Heat stress played a role in Karur TVK rally stampede deaths, finds Poovulagin study

CHENNAI: Even as the political parties are at odds over the reason for the Karur stampede during TVK president Vijay's rally, which claimed 41 lives on September 27, a study conducted by environmental organisation Poovulagin Nanbargal found that Universal Thermal Climate Index (UTCI) had exceeded 38 degrees Celsius at the spot, pinning that as the reason for the deaths.

Based on study findings, the environmental organisation recommended that the government create rules and systems to plan large gatherings based on temperature and weather conditions, and ban roadshows or meetings that make people stand for several hours on narrow roads without shade or water.

As per the study report, Vijay's arrival at the Karur rally was delayed by 7 hours, exposing the people to 6 hours of very high heat stress from 11 am to 5 pm. The solar radiation peaked at 924 W/m² (watts per square metre) at midday, resulting in very high MRT (Mean Radiant Temperature) values (the average temperature surrounding a person).

The crowds exceeded 20,000 by 3.30 pm, resulting in an average crowd density of 2 people per square metre. This crowd effect must have significantly reduced the winds, spiking the heat stress, the report said.

Pointing out that when the density reached four people per sqm, the report explained, everyone in the crowd would have been releasing metabolic heat depending on their activity. If the crowds are static and calm, the metabolic heat released would be less —about 50 watts per person, it clarified. This means that the heat released would be around 200 w/m2. This is quite manageable. But if the crowds are unruly and start pushing and pulling (as in Karur), the heat released can exceed 250 W per person, or 1,000 W/m2, as per the report.

"Normally, a significant part of the metabolic heat would be lost to the surroundings through radiative, conductive, and convective cooling. Heat loss through radiation occurs to colder media around the body, while flowing winds or water provide convective cooling, and colder surfaces in contact provide conductive cooling. Sweating is the primary mechanism to relieve metabolic heat buildup,” adds the report.

But when many people are packed together, these cooling mechanisms are severely impaired. When people pile up, the sky view factor decreases, leading to a significant reduction in radiative cooling. Conductive cooling is minimal due to contact with other human bodies in the crowd. Hence, the only two ways to cool are convective cooling (winds) and sweating, the report said.

Even though strong winds were reported in the area of the Karur tragedy, the presence of large crowds would have largely obstructed them, the report said. Also, urban structures act as barriers to wind flow, a characteristic observed at both event locations, Karur and Namakkal. This means that convective cooling from the wind was relatively low in those locations.