24th March is World TB Day (1)

Airborne infections: Some common terminologies

Tuberculosis (TB) is known to be an airborne nuclei transmitted infection. It spreads by airborne nuclei, which means it spreads one person to another through the air between them. People acquire the infection by inhaling these nuclei containing the TB bacillus (Mycobacterium tuberculosis).

Open cases are considered contagious and just being within close proximity of an infected person who had just coughed, sneezed or spat (even if only while speaking) may expose a person to the risk of acquiring the infection.

Let’s look at some basic definitions and facts in relation to airborne infections.

When you cough or sneeze, you tend to expel out respiratory waste through the nose and mouth, called droplets or droplet nuclei.

·         Droplets: These are more than 5 microns in size; droplets remain suspended in the air only for a limited period and settle down, a distance of 3-6 feet is required to be maintained to prevent exposure. The classical example of droplet infections is flu.

·         Droplet nuclei: These are less than 5 microns in size; they remain suspended in air for several hours and can infect people at a distance even beyond 10 feet, which means there is no distance cut-off to prevent exposure. A person who enters a room, which has been left by a patient long ago is also at risk of acquiring the infection. Examples of airborne droplet nuclei infections are TB, measles, chickenpox and SARS.

·         Air changes per hour: It is the rate at outdoor air replaces indoor air within a room in one hour. Air changes per hour means the number of times that the entire volume of air of a particular space is replaced by fresh outdoor air. For example, an air exchange rate of 2 means that the air is completely changed twice in one hour. Air changes in a room maintain the air quality. Normally, for all spaces in general, the air change rate should be more than 4.

·         Air conditioning: In air conditioned rooms with no air exchange, the infections can spread from one person to another. In an air conditioned atmosphere, the setting of the AC should be such that the same air is not circulated and fresh air is allowed to exchange. In rooms with ACs with no air exchange, the infections can spread from one person to another. In an office with split AC, if one of the employees has any droplet nuclei disease, he/she can transmit infection to others. Split ACs are more dangerous than window ACs.

·         Natural ventilation: Natural ventilation is via doors and windows. Open windows in a house allow constant exchange of air, which prevents spread of infections. Hence, cross ventilation is always preferred as it draws in the outdoor air through one window and pushes out the stale indoor air through another window. Therefore, natural ventilation is the most economical alternative to have clean air quality. High ceilings provide better ventilation. This is why old houses and buildings have high ceilings.

·         Mechanical ventilation: This is via fans and exhaust fans, which blow out air that may contain bacteria. Fans can either be installed directly in windows or walls, or installed in air ducts for supplying air into, or exhausting air from, a room.

Dr KK Aggarwal

Padma Shri Awardee

President Elect Confederation of Medical Associations in Asia and Oceania   (CMAAO)

Group Editor-in-Chief IJCP Publications

President Heart Care Foundation of India

Past National President IMA