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  • 17-Apr-2023

Heatstroke deaths in Mumbai: Why humidity, not heat, is often the critical factor in such events

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The deaths of 11 people in Mumbai on Sunday (April 16), apparently due to heatstroke, while they were attending a government meeting in an open space, is possibly the largest heatwave-related death toll from a single event.

So is it extremely hot in Mumbai right now?

Interestingly, Mumbai is not even facing heatwave conditions right now. The tragedy took place in Kharghar, Navi Mumbai, under the Panvel Municipal Corporation area. The India Meteorological Department’s (IMD’s) Santa Cruz (Mumbai) observatory recorded a maximum temperature of 34.1 degrees Celsius on Sunday, which was 1 degree above normal. The coastal observatory at Colaba (Mumbai) recorded a high of 32.4 degrees Celsius.

 

According to the IMD, heatwave conditions are currently prevailing in some areas of Gangetic West Bengal, coastal Andhra Pradesh and Bihar. In most parts of Mumbai, maximum temperatures on Sunday were in the range of 30-35 degrees Celsius, and it was expected to remain this way on Monday as well, according to the IMD forecast.

So what happened in Kharghar on Sunday, then?

High temperatures in themselves are often not fatal. The combination of high temperature and high humidity, referred to as the “wet bulb temperature”, is what makes heatwaves deadly. Information on wet bulb temperatures in Mumbai on Sunday was not immediately available.

Humidity is a critical factor in heat exposure because high humidity does not allow sweat to evaporate off the surface of the skin, the primary way in which humans lose the heat generated within the body. The cooling effect that the evaporating sweat produces is essential in maintaining a stable body temperature.

Also, the resilience of people to heatwaves varies with age and depends on underlying health conditions.

 

Are heatwave deaths very common in India?

The monitoring and management of heatwaves has undergone major improvements over the last few years, with almost every state coming up with a heatwave action plan consisting of early warning, provision of water availability at public places, and flexible working hours in offices and education institutions. Consequently, heatwave-related deaths showed a sharply declining trend between 2010 and 2020.

 

Heatwave fatalities peaked in 2015, when more than 2,000 deaths were recorded. In 2020, only four heatwave deaths were reported, according to government figures. However, this data is not very reliable, and different government agencies record it differently. In the last few years, these numbers have been showing an upward trend, mainly because of improvements in the reporting and recording of deaths.

In June 2019, more than 100 heatwave-related deaths were reported from four districts of Bihar alone. But unlike the fatalities in Mumbai on Sunday, these deaths did not come from a single event and were spread over several days.

Will things get worse as the summer progresses?

The summer this year is predicted to be excessively hot because of the end of the La Nina phase in the equatorial Pacific Ocean, something that has a general cooling effect on the Earth’s atmosphere. New forecasts suggest that El Nino, which has the opposite impact of La Nina, is likely to kick in from the May-July period itself, earlier than expected.