIOSH TB study. "In that guideline, they suggested ways to use engineering controls to protect health care workers from getting TB, particularly when they work in TB isolation rooms. However, they noticed they didn’t have a lot of data on ultraviolet light (UV), so they decided to put out an RFP [request for proposals] for more data on UV." The existing data at the time were sparse, she says, "so they wanted a detailed, controlled study."
Honing in on UVGI
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 the 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.
However, while it was known that UV light renders bacteria inactive, thereby limiting their ability to grow and multiply when inhaled, most of the experimental data that led to the development of UVGI systems are decades old. Aside from anecdotal observations, little subsequent information existed about the actual performance of these systems in hospital rooms.
The results of the new research would give employers, employees, and facilities managers better data for answering key questions such as: Is a combination of ventilation and UVGI reliable for controlling TB transmission in a given facility? Would an employer need to invest in potentially more costly, time-consuming upgrades to the ventilation system to be safe? Is UV irradiation effective only above a specific intensity?
The study took place between 1997 and 2002. The research team looked at environmental factors that could either enhance or diminish the effectiveness of UVGI, such as high vs. low levels of relative humidity, high vs. moderate ventilation in the room, and air-mixing effects, as well as the actual UV levels coming from the UVGI lamps. They also examined UVGI effectiveness when the lamps were placed at different locations within the room. They used newly designed, commercially available UVGI fixtures consisting of five lamps. Four of the lamps were mounted in each corner of the room. The fifth was mounted from the center of the ceiling.
The mannequin in the test chamber (called "Manny" by the staff) was heated to approximate human body temperature. Heat from the body is one of the subtle factors that influence the movement of air near a person inside a room, affecting the amount of tiny particles such as bacteria that come into the person’s breathing zone.
"The lab was probably roughly the same scale and volume, with a ventilation system similar to that in many hospitals," says Miller. "Temperature and relative humidity were controlled just like a hospital room would be. We also had HEPA filters on our ventilation system because we really needed to keep the test chamber clean," she says.
Examining the results
Findings from the study include:
- 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% 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.
The findings of the NIOSH-funded study provided new data to help scientists in future research projects to evaluate a novel three-dimensional measurement approach to measuring UV radiation. This last finding is particularly interesting and may lead to significant new developments in the future. "One of the more important variables is the actual irradiance level the lamps put out into the room," Miller explains. "I suspect that many times hospitals just hang them and do not measure irradiance, but it is that [irradiance] level that enables you to kill the bugs."
Because the lamps the researchers used were hanging all over the room, they had to measure irradiance from all directions. "So, we tested a new method for getting the levels," says Miller. Miller concedes, however, that more research needs to be done on measuring irradiance in a room. "The way we did it in our project is just too time-consuming and cumbersome for health and safety officers in hospitals," Miller says. "We are working on developing a way for staff to do it easily."
She also is interested in testing UV systems against viruses. "It’s a big concern, especially with the recent SARS outbreak," she notes. Her team is currently developing the protocols for new research, but they have not yet identified a target virus.
Findings generalizable
As for the research that has been completed, Miller is convinced that UV systems offer added protection against TB. "The data definitely showed that," she asserts.
In addition, NIOSH says that while the study used a hospital room as a model, the findings are also applicable to other workplaces such as correctional facilities, homeless shelters, residential care facilities, and nursing homes. "The findings are pretty generalizable to other high-risk environments where you have someone who has TB but doesn’t know it, and there are other susceptible people in that environment," Miller concludes.
[Editor’s note: The recently published article on this study, titled "Efficacy of Ultraviolet Germicidal Irradiation of Upper-Room Air in Inactivating Airborne Bacterial Spores and Mycobacteria in Full Scale Studies," by Xu, et al., appeared in the journal Atmospheric Environment (2003; 37:405-419). Shelly Miller, PhD, can be reached at 427 UCB, University of Colorado, Boulder, CO 80309-0427. Telephone: (303) 492-0587.]
The test lab in the engineering department at the University of Colorado in Boulder looks very much like a hospital room, complete with a patient hooked up to a hanging IV drip. However, the patient is a mannequin being used to study the effectiveness of ultraviolet light to reduce health care employees exposure to tuberculosis.You have reached your article limit for the month. Subscribe now to access this article plus other member-only content.
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