Research Indicates UVGI is an Effective Supplement to Ventilation for Eliminating TB Bacteria
By David R. Linamen, PE, CIPE
In 1994, the Centers for Disease Control and Prevention (CDC) adopted Guidelines for Preventing the Transmission of TB in Healthcare Facilities in an attempt to minimize the risk that TB might be passed from patients to healthcare workers (HCWs) and visitors. Although upper room ultraviolet germicidal irradiation (UVGI) is noted in the guidelines as having some effectiveness in reducing viable TB bacteria in healthcare facilities, the primary focus of the guidelines concerns ventilation systems. The guidelines establish 12 air changes per hour (ACH) as a minimum standard for new or remodeled rooms intended to house TB patients, while 6 ACH is established as the absolute minimum airflow rate for existing facilities.
Recent research conducted at the National Institutes of Health (NIH) by Farhad Memarzadeh, MD, with assistance from Andrew Manning, MD, indicates that UVGI, used in the proper configuration with adequate ventilation flow rate, can have a significant impact on reducing the number of viable TB bacteria in a patient room. Memarzadeh and Manning also found that the UVGI can reduce the total airflow required, resulting in a first-cost savings in the ventilation system, as well as an overall operational cost savings because of the reduced air change requirement. This study further indicates that wintertime baseboard heating in a TB isolation room, in conjunction with a properly arranged ventilation system and effective UVGI, can significantly reduce the number of viable particles in the room. The following summarizes the procedures and some of the more notable results from this research.
The research utilized computational fluid dynamics (CFD) to model the effect of airflow patterns on the distribution and removal of TB bacteria from patient rooms. As part of this research, algorithms were developed to track the bacteria through the room so the UV dosage affecting these particles could be calculated. As a result, the number of bacteria removed by the ventilation system, the number of bacteria destroyed by UV, and the number of remaining viable bacteria in the room at any time could be determined. The study evaluated the effect of several conditions including ventilation flow rate, supply temperature and external ambient condition, exhaust inlet location, baseboard heating influence in winter, pressurization of the patient room relative to surroundings, and location and intensity of UV lamps in the room. The study evaluated 40 different room configurations and three different combinations of lamp intensity and location. The study produced the following conclusions:
- The number of bacteria removed from the room by the ventilation system increases with airflow (ACH). The effect of varying airflow is much greater in winter without baseboard heating than it is in summer.
- At low to medium ventilation rates, a ventilation system utilizing high exhaust grilles removes a greater number of TB bacteria than a system utilizing low exhaust grilles, based on the particle release points modeled in the study. Location of exhaust inlets had less influence at higher airflow rates.
- UVGI is very effective in killing or inactivating TB bacteria. Even at the lowest UVGI intensity levels modeled in this study, UVGI killed more than 50% of the TB bacteria in the room after 5 minutes.
- Baseboard heating enhances the effectiveness of UVGI regardless of airflow rate. The study suggests that baseboard heating should be used during the heating season in TB isolation rooms, especially if airflow rates are low. (In the current edition of the Guidelines for Design and Construction of Hospital Healthcare Facilities published by the AIA and the US Department of Health and Human Services (HHS), gravity-type heating equipment is not permitted in infectious isolation rooms. The authorities having jurisdiction for each facility or project should address this issue.)
- Increasing the pressurization of the room with respect of the surroundings had no influence on the number of viable TB bacteria in the room.
- Use of UVGI is significantly more effective than increasing the airflow rate (ACH) in the room. Increasing airflow rate from 6 ACH to 16 ACH results in a 30% reduction in the viable TB bacteria in the room if UVGI is not utilized. In comparison, utilizing UVGI at 6 air changes per hour results in a 68% decrease in the number of viable bacteria. Both the first cost and the operating cost for utilizing UVGI are significantly less than the associated cost with increasing airflow rates.
- The study suggests that 6 ACH is the optimum ventilation rate in conjunction with UVGI and baseboard heating. In addition to the first cost and operating cost implications, this also has comfort implications for the patient because it is generally difficult to distribute conditioned air at high flow rates and avoid uncomfortable drafts. Without baseboard heating, the optimum winter airflow rate appears to be 10 ACH to 12 ACH.
- Increasing the intensity of the UV does not appear to be cost effective. For instance, doubling the UV intensity decreases the number of viable bacteria by only 20% for summer conditions or winter conditions with baseboard heating.
To date, it is not known if the CDC intends to revise the Guidelines to reflect the findings from this study. A detailed summary of the study procedure and the results are included in the Handbook on Assessing the Efficacy of Ultraviolet Germicidal Irradiation Ventilation in Removing Mycobacterium Tuberculosis, available from the NIH, Bethesda, Md. (ISBN 0-16-061398-1).
David R. Linamen, PE, CIPE, is a principal with Burt Hill Kosar Rittelmann Associates, an architectural and engineering firm with locations in Pittsburgh, Philadelphia, and Butler, Penn; Boston, Mass; Cleveland, Ohio; and Washington, DC.
1. State the requirements of OSHA's Needlestick Prevention Act.
2. Identify safety criteria to use when selecting safety devices for healthcare workers.
3. Recognize the cost/benefit issues in purchasing equipment and supplies that meet infection control standards.