Opening a window the best way of reducing TB spread in hospitals

Keith Alcorn
Published: 22 March 2007

Ventilating tuberculosis (TB) wards by opening windows is more likely to reduce the risk of infection than mechanical ventilation and should be practiced wherever the climate permits, say researchers from Imperial College, London, in the February edition of PLoS Medicine.

Exposure to TB in healthcare settings is a particular risk for HIV-positive people because of the high existing rates of HIV/TB coinfection in resource-limited settings. Transmission within healthcare facilities has come into particular focus in the past year after the finding that the majority of extremely drug-resistant (XDR) TB cases in the first South African outbreak were acquired from healthcare workers.

Experts have since emphasised the need for better infection control procedures in health care facilities, with some advocating greater investment in mechanical “negative pressure” ventilation, which keeps the air pressure in a TB ward lower so that air does not leak into other parts of the healthcare facility, but is instead expelled by a fan.

However, a study carried out at six hospitals at Lima, Peru, shows that the old-fashioned method of ventilation and design of TB wards – high ceilings and large windows that can be opened – is considerably superior to mechanical ventilation and to modern hospital layouts when it comes to preventing airborne infection.

The study compared air flow in five hospitals of old-fashioned design built before 1950, three modern hospitals and Lima’s only hospital with mechanical ventilation, installed in 2000 (follow this link for illustration of each type of facility).

The study measured the rate of air change in rooms by releasing carbon dioxide and then measuring the levels after ventilation began. Air changes per hour were then calculated, based on the rate at which carbon dioxide levels fell.

They found that opening doors and windows resulted in greater ventilation (28 air changes per hour) when compared to mechanically ventilated negative pressure rooms (12 air changes per hour). Facilities with high ceilings and large windows had greater ventilation than modern naturally ventilated rooms (40 versus 17 air changes per hour, p<0.001).

"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," said Dr Rod Escombe of Imperial College, London, who led the study.

The research group also estimated the risk of infection for people who spent 24 hours in each of these environments, based on the air change rate and an accepted formula for calculating the risk of TB infection called the Wells-Riley equation.

The risk of infection was 97% over 24 hours for patients in naturally ventilated facilities when all windows and doors were shut, 39% in a mechanically ventilated negative pressure room, 33% in a modern naturally ventilated facility and 11% in a pre-1950 naturally ventilated facility.

However the investigators caution that the risk of infection increases, even with good ventilation, with the infectiousness of patients. This has particular relevance, they say, for clinical areas that are not designated as high risk, such as waiting areas and non-TB wards, since these are the places that undiagnosed TB patients, who are likely to be most infectious, will predominantly be found. (The infectiousness of a TB patient falls markedly within weeks of starting treatment as the bacterial burden in the lungs falls in response to antibiotic treatment).

If the air change rate in such non-high risk areas is calculated at six air changes per hour with mechanical ventilation (rare in most settings) the TB infection rate would be around 70%, suggesting that the risk is even higher in poorly ventilated waiting rooms and other clinic areas.

The findings also have relevance for the new wave of hospital building that is taking place as a result of PEPFAR funding and other investments in health care infrastructure.

“Probably the main reason that modern building trends increase patient risk is financial: smaller rooms (which more easily become stuffy and overcrowded) are cheaper to build and heat,” the authors remark.

They found that a one metre increase in the ceiling height offered substantially greater protection against airborne infection because it increased absolute ventilation of the room by a quarter, even when using mechanical ventilation.

Addressing concerns that open doors and windows might lead to leakage of airborne TB into other areas of health care facilities, the authors note that it should be possible to locate a TB ward on the top floor of a building and downwind of other rooms.

Historical note

Although the authors say that their findings are not suited to cold regions, natural ventilation was a standard feature of TB sanatoria in Europe and North America, and public health prevention measures in Europe in the 1920s and 1930s commonly urged people always to sleep with the window open. Indeed, some sanatoria accommodated patients not in wards but in huts in the grounds, so beneficial was fresh air considered to be, while in the United States tent sanatoria sprang up in the California desert.

Reference

Escombe AR et al. Natural ventilation for the prevention of airborne contagion. PLoS Medicine 4 (2): e68, 2007. doi:10.1371/journal.pmed.0040068

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