Tuberculosis (TB) is the leading cause of death in the world from a single infectious disease. After a century of decline in the United States, incidences of tuberculosis are increasing, and multiple drug-resistant strains have emerged. This increase is attributable to changes in the social structure in cities, the HIV epidemic, and a failure of some cities to improve public treatment programs. Mycobacterium tuberculosis is the etiologic agent of tuberculosis (TB) in humans. Humans are a reservoir for the bacterium and can harbor the bacteria for many years.
Mycobacterium tuberculosis is a fairly large rod-shaped bacterium. The rods are 2-4 microns in length and 0.2-0.5 microns in width. TB is spread from person to person through the air. When a person with infectious TB coughs or sneezes, tiny particles containing Mycobacterium tuberculosis are expelled into the air. These particles, called droplet nuclei, are about 1 to 5 microns in diameter. Droplet nuclei can remain suspended in the air for several hours, depending on the environment. The most effective droplet nuclei tend to have a diameter of 5 micron.
Droplet nuclei are generated during talking, coughing and sneezing. One cough can generate 3000 droplet nuclei. Talking for 5 minutes can generate 3000 droplet nuclei and singing can generate 3000 droplet nuclei in one minute. Sneezing generates the most droplet nuclei by far (tens of thousands), which can spread to individuals up to 10 feet away. Direct sunlight quickly kills tubercle bacilli, but they can survive in the dark for several hours. The probability that TB will be transmitted depends on three factors: the infectiousness of the person with TB, the environment in which exposure occurred, and the duration of exposure.
The air filter efficiencies prescribed by the United States Department of Health & Human Services, as well as the recommendations published by the American Society of Heating, Refrigerating and Air Conditioning Engineers are sufficient to protect most areas of a medical facility (MERV 7 prefilter and MERV 14 final filter). In areas that directly serve TB patients HEPA filtration is required to protect medical facility employees and patient visitors.
Persons at the highest risk of becoming infected with TB are close contacts; persons who often spend time with someone who has infectious TB. These contacts include family members, roommates, friends, coworkers, or others. These persons are at risk for TB infection because they are more likely to be exposed to TB. Predisposing factors for TB infection include: Close contact with large populations of people (hospitals, schools, nursing homes, dormitories, prisons, etc.) IV drug use HIV infection is the #1 predisposing factor for TB infection. Ten percent of all HIV-positive individuals harbor TB. This is 400-times the rate associated with the general public Only 3-4% of infected individuals will develop active disease upon initial infection, 5-10% within one year. These percentages are much higher if the individual is HIV positive. TB infection progresses to disease when tubercle bacilli overcome the defenses of the immune system and begin to multiply. Infection can progress to disease very quickly or many years after the actual infection. In the United States, of approximately 5% of the people who have been recently infected with TB, the disease will develop in the first year or two after infection. In another 5%, the disease will develop later in their lives. In other words, approximately 10% of persons infected with TB will develop active TB at some point. The remaining 90% will stay infected, but free of disease, for the rest of their lives.
Engineering controls are based primarily on the use of adequate ventilation systems; and may be supplemented with high-efficiency particulate air (HEPA) filtration and ultraviolet germicidal irradiation (UVGI) in high-risk areas. These strategies are designed to reduce the concentration of infectious droplet nuclei in the air, to prevent the dissemination of droplet nuclei throughout the facility, and to render droplet nuclei noninfectious by killing the tubercle bacilli they contain. In infectious patient isolation rooms, special ventilation system controls are necessary to maintain negative pressure within the room and to exhaust the air properly. Isolation rooms should be monitored daily when in use to ensure the negative pressure maintained. Isolation room doors should be kept closed, except when patients or personnel must enter or exit the room, in order to maintain negative pressure. Ventilation systems can also be designed to minimize the spread of TB to other areas of the health care facility.
HEPA filters can be used in ventilation systems to remove droplet nuclei from the air. These filters MUST be installed in ventilation ducts to filter air for recirculation into the same room or recirculation to other areas of a facility. The effectiveness of portable HEPA filtration units has not been adequately evaluated. All HEPA filters must be carefully installed and meticulously maintained to ensure adequate function.
The United States Centers for Disease Control publish a document titled Guidelines for Preventing the Transmission of Mycobacterium tuberculosis in Health Care Facilities. The document includes recommendations for the use of engineering controls to prevent spread of the bacteria. Specifics include direct source capture using local exhaust ventilation, controlling airflow direction to prevent cross-contamination, dilution and removal of contaminated air via general ventilation and air cleaning through air filtration.
Some specifics include following DHHS recommendations for air changes and air filtration, supplying at least 12 air changes per hour to the infected patients room, using HEPA filters in an recirculation system (either within the room or returning air to other conditioned spaces) and ensuring that contaminated air is not exhausted in areas of human habitation.
For a full PDF copy of Guidelines for Preventing the Transmission of Mycobacterium tuberculosis in Health Care Facilities, click here.