How Humidity Is Breaking Your Body’s Thermostat

As India grows warmer, it is also getting more humid, particularly on the coasts and the Northeast region which historically have higher humidity as compared to the plains. It is therefore essential to also monitor and study the impact of humidity on heat outcomes and health for hundreds of millions of Indians.

New research shows that the combined effect of temperature higher than 32 degrees and humidity beyond 60% is hazardous for the human body, especially among outdoor workers with sustained exposure. Many stations in Konkan, Goa, Gujarat, Tamil Nadu, Kerala, Odisha, West Bengal are surpassing this limit on the daily in summer. What’s more is that the research warns that at temperatures above 32°C, each 10% increase in relative humidity resulted in a 22% increase in physiological strain on participants.

Humid heat inhibits the body’s ability to cool down through its natural mechanism of sweating. And humidity has increased across India at the rate of 0.79% per decade during the period of 1969 to 2012.

“Humidity is an almost forgotten factor when it comes to Indian settings. Everybody is talking about heat,” said Vidhya Venugopal, faculty of public health at Sri Ramachandra Institute of Higher Education and Research, Chennai. “The kind of heat one experiences in Rajasthan is very different from Gujarat, Tamil Nadu. There are differences even within a state. How your body reacts to 20% humidity is different from 60% humidity.”

“When you sweat so much, there is cellular dehydration and electrolyte imbalance. We must pay attention to the humidity factor in our approach to extreme heat,” said Venugopal, whose research area is extreme heat and health.

High humidity levels coinciding with high temperatures are likely to create lethal weather conditions and imperil the lives of millions across large parts of the subcontinent, as IndiaSpend reported in August 2017, based on research published in the scientific journal Science Advances. Climate change will make such conditions more frequent as the 21st century comes to a close.

IndiaSpend wrote to the Union Ministry of Health on what steps it is taking to issue advisories to people and various stakeholders to protect from or mitigate the impact of humid heat. We will update this story when we receive a response.

Perspiration nation

As has been the trend for a few years now, warmer temperatures set over India as early as February this year, obscuring the spring. The India Meteorological Department (IMD) has been issuing heatwave alerts right from March.

The IMD measures heat throughout the day for hundreds of weather stations which is marked by the day’s maximum temperature. The same is done for minimum temperature at the end of every night. Each weather station has a ‘normal’, and the departure from this normal shows how much hotter or cooler it is than usual. All weather stations also record relative humidity , which is the measure of how much water vapour is contained in the air as compared to how much it can hold. This is expressed in percentage.

However, when issuing information on extreme heat or record-breaking heat, only temperatures are taken into consideration. The threshold for declaration of a heatwave is also dependent on the same, but there are lower thresholds for coastal locations as compared to the plains. Due to high humidity, there might be a reason to re-draft these guidelines.

In coastal areas, a heatwave is declared if the temperature crosses 37°C for two consecutive days in at least two stations. This threshold is 45°C for a station in the plains. However, if one considers a maximum temperature of 36°C, just one degree lower than the criteria, and factor in 60% humidity, the ‘felt temperature’ or ‘heat index’ as per this calculator is 48.1 degrees, higher than the ‘mark’ for a severe heatwave.

In other words, the human body will experience thermal discomfort high enough to be categorised into a ‘severe heat wave’ and yet, the station will not even record a ‘heatwave’. (It must be noted that relative humidity varies as per the time of the day, location and other factors whereas maximum temperature is one value at one time of the day).

Declaration of a heatwave means that various stakeholders from the Union ministries to the concerned local authorities can take measures to prepare for extreme heat outcomes. This includes taking care of those most vulnerable to heat (infants, elderly, people with chronic diseases), issuing alerts and advisories to people. In case of a red alert, there are chances of a heat stroke and extreme care is needed for the most vulnerable.

Our survival depends on our bodies staying at a consistent temperature. Normally, our core temperature--from the top of the head to mid-chest, encompassing the brain, lungs and chest--is regulated at 37°C, and our skin at 35°C. Sweating helps us shed excess heat--sweat beads cool the skin, and evaporation of sweat gets rid of heat, restoring the equilibrium.

This process of thermoregulation occurs efficiently only when ambient air is favourable for us. Air can hold a limited quantity of water until it saturates. In dry air without much water content, your sweat evaporates quickly, causing cooling. In humid air that has much moisture already, sweat does not evaporate as easily, leading to overheating of the body.

Physical labour--such as on farms, where 48.8% of Indians work--becomes unsafe in such conditions, and can prove fatal, as IndiaSpend reported in July 2021. And this is not in the future. India lost around 259 billion hours of labour annually between 2001 and 2020 due to the impacts of humid heat, as IndiaSpend reported in January 2022 based on a study from researchers at Duke University. The loss of these productive hours cost India $624 billion or about 7% of its 2017 gross domestic product.

Felt temperature is the thermal discomfort the human body feels and is a combination of maximum temperature and relative humidity. If we add solar radiation and wind speed to it, it gives us a ‘wet bulb globe temperature’. There are discussions globally on whether the WBGT might be the most accurate depiction of temperatures. Either way, meteorologists around the world are now talking about extreme heat not just in temperature numbers but in terms of ‘heat stress’.

IMD has been issuing a forecast for ‘hot and humid weather conditions’ for a few years now. For example, on April 17 it forecast that such weather is likely to prevail over Gujarat that day; and in Marathwada and central Maharashtra during April 17-21. ‘Hot and humid weather’ is when maximum temperatures remain 3°C above normal along with above-normal relative humidity. However, these alerts are different from the colour-coded alerts issued by IMD, such as yellow, orange and red to denote heatwaves in summer.

“Humidity is more dynamic than temperature,” explained an IMD scientist who did not wish to be named. “Humidity cannot have a national mean because it changes throughout the day. As temperature rises, the air’s capacity to hold moisture also rises. You cannot have one figure for the entire day also.

“For our experimental heat index, we are using temperature and humidity figures of 2.30 p.m. but for this heat index or wet bulb globe temperature to become the norm, data is not available in India. We need more studies and data on physiological response from India. It is a multiple stakeholder process beyond IMD,” they added.

The officer was referring to an experimental heat index that IMD has been issuing for the last three years wherein it measures ‘apparent temperature’. Just like the calculations for ‘felt temperature’ used earlier, this heat index measures the heat stress the body is expected to feel and is a combination of air temperature and humidity. It is derived using a formula similar to the one being used in the USA but it is not validated across India.

Senior IMD scientist R.K. Jenamani said that the IMD is trying a number of new things.

“In order to look at different thresholds, heat index, we also need all-cause mortality data. But we have started giving hot and humid weather alerts. Everyone is considering humidity, it is definitely a parameter. We are starting a lot of new things. In our daily forecast, we are now forecasting relative humidity also,” said Jenamani.

The scientist was referring to an annexure in the daily press communication wherein based on the day’s humidity, IMD has been forecasting the next day’s humidity levels across India as a percentage range.

Stranded on a heat island

The phrase ‘urban heat island effect’ has become ubiquitous in Indian cities. In simple terms, it is the warming effect felt in cities as compared to their nearby rural areas because of construction material used, concretisation, greenhouse gas emissions etc. trapping more heat within those cities. A 2024 report by Delhi-based Centre for Science and Environment analysed three factors that contribute to heat stress in six megacities of India during summer--air temperature, land surface temperature and relative humidity. Its findings showed that humidity had contributed to a significant increase in these cities’ heat stress.

In Chennai, one of the hottest and most humid places in India, humidity was responsible for adding on average 6.3°C heat stress to the city. That is because Chennai’s summertime has registered 0.4 degrees increase in ambient temperature as decadal average, while its relative humidity has increased by 5% between 2001-10 and 2014-23.

“There is direct co-relation between increase in built up area and increase in urban heat stress,” the CSE analysis on Chennai found, adding that the city’s built up area has increased from 30% in 2003 to about 74% in 2023, while green cover fell from 34% to 20% in the same time.

Kolkata also had identical numbers but less of an urban heat island effect.

In Mumbai, summertime has registered a 0.6°C increase in comparison while relative humidity has increased by 7% between 2001-10 and 2014-23. High humidity was responsible for adding, on average, 5°C of heat stress to the city. Mumbai’s built up area has increased from 38% in 2003 to 52% in 2023 while its green cover has decreased from 36% to 30% in the same time period.

And across India, as we said, humidity increased at the rate of 0.79% per decade during the period of 1969=2012. Global warming has a role to play here as well. A 2011 research paper stated that “there is a direct influence of global warming on precipitation. Increased heating leads to greater evaporation and thus surface drying, thereby increasing the intensity and duration of drought. However, the water holding capacity of air increases by about 7% per 1°C warming, which leads to increased water vapor in the atmosphere.”

Outdoor workers swelter

Exposure to extreme heat can cause fatigue, heat cramps, dehydration, dizziness and even fatal sunstroke. But how does humid heat impact health?

A new paper published by researchers from Sri Ramachandra Institute of Higher Education and Research, Chennai found that workers exposed to high humidity were 150% more likely to experience more physiological strain compared to those in low humidity conditions. Here, physiological strain is calculated using core body temperature and heart rate. It identified a threshold of 32°C temperature and 60% relative humidity and stated that “this highlights a critical point beyond which thermoregulatory mechanisms become inefficient”. The researchers surveyed 1,400 labourers in various outdoor occupations such as agriculture, brick kilns, salt pans for this paper.

Tanya Isaac, lead author of the paper, said, “Temperature and humidity by nature are tough to quantify (their impact on physiological strain). Even then, humidity is very important to track because it limits evaporative cooling from your skin. Even if a person is experiencing a low temperature like 35°C but very high humidity, they are suffering from heat as good as 45°C. You need alerts that combine multiple factors and not just one,” she said.

The research has recommended that policymakers must integrate humidity-adjusted thresholds into heat action plans and occupational safety guidelines. Industries such as agriculture, brick kilns, construction, and salt pans require specific protections since they are at extreme risk of heat-related illnesses when this threshold is breached.

Some examples of protections in agriculture could be mandatory rest breaks, adjusting work hours, cooling vests, wide brimmed hats and hydration stations. In salt pans, protections could be reflective clothing, mechanised salt harvesting to reduce labour intensity, emergency cooling tents and saline IV drips near worksites, recommended Isaac.

Ashwin Sreekumar, intern with IndiaSpend, contributed to this report.

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