When you think of a natural disaster, a heat wave might not be the first occurrence that comes to your mind. It's mostly earthquakes or floods or hurricanes - which are big, dramatic and cause widespread visible damage to life and property.

But heat waves rarely make headlines and the associated deaths are grossly underreported.

A recent study published in The Lancet Planetary Health found that globally, more that 5 million excess deaths were associated with non-optimal temperatures per year. The study looked at mortality and temperature data from 2000 to 2019.

The study is significant, as much of the world, including parts of India are baking under intense prolonged heat, with dozens of deaths recorded in the US and Canada.

And the heat-related deaths are not just a result of high temperatures. There are a number of factors at play that make it deadly.

Historically, too, heat waves have been responsible for a large number of deaths. The most catastrophic recent events are the 2003 European heat wave that recorded over 70,000 additional deaths and the 2010 Russian heat wave that killed about 11,000.

The abysmal data on heat waves, the associated deaths and our understanding of it can be explained with the following example.

In May 2010, Ahmedabad in Gujarat witnessed a record-breaking heat wave where the temperatures reached a high of 46.8 degrees Celsius with an apparent increase in mortality.

A study found that the mortality rates in the city during the heat wave were 43 percent higher in comparison to the same days in 2009 and 2011. It said 1,344 additional deaths were registered in the city.

What the study revealed was that the heat wave had a substantial effect on the excess mortality, which could've been argued otherwise without knowing the cause of deaths.

"That's the thing with heat, the deaths are not visible. They don't make a spectacle out of them," Dr. Gulrez Shah Azhar, a researcher at US think tank Rand Corporation said.

This is the first issue – Even if the numbers are huge, they're not visible.

The second issue is how the deaths are recorded.

In India, the medical certification of the cause of death is itself not high and uniform, and the immediate cause of death is often not listed as heat wave or high temperatures.

India counts only deaths due to heat stroke as those caused by direct exposure to the sun and not those indirect deaths due to high ambient temperature.

The vulnerability to extreme heat depends on a number of factors.

It could be the neighborhood you're living in, whether you have water supply, if you have air-conditioning, electricity connection, access to healthcare services, the kind of occupation, underlying health status, among other things.

It can be broadly classified into:

• Hazard probability

• Exposure to hazard

• Susceptibility to heat illness

• Adaptive capacity

These come together to eventually decide how many people might be affected, and to what degree they might be affected, experts told FIT.

• Age of the population

In heat waves that occurred in Europe, the main issue was that there was a disproportionately large elderly population, which is more susceptible to heat stress, Dr Adithya Pradyumna, faculty member, Azim Premji University in Bengaluru, said.

The elderly, not in a position to adequately recognise the symptoms, regulate temperature or seek help is a cause for many isolated, lonely deaths.

Mr Azhar, however, said that according to data from the National Disaster Management Authority, the deaths in the productive age is more than the elderly. "It's because of the unique work profile of the population."

• Susceptibility to non-communicable diseases

"Generally, people with non-communicable diseases like diabetes, hypertension, heart disease etc., are also more vulnerable to heat stress. This is also a problem because the prevalence of all of these diseases is only increasing in India," Dr Pradyumna said.

So, the population overall is becoming more susceptible to heat.

• Are women more at risk?

While data tells a different story, experts think women are more likely to be affected by heat stress than men, but less of it is reported.

Stuck indoors, wearing layers of heaving clothing and drinking less water to avoid frequent toilet trips - this is the lived experience of many Indian women who are more at risk.

• Rural vs Urban

"When we talk about heat waves and the related deaths, it is mostly in urban contexts. But there is increased exposure to stressful conditions everywhere," Dr Pradyumna said.

This might be more in urban areas simply because of the nature of the city, which has a tendency to get hotter due to infrastructure and urban planning. In addition, there is also high population density in urban areas, leading to relatively higher case numbers despite the smaller geographic areas.

"It’s not as hot as urban areas, but people are working in the fields, water supply might be an issue, they might not have easy access to healthcare facilities," Dr Azhar said.

• Perception: 'Indians know heat'

Many Indians have a notion that their bodies and physiologies are adapted to heat. So, there is complacency that it's not really something one need to be worried about.

But this perception works only up to a degree of human tolerance and adaptation.

"With every increasing temperatures, you also find increasing humidity in the atmosphere. So, in cases in cases of high humidity, sweating mechanism doesn't work," Dr Azhar said.

Without air conditioning, it would be incredibly hard for the body to cool down.

Studies show that and in vast parts of India, humidity combined with heat is deadlier.

Even if the heat doesn't kill you, it can have a bunch of other effects on the human body. Everything from mental health to fertility to renal failure to dehydration, he added.

There is no doubt that the frequency and the intensity of heat waves have been increasing and will only get worse in the decades to come. There is no doubt that heat-related deaths are not easy to identify. But they are definitely preventable with timely action, experts said.

The Ahmedabad Heat Action Plan, which was launched in 2013 after the silent disaster in the city, reportedly saves hundreds of lives every year. This heat action plan and early warning system in south Asia has served as a template for other plans in the country.

Experts said, health vulnerability assessments in various parts of India, locally at small scales are needed.

"Once these places are identified, it might be as simple as starting with informing people before the summer and what the signaling might look like," Dr Pradyumna said.

Colour-coded alerts or heat warnings based on weather forecasts can be useful.

Specifically looking at these vulnerable groups is important in order to address these larger structural and social determinants of health, Dr Pradyumna said.

Capacity-building of health-care professionals to treat people with heat-related complications is another aspect.

Opening drinking water centres, resting facilities in the city, and altering school and college timings become important.

At an individual level, lifestyle and behaviour modifications would also have a strong impact in reducing heat vulnerability.

Heat wave is dubbed as a silent killer for a reason. It might not cause an immediate, visible damage to property, but it is deadly. Despite this, we take it lightly and remain woefully unprepared for the increasing threat.

“You cannot build the well once the fire starts. You need to have already dug the well, bought the pump, and put everything in place beforehand. By planning ahead, we can prepare for heat waves before they hit and save many more lives,” Dr. Dileep Mavalankar, Director, Indian Institute of Public Health-Gandhinagar, says.