Key Point

Natural, rather than mechanical, ventilation in hospitals may provide better protection against the airborne spread of TB, and naturally ventilated rooms with high ceilings and large windows on opposite walls may offer the best protection.

LIMA, PERU—The spread of tuberculosis (TB) in hospital rooms may be minimized most effectively by the simplest of strategies: opening windows and doors. A. Roderick Escombe, MD, PhD, and colleagues found that the risk of airborne TB contamination was significantly lower in naturally ventilated rooms than in rooms with mechanical ventilation, and rooms with high ceilings and large windows on opposite walls provided the best protection against TB infection.

Dr. Escombe, Honorary Research Fellow in the Division of Investigative Science, Imperial College London, and his team conducted their study in eight hospitals in Lima, Peru. Five of these hospitals were built before 1950, while the remaining three were built between 1970 and 1990. The older hospitals had rooms with higher ceilings and larger windows than those in the newer buildings; the rooms in the older facilities were also more likely to have windows on opposite walls. From this group of hospitals, 70 naturally ventilated clinical rooms—including respiratory isolation rooms, TB wards, respiratory wards, general medical wards, outpatient consulting rooms, waiting rooms, and emergency departments—were compared with 12 mechanically ventilated negative-pressure respiratory isolation rooms that were built after 2000. Ventilation was measured in air changes per hour (ACH), calculated by dividing absolute room ventilation (m3/h) by room volume (m3). Guidelines recommend six to 12 ACH for the control of TB transmission in high-risk health settings.

Open windows and doors provided median ventilation of 28 ACH, more than twice the 12 ACH provided in the mechanically ventilated negative-pressure rooms. Older naturally ventilated rooms had a median of 40 ACH, compared with 17 ACH in newer naturally ventilated rooms. In addition, open windows and doors provided 18 times more ACH than did naturally ventilated rooms with all windows and doors closed; these closed-up rooms had the highest risk of infection spread. Using the Wells-Riley equation, a standard model of airborne infection, the researchers predicted that 39% of susceptible persons in mechanically ventilated rooms would become infected after one day of exposure to untreated TB patients, while in modern naturally ventilated rooms and older naturally ventilated rooms, 33% and 11% of such persons, respectively, would become infected with TB.

POTENTIAL DRAWBACKS

Dr. Escombe and colleagues pointed out that the frequent inability of respiratory isolation rooms to deliver the recommended number of ACH, to maintain negative pressure, or to prevent positive pressure has been blamed for many TB outbreaks. The installation and maintenance expenses that are necessary for mechanically ventilated hospital rooms are additional drawbacks not associated with natural ventilation, which costs little and requires no maintenance.

“The enormous dilution resulting from release of contaminated air into the outside atmosphere prevents natural ventilation from contaminating the immediate environment significantly,” the investigators pointed out. “Consequently, opening the windows releases the same number of infectious particles into the atmosphere as mechanical ventilation without causing significant risk to those outside, but does so with greater protection for people inside the rooms.”

However, without negative pressure, it is difficult to control the direction of airflow within naturally ventilated rooms; this raises the risk of contamination of corridors and adjacent rooms, especially on days with no wind. To counteract this elevated risk of infection, Dr. Escombe’s team suggested locating TB wards on the top floor of the building, downwind of other rooms and the nursing station. They also recommended that corridors be open at both ends, so that large amounts of fresh air can pass through the hospital rooms.

It may not be feasible to provide natural ventilation in cold environments, but “in temperate or tropical climates with a high prevalence of TB, it may be safer for patients, visitors, and staff to wear extra clothing in open-windowed, naturally ventilated wards and waiting rooms than to be warm in stuffy, low-ceilinged rooms with increased risk of nosocomial airborne disease transmission,” the researchers stated. Furthermore, the study’s findings indicated that although cultural traditions or security concerns may necessitate closed windows at night, using natural ventilation with partially open windows can still result in protective rates of ACH.

“These findings throw the old practice of situating TB wards at the top of a building and open to the elements in a new light,” Peter Wilson, MD, FRCP, a consultant microbiologist at the Windeyer Institute of Medical Sciences, University College London Hospitals, pointed out in an accompanying editorial. “The current practice of sealing in the local environment is probably the wrong route for hospital wards.” However, Dr. Wilson also noted that natural ventilation “is not an easy solution” for hospitals located in countries with cold winters, adding that “cheap and reliable ventilation for rooms that promote[s] rather than prevent[s] passage of air yet allow[s] thermal control” is needed.            

—John Merriman

Suggested Reading
Escombe AR, Oeser CC, Gilman RH, et al. Natural ventilation for the prevention of airborne contagion. PLoS Med. 2007;4:e68.
Wilson P. Is natural ventilation a useful tool to prevent the airborne spread of TB? PLoS Med. 2007;4:e77.

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