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When employee health professionals work to reduce the occupational risk of tuberculosis, respiratory protection is a major part of their efforts. But as a health hazard evaluation shows, improving ventilation is another important consideration.
In 2010, managers at a Texas hospital became concerned when 35 employees on a single medical-surgical unit tested positive for exposure to Mycobacterium tuberculosis. Local health department officers discovered that a patient with undiagnosed active TB had been treated on the unit for a month and was not placed in isolation.
Still, the hospital wanted to check its ventilation system. The Centers for Disease Control and Prevention recommends any airborne infection isolation rooms constructed or renovated before 2001 to have at least six air changes per hour. For optimum air flow, the recommended level for newer construction is 12 air changes per hour, the agency says. Newer anterooms should have 10 air changes per hour.1
Two industrial hygienists from the National Institute for Occupational Safety and Health (NIOSH) investigated the ventilation. They found that all the airborne infection isolation rooms had the appropriate negative pressure, except that the flow in one room was affected by a television located below the exhaust air return. (The hospital immediately moved the television.)
Two of the seven measured rooms had fewer than six air changes per hour, and three had more than 12 air changes per hour. None of the ante rooms were at the recommended level.
Nurses or maintenance workers should check airborne isolation rooms every day while there is a patient in them, says Todd Niemeier, MS, CIH, an industrial hygienist with NIOSH in Cincinnati. That can be done simply by gently squeezing a smoke tube outside the room, two inches from the bottom of the closed door, and watching the trail. With negative pressure, the smoke should flow from outside the room under the door. Or a thin strip of tissue can be held parallel to the door to determine the direction of the flow of air.2
When isolation rooms were full, the hospital used a portable air cleaner with a HEPA filter and ultraviolet germicidal irradiation (UVGI). However, Niemeier cautioned that the air cleaner would not create negative pressure unless it exhausted air outdoors.
"If your return air is not exhausted directly outside, you could still be spreading infectious aerosols to other spaces in the hospital," he says. Niemeier suggests consulting an engineer or industrial hygienist if a portable air cleaner is to be used to create an airborne infection isolation room.
In the health hazard evaluation report, he states, "Though these recirculating units have been demonstrated to be effective in removing bioaerosols from room air, their effectiveness can vary, and they are considered supplemental ventilation units. They should not be relied on in place of maintaining a sufficient number of AII rooms that meet the CDC guidelines."
"The best solution is to have the appropriate number of airborne isolation infection rooms to meet the needs of your patient population," he says.
Meanwhile, Niemeier stresses that proper ventilation is just one preventive measure to prevent the spread of tuberculosis.
"It's important to understand that even meeting this guidance doesn't preclude the fact that if a nurse walks into the room, they still need to wear respiratory protection," he says. "They're not protected from exposure to TB if they don't have respiratory protection."
Employees who could be exposed to infectious aerosols should be fit-tested annually, if they use N95s, and should receive training in the proper use of the respirator, he says. "That's your last line of defense, wearing that respirator correctly," he says.
[Editor's note: The Health Hazard Evaluation on "Evaluation of Ventilation Controls for
Tuberculosis Prevention at a Hospital" is available at www.cdc.gov/niosh/hhe/reports/pdfs/2010-0092-3188.pdf.]