It turns out that helping prevent the spread of tuberculosis (TB) may be as simple as opening a window. Researchers in Peru have found that natural ventilation can move more than twice as much air through hospital wards than expensive, hard-to-maintain fans can. In principle, such ventilation dilutes the concentration of TB (not to mention of other airborne diseases) in the air, reducing the risk of infection for hospital workers and other patients.

The World Health Organization recommends natural ventilation as one way to limit TB transmission in impoverished areas, but until now there was no evidence to support the idea or guidelines on how to implement it, says infection control specialist Rod Escombe of Imperial College London. Escombe and his colleagues examined the airflow in 70 naturally ventilated rooms from eight hospitals in Lima, Peru, built before 1990.

With windows and doors open, the air inside the rooms was, on average, completely changed out 28 times an hour. Fresh and stale air were also exchanged in 12 more modern hospital rooms ventilated with powerful fans, but the air changed at only 12 times an hour, which is the recommended frequency. Five of the older Lima hospitals, built before 1950 with high ceilings and large windows, had 40 air changes per hour. "Simply opening the windows can give you phenomenal air exchange," Escombe says.

Using a mathematical model of airborne infection, the researchers report in PLoS Medicine that the risk of TB infection should fall from 39 percent per day in a mechanically ventilated room to 33 percent in modern, naturally ventilated rooms, and to 11 percent in the older style rooms.

Escombe notes that mechanical ventilators can be prohibitively expensive for poorer countries, costing tens to hundreds of thousands of dollars and requiring frequent maintenance to keep air from becoming stagnant. "For low-resource settings, which is most of the world, mechanical ventilation really isn't an option," he says. More than three quarters of TB cases, he notes, occur in the tropical regions of South America, Africa and Asia.

Escombe says the riskiest areas of hospitals are waiting rooms and other places where people congregate before being diagnosed. In a prior study, he and co-workers found that 30 percent of the emergency department staff in a Lima hospital became infected with TB during a year.

His recommendations: put waiting rooms outside; install skylights and bigger windows; and place TB wards on top floors downwind of other wards. Escombe says he is currently retrofitting 18 hospitals around Peru, and plans to work on prisons and homeless shelters, too.