‘Integrate health, physiology to assess heat impacts’
When we think of extreme heat, the discussion often focusses only on the mortality it causes. However, it affects individuals differently. Joy Merwin Monteiro, assistant professor at the Department of Earth and Climate Science at the Indian Institute of Science Education and Research, Pune, is studying how a combination of environmental and individual factors determine the risk of heat-related health impacts. In an interview with Down To Earth, Monteiro discusses the concept of heat strain—the physiological burden extreme heat places on the body—and why understanding it is crucial to safeguard the population. Excerpts:
Himanshu Nitnaware (HN): How does extreme heat impact India?
Joy Merwin Monteiro (JMM): Even though India is intuitively perceived as a hot country, we lack a quantitative understanding of its actual impact on our people. Over the past 10-15 years, epidemiological studies have established that extreme heat contributes to mortality in certain cities and regions. Heatwaves in 2010, then in 2015 and 2019, made headlines due to clear links with mortality. However, heat stress affects us throughout the warm season and likely during the monsoon as well, which is particularly uncomfortable in northern India. It impacts physical work capacity, mental health, crop growth and, through reduced labour productivity and agricultural yields, overall economic output. Heat-related illnesses often go unnoticed, even though most of us have likely seen domestic workers or those ironing clothes fall ill during the summer.
Lifestyle changes have also increased our susceptibility. As more people work indoors and lead sedentary lives, acclimatisation to heat has declined. Additionally, lifestyle-related diseases such as cardiovascular diseases and diabetes heighten vulnerability. Studies show India accounts for nearly half of the world’s labour productivity loss due to heat stress. It is time the country examined heat’s impact not only on mortality but also on health, productivity and the economy as a whole.
HN: Wet-bulb temperature and humidity are often used to assess heat impacts, especially with respect to climate change. How crucial are these parameters?
JMM: Wet-bulb temperatures and humidity are frequently discussed in the context of heat stress and survivability. While concerns about rising wet-bulb temperatures are well-founded, our research shows that all documented high-mortality heatwaves in South Asia have been characterised by lower-than-usual humidity in the affected regions.
The human body responds to temperature and humidity differently from a wet-bulb thermometer. For instance, wet-bulb temperatures do not account for the effects of radiation from the sun and surrounding surfaces. A recent study found that wet-bulb temperatures under-estimate health risks at high air temperatures. It is therefore crucial to move beyond the sole focus on wet bulb temperatures and integrate insights from health and physiology into discussions on climate change and human survivability.
HN: Mortality is linked to heat stress. But you speak of heat strain. How is it different?
JMM: Heatstroke is an extreme consequence of heat exposure, occurring when a combination of high air temperature, humidity and radiation causes the body’s core temperature (the same temperature monitor during a fever) to rise to dangerous levels. A heat stress index combines air temperature, humidity, radiation, and wind speed to estimate the actual heat burden on the body.
However, extreme heat stress is not always necessary for heat-related illnesses and fatalities to occur—they can happen at lower levels as well. This is where heat strain becomes critical. The body responds to heat stress by shifting blood closer to the skin’s surface to dissipate heat, increasing cardiovascular load to maintain blood pressure. Sweating also leads to water loss. For individuals with cardiovascular conditions, this added strain can be fatal. Dehydration or reduced sweating ability (due to diabetes or old age) heightens vulnerability to heat-related illnesses and death.
Thus, the same level of heat stress may be safe for one person but deadly for another. This variation is due to differences in heat strain and each individual’s capacity to cope with it.
HN: Do we need tailored heat advisories to ensure that a large part of the population can make informed decision on their work routines during heatwaves or peak summer months?
JMM: Yes, to be able to protect our citizens from the impacts of heat stress, we need to be able to provide advisories that take into account individual risk factors, such as pre-existing conditions, age, work profile (sedentary v hard labour) and access to cooling facilities. Unless we create personalised recommendations, we will overestimate heat risk for some groups and underestimate it for other groups. The former leads to reduced economic activity and the latter leads to additional illness and death.
HN: What changes need to be made at the policy level to mitigate the impacts of heat?
JMM: Policy needs to be informed by data. We should start by funding research that will provide the data needed to formulate policies that can actually protect our citizens. Extreme heat is still not a notified disaster and therefore funding protective measures at the administrative level is a challenge.
We also lack insight into why people continue working in extreme heat, despite being aware of the risks. Understanding and addressing these underlying causes should be a key policy focus. For example, heat insurance could enable daily-wage workers to forgo work on dangerously hot days. Similarly, inadequate workplace sanitation forces women to remain dehydrated, increasing their vulnerability to heat stress. Protection from heat is not purely a climate issue, it is intertwined with medical, social and economic considerations. So, it requires policy interventions and cannot be addressed by fragmented interventions at the local level.
This was first published in the 1-15 April, 2025 print edition of Down To Earth