Photo: Jordan Opel/Unsplash.
Both COVID-19 and tuberculosis spread through the air. A person who sneezes, coughs or simply talks in an enclosed space without masking themselves puts others in the room at risk of sharing the air that they have exhaled. This air contains microscopic droplets that contain the disease-causing pathogen.
So the measures we put in place and habits we adopt to deal with COVID-19 could in fact help our response to measles, influenza and tuberculosis as well.
Good ventilation has always been considered a virtue for buildings, as has copious sunlight indoors. However, builders don’t always or by default consider improving access to sunlight and ventilation in their designs. Most office buildings today are centrally air-conditioned with large unopenable glass panes – thanks to the focus on providing islands of thermally comfortable built-spaces. There is no inlet for outside air; the central AC system has a provision for outside air to enter the space but due to efficiency concerns, this supply is limited.
To repeatedly cool outside air before supplying it to the inside requires more energy, so a more economical way is to recirculate the same air over and over again. This saves energy, for sure, but at the risk of exposing occupants to airborne infections.
This problem is worst in rooms that use split ACs. These machines have narrow copper pipes to transport the refrigerant between the indoor and outdoor units. The indoor unit supplies cool air and keeps the occupants comfortable. But this air is also constantly recirculated. There is no provision to transport fresh air from the outside to within the room.
Our general lack of awareness also means we close the windows and the doors to seal in the cool air and reduce power consumption. The better case scenario is one in which the room has some leaks – like cracks under the doors and around window frames – through which air from the outside can enter the enclosure.
Our preference for ventilated spaces is a preference for dilution ventilation. That is, the concentration of pathogen-laden aerosolised droplets in the air will drop if we allow outside air to ‘dilute’ the air within the room and decrease the probability of infections.
In 1957, researchers established this empirical relationship between infection probability and air diffusion with guinea pigs supplied air drawn from a tuberculosis ward. This work led to the Wells-Riley equation, which researchers have tweaked and improved over the years and have used it to develop subsidiary models, like the Rudnick-Milton model. And these models together imply that a building must have appropriate opportunities for air from the outside to diffuse inside and have large volumes of space in which the air can mix.
Comfort in buildings shouldn’t, rather doesn’t, have to come in exchange for a heightened risk of spreading airborne pathogens. COVID-19 has put indoor ventilation in the spotlight, among other things, and architects, urban planners and healthcare workers thus have an opportunity to use this intervention to help fight the spread of tuberculosis, measles, influenza and other airborne diseases.
Raja Singh is a PhD research scholar and Anil Dewan is professor – both at the department of architecture, School of Planning and Architecture, New Delhi.