A study of eight hospitals in Peru has shown that opening windows and doors provided ventilation more than double that of mechanically ventilated negative-pressure rooms and 18 times that of rooms with windows and doors closed.

The researchers, led by Rod Escombe from Imperial College London, compared the airflow in 70 naturally ventilated clinical rooms such as respiratory isolation rooms, TB wards, respiratory wards, general medical wards, outpatient consulting rooms, waiting rooms, and emergency departments with 12 mechanically ventilated negative-pressure respiratory isolation rooms built after 2000.

Even at the lowest of wind speeds, natural ventilation exceeded mechanical ventilation. Facilities built more than 50 years ago, characterized by large windows and high ceilings, had greater ventilation than modern naturally ventilated rooms. The authors went on to calculate what these results might mean for transmission of infection and estimated that in mechanically ventilated rooms 39% of susceptible individuals would become infected following 24 h of exposure to untreated TB patients compared with 33% in modern and 11% in pre-1950 naturally ventilated facilities with windows and doors open.

The authors conclude that opening windows and doors maximizes natural ventilation and that the risk of airborne contagion is lower than with mechanical ventilation systems. Old-fashioned clinical areas with high ceilings and large windows provided the greatest protection.

In a related perspective article Peter Wilson, from University College London, concludes that although natural ventilation is not an easy solution for patients in countries where winters are cold the current practice of sealing in the local environment is probably the wrong route for hospital wards.