GUEST COLUMNIST: This is how TB spreads in the air

Tuberculosis (TB) is an airborne disease that is transmitted from one person to another through small air droplets also known as droplet nuclei.

These droplets can remain suspended in air from few minutes to two hours. They are produced in massive amounts when a patient who can potentially infect another coughs and sneezes.

Presence of good ventilation systems in indoor location ensures that air is constantly mixed and that you have “new” clean air from outside that is exchanged with indoor air.

Locations with poor ventilation increase the time for the droplet nuclei carrying TB particles suspended in air and this may result in increased probability of being rebreathed by the next person in the room.

We carried out a study and found that certain locations have the potential for being transmission hotspots due to poor air ventilation as captured by our new method and the large number of people whom we interacted with at such locations in one year.

Prisons in particular are a problem because most people who end up in jail are from a low socio-economic status, succumb to poor nutrition while in jail (both factors are epidemiologically related to TB disease) and lastly unfortunately in developing countries prisons are mostly overcrowded making TB transmission rates higher than in the general population.

However, congestion alone is not a problem as long as there is adequate system to support good ventilation i.e., air exchange between the indoor location and the “air outside the particular room/building”.

However, policies which address both congestion in public locations e.g., transport, prisons, night clubs etc., with clean air quality policies will result into better control of TB by reducing significantly transmission from such locations.

What we already know from studies elsewhere; is that only approximately 20 per cent of TB is transmitted at household level, meaning about 80 per cent of TB is transmitted in other locations apart from ones household.

The challenge ahead is to appropriately estimate the proportion of TB transmission in different locations and to highlight locations with highest probability of transmission at the population level.

The gaps in terms of prevention needed are mainly due to lack of policy for indoor air quality addressing ventilation status. Once we have this policy, we can then build tools and basis of enforcing this policy.

We were successful in introducing smoking ban under the Tobacco Products (regulation) Act of 2003 which made it illegal to smoke inside public transport, hospitals, schools etc., we need policies which prevent/control from congestion in public places and installation of ventilation system in infrastructures which are accessed by the mass.

We are actually focusing on Dar es Salaam for clear reasons. It’s a city with the highest number of people in the country. It is leading in TB notification each year accounting for about of 22 percent of all TB patients notified to the program each year.

Suffice to mention, Dar es Salaam is among the fastest growing cities in Africa which unfortunately like elsewhere is faced with uncontrolled urbanization resulting into many slum like dwellings which are notoriously known for fueling TB transmission.

Drivers are at a greater risk than passengers. Here, one needs to take into consideration the time of exposure simply meaning the total time that one is exposed to poor ventilation conditions.

When you think about it, passengers spend less amount of time in the bus during one day as compared to drivers. If you model this relationship during one year, the risk of TB transmission is higher among drivers. For example the probability of a passenger to be in a bus with one TB patient during one route e.g. from Temeke to Mwananyamala is the same for the passenger and the driver.

However the driver will have many trips increasing his probability during the day and the year. A good intervention will be to ensure public transportation is not congested which will result into very low risk of transmission for drivers and even lower for passengers.