The index considered various factors, including a person's age, caste, income and health, as well as the green cover in an area, as having a bearing on people's heat vulnerability, and its analysis of 640 (of the 707) districts in the country finds 10 districts to be "very high risk".
Six of these are in relatively underdeveloped areas in Madhya Pradesh and Chhattisgarh. In contrast, the 20 districts in the "very low risk" category are mostly in the relatively developed states of Kerala and Goa and the Union territory of Lakshadweep.
The Index was published in The International Journal of Environmental Research and Public Health, a peer-reviewed scientific journal.
Researchers used data from the 2011 Census of India, the District Level Household Survey-3 and the Indian Space Research Organisation, combining it statistically to create the index, according to a post by two of the authors, Gulrez Shah Azhar and Jaime Madrigo.
The very high-risk areas, concentrated in Madhya Pradesh and Chhattisgarh, are less urbanised and people there have fewer household amenities as well as lower rates of literacy and access to water and sanitation.
These high-risk areas are not the same as areas with high or rising temperatures -- the correlation between the HVI and average land surface temperatures is moderate.
"When we think of temperature, there are several issues to consider -- in addition to temperature there is effect of humidity and radiant heat on mortality. At the same time, local populations are adapted to (used to) the prevailing temperatures. Deaths happen when heat exceeds these adaptation thresholds," Azhar told IndiaSpend.
Azhar gave the example of Dehradun, which reported heat wave deaths at lower temperatures than neighbouring cities in Uttar Pradesh last year. "In Uttar Pradesh there were much higher temperatures but no reported deaths, since the much higher temperatures are normal," he said, adding, "If we include temperature as a variable in the index itself, it would bias the results towards places with higher normal temperatures."
The number of deaths due to heat waves recorded in 2015 -- more than 2,400 -- was higher than the number of deaths caused by any other natural disaster, according to the Ministry of Home Affairs (MHA). In 1998 the death toll was 3,000 and in 2002, over 2,000, according to data from the National Disaster Management Authority (NDMA) 2016 action plan for the prevention and management of heat waves.
A heat wave is a period of abnormally high temperatures (more than the normal maximum temperature) that occurs during summer, with the resultant atmospheric conditions causing physiological stress, and sometimes death, among affected populations, according to the NDMA.
Despite casualties, and the fact that heat wave deaths occur due to atmospheric conditions, heat waves are not included in the notified list of natural disasters, according to a MHA response in the Rajya Sabha.
In 2016, the NDMA prepared guidelines for state governments to formulate action plans for the prevention and management of heat waves, outlining four key strategies: Forecasting heat waves and enabling an early-warning system; building capacity of healthcare professionals to deal with heat wave-related emergencies; community outreach through various media; and inter-agency cooperation as well as engagement with other civil society organisations in the region.
The NDMA says adopting these measures has brought heat wave-related mortality down in many places such as Ahmedabad. The city experienced a major heat wave in 2010, and registered over 1,000 additional deaths in May that year that could be attributed to heat wave conditions.
In 2013, the city adopted a "Heat Action Plan" (HAP) that has reduced mortality in succeeding years. The primary objective of the HAP is to alert populations most at risk of heat-related illness to existing or impending extreme heat conditions, and to take precautions.
At the same time, the India Meteorological Department (IMD) publishes daily bulletins containing information about the prevailing temperature and a warning for coming days. For example, on May 17, severe heat wave conditions were observed at isolated places over coastal Andhra Pradesh, and heat wave conditions at isolated places over Odisha, Vidarbha and Tamil Nadu, according to an IMD bulletin.
"The combination of exceptional heat stress and a predominantly rural population makes India vulnerable to heat waves. Vegetable vendors, auto repair mechanics, cab drivers, construction workers, police personnel, roadside kiosk operators and mostly weaker sections of the society have to work in the extreme heat to make their ends meet and are extremely vulnerable to the adverse impacts of heat waves such as dehydration, heat and sun strokes," the NDMA guidelines say.
The HVI confirmed as much. Pointing out that the high and very high HVI districts are in the central part of the country, the paper said that with a higher tribal population, these states have been at the lower end of various health, education, economic and population growth indicators.
Among the solutions offered in the report were: to provide separate indices for urban and rural areas to enable more targeted interventions; deeper analysis of urban ward-level data to provide intra-city vulnerability patterns; public messaging (radio, TV), mobile phone-based text messages, automated phone calls and alerts; popularisation of simple design features such as shaded windows, underground water storage tanks and insulating housing materials; and provision of drinking water within housing premises and indoor toilets.