Should natural ventilation be a part of preventive efforts to control TB?
Tuberculosis (TB) is known to be an airborne transmitted infection. It spreads by airborne droplets, which means it spreads one person to another through the air between them. People acquire the infection by inhaling these droplets containing the TB bacillus (Mycobacterium tuberculosis).
Re-circulated indoor air has long been recognized as a mode of transmission of infectious disease. And the vehicle for release of airborne infectious particles is the exhaled air (PLoS One. 2014 May 7;9(5):e96334).
The risk of disease transmission is particularly high in overcrowded conditions such as schools, home shelters, prisons, waiting rooms and health care facilities. The bacteria survive for long especially in poorly ventilated settings, both in household and healthcare.
Good ventilation, natural or mechanical, can reduce the risk of transmission.
The effect of natural ventilation for the prevention of airborne contagion was examined in a study reported in the journal PLoS Medicine.
The study was conducted in eight hospitals in Lima, Peru – five older and three newer hospitals. It was found that natural ventilation provided 28 air changes per hour vs 12 in negative pressure rooms. The high ceilings and large windows of old buildings allowed 40 air changes per hour. Rooms with open windows showed 6-fold greater absolute ventilation than that calculated for mechanically ventilated rooms. The model of infection risk suggested a 39% infection rate in 24 hours of exposure to open TB in negative pressure rooms and 97% in closed unventilated rooms (13 infectious quanta per hour). On the other hand, the infection rate with natural ventilation was 33% in newer buildings and 11% in older buildings.
Based on these findings, the study concluded that the risk of airborne contagion is much lower with natural ventilation compared to the much more expensive mechanical ventilation systems. Opening windows and doors maximizes natural ventilation.
Compared to other methods, airborne infection control has not garnered much attention as a component of TB control.
Natural ventilation requires no maintenance, which makes it particularly suitable for low-resource settings like India, which continues to have the highest global burden of TB.
On the other hand, installation and maintenance of mechanical ventilation systems is expensive and is not a feasible option for all.
However, natural ventilation is also not a simple and straightforward solution as windows of households are mostly kept closed, even during the day, especially in cold areas and because of overcrowding in a closed space, the risk of transmission of infection among contacts is increased. Also, natural ventilation is not appropriate for all hospital settings.
But, there is still a need for creating awareness about the importance of free air circulation, especially by opening windows and allowing natural ventilation in households.