I have vivid memories of my parents taking extraordinary measures to prevent us from the common cold at the onset of winter. They would boil the water before drinking and serve us hot turmeric milk every evening. While the scientific nature of their measures can be debated, their observation of the seasonal nature of diseases is spot on. Climate influences the survival of disease-causing microbes, how our body responds to diseases, and how diseases spread from one person to another. However, the specifics of how this plays out in various diseases remain largely unclear.
In a recent study, researchers from the National Institute of Tuberculosis Research in Chennai examined the relationship of climatic factors with the occurrence of tuberculosis (TB) in India. Their results show that TB infection increased during summer and monsoon when the dew point rose. The study was published in the journal Scientific Reports.
Tuberculosis is caused by a bacterium (Mycobacterium tuberculosis) that spreads through the air when an infected person coughs, speaks, sneezes or spits in the proximity of another. As TB spreads through the air, the researchers hypothesised that the local climate conditions might influence the progression of the disease. Hence, they set out to examine the relationship between TB infections and changing climate conditions.
Meteorologists routinely measure three climate metrics to assess the local weather in a place – temperature, relative humidity and dew point temperature (or dew point). While temperature reflects how hot or cold it feels, dew point reflects how humid it feels outside.
Dew point: A measure of moisture in the air
The air around us can hold a certain amount of water vapour, beyond which the vapour will condense to form water droplets or dew. The temperature at which this transition happens is known as the dew point. Water droplets on the outer surface of a glass of cold water, on the inner surface of a lid of a hot cooking utensil, and on the grass of the garden early in the morning are some examples of this phenomenon.
The amount of vapour in the air is usually linked to relative humidity, which is an estimate of how much water vapour is present in the atmosphere as a percentage of the air’s maximum capacity to hold vapour. However, the capacity of air to hold vapour increases with temperature. So in absolute terms, for the same relative humidity, there is more vapour in the air at a higher temperature. Dew also starts forming at a higher temperature when the air temperature is higher as compared to air with a lower temperature. Thus dew point can be an indicator of the combined effect of temperature and relative humidity and indicate the absolute amount of vapour present in the air.
The higher the amount of vapour in the air, the stickier it feels. Thus, a higher dew point reflects a more humid air.
Dew point: A predictor of Tuberculosis pattern
In the current study, the researchers examined 5777 TB infections in Chennai between 2008 and 2013 using data obtained from daily patient records maintained at the National Tuberculosis Elimination Program centres. They also examined daily weather data for the same period, obtained from the Regional Meteorological Centre in Chennai. They then assessed the relationship between the occurrence of active TB cases and the local climate by utilising three metrics present in meteorological records – relative humidity, temperature, and dew point.
To examine seasonal changes in TB infections, researchers grouped their data into four seasons: winter (January, February), summer (March, April, May), monsoon (June, July, August, September), and post-monsoon (October, November, December). They observed that there were three peaks of infection – first in Summer (March-April), second in Monsoon (May-June) and the third in Post Monsoon (November-December) season. They then used a mathematical model to assess the relationship between the peak of infections and the local climate.
The results showed that different climate factors were linked to the progression of TB cases in different seasons. In the summer and monsoon seasons, the pattern of TB cases mimicked the pattern of dew point temperatures – that is, cases increased as dew point temperature increased and dropped as dew point temperature dropped. On the other hand, in the post-monsoon and winter seasons, the pattern of cases mimicked the pattern of relative humidity.
Since most TB cases were reported in the summer and monsoon season (58.2%) when the case pattern followed the dew point temperature pattern, the researchers hypothesised the importance of dew point in the progression of TB. Dr K Rajendran, the lead author of this study, speculates that dewdrops may support the survival of Mycobacterium tuberculosis by providing a safe harbour on its way to another host.
The current study is the first one in India to examine the link between climate change and TB infection. It will be interesting to test if TB progression in less humid places like Delhi also follows the dew point temperature pattern.
This article has been run past the researchers, whose work is covered, to ensure accuracy.