- A recent study finds that humid heat stress during India’s monsoon season will rise sharply as global temperatures increase.
- As global warming reaches 2°C, hot-humid heat stress during the monsoon season will surge to affect about half of the country, the study notes.
- The study authors and public health experts point out the need to update heat action plans and policies to include the impacts of humid heat.
In Kothigal, a village situated in the coastal district of Ramanathapuram, Tamil Nadu, 30-year-old V. Kalaiselvi’s workday is governed by a clock that the climate change impacts are actively resetting.
Her family cultivates green gram, cotton, and urad dal across three acres. Even during the peak of summer, Kalaiselvi could reliably work in her fields until 10 a.m. But with the arrival of the monsoon, the air has turned heavy, wet, and unbearable. By 9 a.m., the sheer humidity forces her to retreat.
“We cannot stay in the field after 9 a.m.” Kalaiselvi says. “We keep sweating relentlessly and suffer from itching and skin allergies. I frequently feel dizziness, headaches, and sinus pressure. Our children get eye sores,” she shares.
Kalaiselvi now splits her shift — working from 6 a.m. to 9 a.m., then stepping back onto the field from 3 p.m. to 6 p.m. Between 9 a.m. and 3 p.m., her family stays indoors under ceiling fans. “It is the only respite,” she says, adding that it is tough to stay inside a room because of the indoor heat as well. “But we have no other option.”
Kalaiselvi’s daily ordeal is not an isolated weather anomaly; it is the frontline of what could be a dangerous climate transition.
In the last decade, public health policies and Heat Action Plans (HAPs) across India have treated extreme heat as a purely summer phenomenon — a dry, scorching emergency in May and June that disappears once monsoon rains arrive. Once July hits, heat alerts cease and emergency protocols shut down.
However, a recent study published in AGU Advances projects that humid heat stress during India’s monsoon season will rise sharply as global temperatures increase. What Kalaiselvi faces in coastal Tamil Nadu today is a preview of what over half of India could experience if global temperatures exceed a 2°C rise (relative to the preindustrial period).
“Until now, our Heat Action Plans issued warnings based strictly on dry air temperatures,” explains Aditya Valiathan Pillai, a doctoral researcher at King’s College London and a visiting fellow at the Sustainable Futures Collaborative where he studies climate adaptation governance. “However, the rate of climate change has accelerated over the past decade. We are witnessing intense precipitation variations — like cloudbursts — and now, an emerging hazard in the form of compound hot and humid conditions.”
This systemic gap was highlighted by Pillai and his team in a 2023 Centre for Policy Research assessment which evaluated 37 Indian heat action plans. The assessment revealed that existing state frameworks operate on outdated summer baselines, largely ignoring humid heat stress. “We know this trend will escalate, but predicting its exact timing is impossible because climate change is inherently unpredictable.” Pillai was not associated with the recent humid heat study.
How humid heat during monsoons can affect India
The recent study notes that the actual physical land area in India which experiences uncoolable heat has grown by at least four times since the 1980s. The study finds that in the observed period (1979-2021), Uncompensable Heat Stress (UHS) affected 8% of India during the summer, but a negligible 1% during the monsoon. UHS is the physiological threshold where the human body loses its ability to cool itself.
However, under a 2°C warming relative to the preindustrial period scenario, hot-humid heat stress during the monsoon season will surge to affect about half of the country — nearly the 60% that will be affected during summer.
In India, the eastern coast is projected to face a humid UHS across both seasons as warming progresses. During the rainy season, the danger shifts entirely to Punjab and the northwestern regions and this happens during the monsoon break.
“Both the land and atmosphere play a crucial role here,” explains Vimal Mishra, co-author of the study, who works with the Civil Engineering department at IIT Gandhinagar. He explains, the two key variables are relative humidity and air temperature. During the monsoon, relative humidity is inherently high. “When a dry spell occurs, air temperatures shoot up, particularly in the Indo-Gangetic Plain and western India. Moisture stored in the soil from previous rains continues to evaporate, pumping humidity into the air even while the skies remain clear.”
This combination creates a dangerous feedback loop. “If high humidity occurs when temperatures are lower, such as late August, the wet-bulb temperature remains manageable, below 25°C or 27°C,” Mishra adds. “But in June and July, when high temperatures collide with elevated soil moisture, wet-bulb levels easily cross 30°C, creating unbearable, life-threatening heat stress.”
At a 4°C global temperature increase, the northern Gangetic Plain will suffer 75% more uncoolable days during the rainy season (28 days) than in the summer (18 days).
The occupational toll
The researchers also evaluated a wide spectrum of physiological thresholds. They applied lower limits for vulnerable groups — including the elderly and manual labourers — and upper limits for acclimatised tropical populations. These thresholds were decided by combining meteorological data (temperature, humidity, wind, and radiation) with biophysical human heat-budget equations. By varying metabolic activity rates (heavy exertion vs. light exertion) and physiological cooling capacities (sweat rate limits and acclimatisation status), they derived realistic lower bounds for vulnerable outdoor labourers and upper bounds for acclimatised populations.
“Occupational heat stress guidelines account for work intensity by recommending lower Wet-Bulb Globe Temperature (WBGT) limits as exertion increases,” says Dr. Vidhya Venugopal, Professor and Head of Occupational and Environmental Health at Sri Ramachandra Institute. She is not associated with the study.
“For continuous heavy work like agriculture or construction, the recommended WBGT limit is around 28°C for acclimatised workers, dropping even lower for unacclimatised seasonal labourers,” she adds. “When air relative humidity spikes past 60%, sweat remains on the skin without evaporating. Core body temperature rises, forcing the heart to pump frantically to move blood to the skin for cooling. As continuous sweating depletes fluids, dehydration sets in and blood volume falls,” Venugopal explains.
Workers begin to experience fatigue, dizziness, muscle cramps, and reduced concentration, increasing workplace injury risks, she says and warns that if exposure continues, it progresses to heat exhaustion and severe heatstroke, where core temperature surges above 40°C. “Our own research, along with findings from other studies in hot working environments, shows that repeated heat exposure and dehydration can contribute to long-term kidney damage if workers are exposed day after day without adequate recovery,” Venugopal shares, emphasising that heat ultimately affects nearly every major organ system.
Despite growing health evidence, administrative frameworks have been slow to link climate modeling directly to public health protocols, public health experts note. “There is a clear connection between climate stress and systemic health decline, but administrators have historically been reluctant to acknowledge it,” says Dr. Rajendran Krishnan, a scientist at the ICMR-National Institute for Research in Tuberculosis (ICMR-NIRT), who is also not associated with the study. “While developed nations began integrating climate-health studies decades ago, India is only now catching up.”
“For their long-term health and immune resilience, healthy and nutritional food would have been the solutions to meet their vitamin deficiency. It will build a good immune system because climate impacts the immunity,” he adds.
The data gap and undercounting
While the researchers of the recent study drew upon four decades of weather and mortality records maintained by the India Meteorological Department (IMD) and National Disaster Management Authority (NDMA), the study explicitly noted the scarcity of public data dedicated to tracking heat-related deaths.
“Heat-related deaths are very complicated because we don’t have cause-based mortality data in our country,” explains study author Mishra. “We have to correlate overall mortality data with temperature indices to figure out how many deaths can be attributed to heat. It is a complex, indirect process.” India lacks a comprehensive system to monitor heat-attributable deaths or track emerging mortality trends.
“India struggles with death registration, particularly when accurately classifying underlying causes of death,” says Dileep Mavalankar, the former director of Indian Institute of Public Health and an honorary professor of Public Health, not connected to the study. Scientifically, mortality during extreme weather is best analysed using excess all-cause mortality. calculations. This is hardly being done in India except in Ahmedabad.”
He also notes that elderly individuals with comorbidities succumb to cardiac or respiratory failure brought on by heat stress, which are rarely recorded as heatstroke.
“When the Indian Institute of Public Health (IIPH) analysed daily all-cause mortality counts in Ahmedabad during the May 2010 heatwave, excess mortality data revealed total deaths were ten times higher than official heatstroke figures — accounting for nearly 800 excess deaths alongside just 75 direct heat stroke cases,” Mavalankar shares. “That stark gap is precisely why Ahmedabad launched the country’s first Heat Action Plan in 2013,” he adds.
The policy conclusion and call to action
Uncompensable Heat Stress (UHS) during the monsoon season (July–October) occurs at comparatively lower air temperatures than peak summers — typically between 35°C and 38°C — but is driven by extreme humidity that prevents the body from cooling through sweat evaporation.
“Authorities do not currently have a mechanism to monitor heat stress properly because of its atmospheric complexity,” explains Mishra. “That is why we have heat wave early warning systems, but no heat stress early warning systems,” he adds. Humid heat can also be highly localised and brief — spiking for just two hours in the afternoon. “Yet during those windows, wet-bulb temperatures soar, triggering drastic health impacts.”
He points to a grim historical precedent, “If you recall the 2015 heatwave that claimed over 2,000 lives, mostly in Andhra Pradesh, the driver was not dry heat alone; it was the combination of high humidity and high temperature. In those conditions, drinking water alone cannot save a worker, because the body stops losing moisture and can no longer cool down. It becomes unbearable. That is why it is critical to measure, monitor, and issue dedicated early warnings for humid heat stress,” he explains.
Translating this atmospheric reality into policy requires restructuring India’s Heat Action Plans (HAPs). “There are no legal roadblocks to updating HAPs, but there is a massive implementation gap,” notes Pillai. “The lack of a legislative structure does not bind HAPs as a legal duty of state governments. It could be brought under the National Disaster Management Act as sub-legislation.”
Beyond policy frameworks, public health experts demand immediate transparency in mortality tracking. Mavalankar suggests that municipalities publicly release daily death counts. “We need to analyse baseline trends over the last five to 10 years to establish a clear mortality background level then develop local thresholds based on increase in all-cause mortality as the temperature rises. Granular, real-time mortality data will help us identify emerging trends early and refine our public health interventions,” he says.
“Compounding this is night-time warming,” adds Mavalankar. “During these humid stretches, ceiling fans and evaporative air coolers also fail completely — only air conditioning provides relief which very few can afford in India.”
While the National Center for Disease Control has issued guidelines for treating heat illnesses, Mavalankar says that Primary Health Centers (PHCs) must proactively stock ice-packs, cold water, and IV fluids. “When treating severe heat stroke cases, time is the single most critical factor in averting death.”
Ultimately, tackling this emerging impact of climate change will require the same level of policy focus that India previously applied to dry summer heatwaves. “We did a good job dealing with dry heat once we understood its economic, physical, and transport impacts,” Pillai reflects. “That same knowledge gap now exists with humid heat. Policies tackling this issue cannot be built solely on abstract literature — they require a deep, ground-level understanding of human vulnerability.”
Banner image: Residents sleep on the roof of their home near Versova beach to escape the indoor heat as humid conditions persist amid a delayed monsoon in Mumbai, Maharashtra in June this year. (AP Photo/Rafiq Maqbool)
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