Dry, hot waiting room? Plug in that UV fixture
Study confirms what had been suspected
New studies on ultraviolet germicidal radiation confirms that UV disinfection doesn’t work as well at killing M. tuberculosis under conditions of high humidity. The research, as yet unpublished, was carried out by Gwangpyo Ko, ScD, a research associate at Harvard University’s School of Public Health.
Ko also found UV’s kill-power increases as room temperature rises, which is an interesting finding but may not have many practical consequences, he adds, because comfort will probably continue to dictate the thermostat setting in most facilities.
Ko and his colleagues also refined an existing computer model aimed at establishing whether it’s cost-effective to install UV disinfection. So far, the retooled model shows that installation is more cost-effective in places where TB cases are more likely.
When they began their work, the researchers already knew that under conditions of high humidity, UV didn’t work as well at zapping Serratia marcescens, that familiar pink-colored denizen of shower stalls. "People had speculated the same might be true for TB, but we confirmed it," says Ko. To do so, he constructed a chamber in which he could control humidity, temperature, and air flow. Then he took BCG and exposed it to varying levels of humidity and temperature.
The reason for the relationship is simple, Ko discovered. Apparently, as humidity rises, aerosol particle size increases, owing to water exchange between the aerosol and the surrounding air. Because UV has limited powers of penetration, the larger-size particles are better able to resist penetration. As for temperature’s relationship to UV’s kill-power, the likely explanation has to do with the fact that the radiation kills microorganisms by causing mutations to their DNA. Because microorganisms are more metabolically active at higher temperatures, rising temperatures probably make the bugs more susceptible to destruction.
As part of the same research, Ko set out to customize an existing computer model with the goal of producting software that hospitals can use to predict whether installing UV fixtures will be worth the cost.
"We used a computer simulation to hypothesize in a hospital waiting room what the risk for exposure would be without UV," says Ko. "Then we went to hospital waiting rooms in the Boston area and tried to simulate their conditions as closely as possible in the computer simulation."
He factored in air mixing, numbers of people, and other conditions, and then calculated for cost-effectiveness. The model showed what one would have expected: Installing UV made good economic sense at an annual rate of 50 TB cases per year, but not at five. "We hope eventually to develop the software so it can be used by any hospital" to make the same kind of calculation, Ko adds.
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