Institutional tuberculosis (TB) transmission is an important public health problem highlighted by the HIV/AIDS pandemic and the emergence of multidrug- and extensively drug-resistant TB. Effective TB infection control measures are urgently needed. We evaluated the efficacy of upper-room ultraviolet (UV) lights and negative air ionization for preventing airborne TB transmission using a guinea pig air-sampling model to measure the TB infectiousness of ward air.
Methods and Findings
For 535 consecutive days, exhaust air from an HIV-TB ward in Lima, Perú, was passed through three guinea pig air-sampling enclosures each housing approximately 150 guinea pigs, using a 2-d cycle. On UV-off days, ward air passed in parallel through a control animal enclosure and a similar enclosure containing negative ionizers. On UV-on days, UV lights and mixing fans were turned on in the ward, and a third animal enclosure alone received ward air. TB infection in guinea pigs was defined by monthly tuberculin skin tests. All guinea pigs underwent autopsy to test for TB disease, defined by characteristic autopsy changes or by the culture of Mycobacterium tuberculosis from organs. 35% (106/304) of guinea pigs in the control group developed TB infection, and this was reduced to 14% (43/303) by ionizers, and to 9.5% (29/307) by UV lights (both p < 0.0001 compared with the control group). TB disease was confirmed in 8.6% (26/304) of control group animals, and this was reduced to 4.3% (13/303) by ionizers, and to 3.6% (11/307) by UV lights (both p < 0.03 compared with the control group). Time-to-event analysis demonstrated that TB infection was prevented by ionizers (log-rank 27; p < 0.0001) and by UV lights (log-rank 46; p < 0.0001). Time-to-event analysis also demonstrated that TB disease was prevented by ionizers (log-rank 3.7; p = 0.055) and by UV lights (log-rank 5.4; p = 0.02). An alternative analysis using an airborne infection model demonstrated that ionizers prevented 60% of TB infection and 51% of TB disease, and that UV lights prevented 70% of TB infection and 54% of TB disease. In all analysis strategies, UV lights tended to be more protective than ionizers.
Upper-room UV lights and negative air ionization each prevented most airborne TB transmission detectable by guinea pig air sampling. Provided there is adequate mixing of room air, upper-room UV light is an effective, low-cost intervention for use in TB infection control in high-risk clinical settings.
Using a guinea-pig detection model, Rod Escombe and colleagues find that upper-room UV lamps in hospital rooms can substantially reduce airborne transmission ofMycobacterium tuberculosis.
Tuberculosis—a contagious infection, usually of the lungs—kills nearly 2 million people annually. It is caused by Mycobacterium tuberculosis, bacteria that are spread in airborne droplets when people with tuberculosis cough or sneeze. Most people infected with M. tuberculosis do not become ill—their immune system contains the infection. However, the bacteria remain dormant within the body and can cause disease years later if immunity declines because of, for example, infection with human immunodeficiency virus (HIV), the cause of acquired immunodeficiency syndrome (AIDS). The symptoms of tuberculosis include a persistent cough, weight loss, and night sweats. Infection with M. tuberculosis is diagnosed using the tuberculin skin test. Tests for tuberculosis itself include chest X-rays and sputum cultures (in which bacteriologists try to grow M. tuberculosis from mucus brought up from the lungs by coughing). Tuberculosis can usually be cured by taking several powerful antibiotics daily for several months. Drug-resistant tuberculosis is much harder to cure, requiring multiple second-line antibiotics for up to two years or more. Tuberculosis transmission can be reduced by, for example, hospitalizing people with tuberculosis in isolation wards in which negative-pressure mechanical ventilation is used to reduce the concentration of infectious airborne droplets.
Why Was This Study Done?
After the development of antibiotics capable of killing M. tuberculosis in the mid 20th century, it seemed that tuberculosis would become a disease of the past. But in the mid 1980s, drug-resistant M. tuberculosis strains began to emerge, the HIV/AIDS epidemic took hold, and tuberculosis resurged to today's worrying levels. New ways of reducing tuberculosis transmission, particularly in health care settings and in resource-limited settings, are now urgently needed. The need for effective infection control measures is especially urgent in HIV care programs where highly susceptible individuals frequently mix with people with tuberculosis. In this study, the researchers use a guinea pig air-sampling model (which was first used in the 1950s to show that tuberculosis is an airborne infection) to investigate whether upper-room ultraviolet (UV) lights in patient rooms and negative air ionization can prevent airborne tuberculosis transmission. UV light kills M. tuberculosis; negative ionization gives airborne particles a charge that makes them stick to surfaces.
What Did the Researchers Do and Find?
The researchers exposed a group of control guinea pigs kept in a special air-sampling enclosure to untreated air from an HIV–TB ward in Lima (Perú). Another group of animals (the UV group) breathed air from the same ward, but only on the days that UV lights suspended near the ward's ceiling were turned on, together with mixing fans to mix up the room air. The “ionizer group” had a negative ionizer switched on in their enclosure when they were exposed to ward air (each group of animals was exposed to ward air every other day). The animals were tested monthly with the tuberculin skin test and all were examined for tuberculosis disease when they became infected with tuberculosis or at the end of the 535-day experiment. 35% of the control animals, 14% of the ionizer group animals, and 9.5% of the UV group animals developed M. tuberculosis infections. Tuberculosis disease was found in 8.6% of the control animals but in only 4.3% and 3.6% of the animals in the ionizer and UV groups, respectively. A “time-to-event analysis” also showed that UV lights and ionizers reduced tuberculosis infection and disease. Finally, an analysis of the data using an airborne infection model indicated that ionizers and UV lights prevented 60% and 70% of tuberculosis infections, respectively.
What Do These Findings Mean?
These findings indicate that upper-room UV lights, combined with adequate air mixing, or negative air ionization with special large-scale ionizers can prevent most airborne tuberculosis transmission to guinea pigs exposed to hospital room air. The effectiveness of these approaches in reducing tuberculosis transmission between people is likely to be similar, although remains to be tested. Nevertheless, this first study of the effect of upper-air UV light and of negative air ionization on airborne transmission in a clinical setting suggests that both approaches could be potentially important tuberculosis infection control measures. Furthermore, the UV light approach might provide a relatively low-cost intervention for possible use in waiting rooms and other overcrowded settings where patients with undiagnosed, untreated tuberculosis—individuals who tend to be highly infectious—are likely to come into contact with other susceptible patients, health care workers, and visitors.
Please access these Web sites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.1000043.
The US National Institute of Allergy and Infectious Diseases provides information on all aspects of tuberculosis, including multidrug-resistance tuberculosis, and on tuberculosis and HIV
The US Centers for Disease Control and Prevention provide several fact sheets and other information resources about all aspects of tuberculosis, including Guidelines for preventing the Transmission of Mycobacterium tuberculosis in Health-Care Settings, 2005 (some information in Spanish is also available)
The World Health Organization's 2008 report “Global Tuberculosis Control—Surveillance, Planning, Financing” provides a snapshot of the current state of the global tuberculosis epidemic and links to information about all aspects of tuberculosis and its control (in several languages)
Tuberculosis Infection-Control in the Era of Expanding HIV Care and Treatment is another report from the World Health Organization
HIVInsite provides detailed information about the combination of HIV infection and tuberculosis
Avert, an international AIDS charity, also provides information about the interaction between HIV and tuberculosis
GHD (Global Health Delivery) Online is an online resource dedicated to TB infection control, and is moderated by world experts