Device traps, disables harmful bacteria

June 8, 2005 -- A team of engineers from Washington University in St. Louis and the University of Colorado at Boulder has removed bioaerosols — airborne biological particulate matter — from the air of a hospital therapy pool using a new generation of hybrid filters.

The bioaerosols identified in the unnamed Midwestern hospital pool had sickened nine lifeguards who had become ill with hypersensitivity pneumonitis, a lung condition that mimics pneumonia symptoms. This forced the pool to shut down. It is now reopened.

Photo by David Kilper

Lars Angenent guides graduate student Rebecca Hoffman as she examines bacteria from air digesters under a high-powered epifluorescence microscope.

Lars T. Angenent, Ph.D., Washington University assistant professor of chemical engineering, and the Colorado engineers mounted three high-efficiency particulate air (HEPA) ultraviolet (UV) air filters on the ceiling of the pool room. They compared concentrations of total bacteria, culturable bacteria and airborne endotoxin — a poison present in gram-negative bacterial cell walls that can cause severe inflammatory responses — with and without the air filters operating under similar conditions. They compared the performance of the filters twice, one year apart, and found that the filters reduced concentrations of culturable bacteria by 69 and 80 percent during monitoring periods executed in respective years. The filters reduced concentrations of total bacteria by 12 and 76 percent, respectively, over the same span. But the filters did not affect airborne endotoxin concentrations.

"This specific filter has a blower that takes a high volume of air and puts it through a filter that screens bacteria and even smaller particles," said Angenent, a member of Washington University's Environmental Engineering Science Program. " And after the filter there is UV light; if something passes through the filter, the UV zaps it. It's a combination approach that appears very effective. The bacterium that had been causing illness was Mycobacterium avium, which can make immunocompromised people ill, and since a lot of elderly use therapy pools, that's a concern. However, since using the filters, no one has gotten sick."

The researchers also tested the hybrid filters in an environmentally controlled laboratory chamber to come up with the air-exchange rate for the therapy pool assays, among other parameters.

The researchers published their results in the Feb. 2005 issue of the Journal of Air & Waste Management Association. The study was supported by funds and equipment provided by Honeywell, Inc., UlraViolet Devices, Inc., and the National Science Foundation.

Angenent's collaborators are: Elmira Kujundicz, David A. Zander, and Mark Hernandez, of the University of Colorado Department of Civil, Environmental, and Architectural Engineering; and David E. Henderson and Shelly L. Miller, of the Department of Mechanical Engineering, University of Colorado.

Bioaerosols pose a threat to public health through infectious and toxic diseases. Today there are increased settings where people gather and can be affected by such bioaerosols. Among these "high-exposure environments" are correctional facilities, homeless shelters, healthcare facilities, and public transit systems. Additionally, the advent of hot tubs, hospital therapy pools and other warm-water leisure and therapy pools, many bioaerosols researchers believe, creates harbors that enhance the aerosolization of microorganisms, including strains of Legionella and Mycobacterium, that can cause diseases such as "lifeguard lung."

"The results of this study suggest that a reasonable reduction in bioaerosol concentrations can be achieved by installing this new generation of hybrid air filters," the authors conclude. "Engineering control methods must be balanced with constraints such as occupant comfort, economic factors, and building management strategies to ensure that the health risks associated with bioaerosol exposure are as low as practical."