Opening windows effective way of preventing transmission of TB

The spread of airborne infections in institutional settings such as hospitals, waiting rooms, prisons and homeless shelters is an important public health problem. This is especially true in resource poor settings, where the prevalence of TB is highest, and where preventive measures such as negative-pressure isolation rooms are hardest to implement.

A study published today in the freely available journal PLoS Medicine and carried out by researchers from the Department of Infectious Diseases and Immunity and the Wellcome Trust Centre for Clinical Tropical Medicine, Imperial College London, is the first to investigate the role of natural ventilation by opening windows and doors for preventing the transmission of airborne infection. This simple and low-cost measure is recommended by the WHO for preventing TB transmission in healthcare settings, but until now there have been no studies to support these guidelines.

The researchers, led by Dr. Rod Escombe from Imperial College London, studied eight hospitals in Lima, Peru, and measured natural ventilation in 70 different rooms where infectious patients are likely to be found, including respiratory isolation rooms, TB wards, respiratory wards, general medical wards, outpatient consulting rooms, waiting rooms, and emergency departments. They compared these with 12 modern mechanically-ventilated, negative-pressure respiratory isolation rooms.

The study found that natural ventilation when windows and doors were opened was more than double that of mechanically-ventilated, negative-pressure rooms functioning at the high rates recommended by guidelines, and 18 times that of rooms with windows and doors closed. Even at the lowest wind speeds, natural ventilation exceeded mechanical ventilation.

“We were surprised by how effective simply opening windows and doors was at generating high rates of ventilation, which would theoretically reduce the risk of transmission,” says Dr Escombe. “Using an airborne infection model to predict the effect of natural ventilation on TB transmission, we estimated that in mechanically-ventilated rooms, 39% of susceptible individuals would become infected following 24 hours of exposure to untreated TB patients. This compared with 33% in modern and 11% in pre-1950 naturally ventilated facilities with windows and doors open.”

Facilities built more than 50 years ago, characterized by large windows and high ceilings, had greater ventilation than modern naturally ventilated rooms. Some of these older facilities had been built especially for TB patients, along the design principles of TB sanatoria when fresh air was part of the treatment for TB in the pre-antibiotic era.

“We concluded that opening windows and doors provides high rates of ventilation and may therefore provide the most effective protection against airborne infection in low resource settings,” says Dr Escombe. “Old-fashioned clinical areas with high ceilings and large windows provided the greatest protection. Mechanically ventilated negative-pressure isolation rooms are very expensive to install and maintain, and are limited to certain high risk areas such as TB isolation rooms. In contrast, natural ventilation is a low-cost, low-tech intervention that can be applied in many different areas, including waiting rooms, out-patient clinics and emergency departments, where many infectious patients are found. Whilst not suited to cold climates, natural ventilation offers an important infection control measure in the tropics, where the greatest burden of TB is found.”