Physiological Effects of Hot Climates

Temperatures in Delhi reached 46 °C this week. How does this kind of heat affect people?

In my last post “Why do most hot countries remain poor?” I summarised explanations by influential economists and geographers. I reviewed the quantitative evidence that demonstrates the strong inverse link between climate temperature and economic productivity.

Of course, a correlation does not necessarily imply a cause.

23 years on, we now have research on physiology that has helped us understand a little more on how people are affected by hot climates. While there are still large gaps in our understanding, there is now little doubt that cooling is essential for economic and social development, even more so as climate warming raises temperatures everywhere.

This is a long post. However, you’re reading something new here. As far as I am aware, no one has put all these different ideas together into a compelling argument. I hope you have the patience to read it all and I welcome your comments and suggestions.

The photo featured for this piece is by Alev Takil at

For a while at the start of my engineering career in the 1970s I developed an interest in ergonomics, the study of humans at work and how to design for real people. The effects of heat were well-known at the time, of course. We knew that one should keep people comfortable around 20 – 24 °C to ensure they can work productively.

Living in Perth, Australia, I became accustomed to hot weather in summer and the joys of spending late afternoons at the beach in the surf. However, it was only when I started visited Pakistan in June that I really experienced living in a hot climate. Of course, I had a bedroom with an air-conditioner.

Around 2004, regular power interruptions (called load shedding) started in Pakistan as the demand for electricity to run air-conditioners overwhelmed the grid’s capacity. I woke as my window air-conditioner thudded to a halt in the pitch dark… no fan, no light, and the faint whine of mozzies circling as they searched for the carbon dioxide in my breath. With the heat stored in the masonry walls and reinforced concrete floor and ceiling, the bedroom temperature climbed to about 40 degrees in a few minutes. Beads of sweat started running down my face. That was when the idea of a miniature air-conditioner came to me, just sufficient cooling to keep my face feeling dry and refreshed might make the heat bearable.

Without air-conditioning, summertime indoor temperatures in buildings across South Asia hover around 40 °C, day and night. Bricks and reinforced concrete form most urban and village buildings as termites quickly attack most wooden structures. So how do people manage to sleep under these conditions?  The monsoon season that comes with the torrential rains sweeping in from the Indian ocean in from June till September brings lower temperatures but with humidity around 70 – 90%, so the heat is even more uncomfortable.

So, what does the physiology research tell us?

Effect of temperature on day-time productivity

A recent and widely cited meta-study has provided data showing the effect of temperature on labour productivity (Day et al., 2019). The authors analysed data from 13 empirical sources and showed a sharp decline in productivity above 27 °C Wet Bulb Globe Temperature (WBGT).

We calculate WBGT this way: 0.7 * (Wet bulb temperature) + 0.2*(globe temperature) + 0.1*(dry bulb temperature). A thermometer with the liquid bulb wrapped in a wet tissue provides a measurement of wet bulb temperature. Globe temperature measures the temperature of a black sphere, about 2 cm in diameter, exposed to radiant heat from the sun or other sources.  The graph below summarises the findings.

The next graph shows weather data collected from several locations in South Asia superimposed on a chart of temperature and relative humidity showing the sloping region where WBGT is high enough to cause productivity losses.

The curved lines for each location show the change in daytime temperature and humidity between mid-afternoon (typical maximum temperature) and 2 am (typical minimum temperature), adjusted for 3 °C urban heat island effect. This graph shows that most urban centres experience day-time conditions through summer months that cause significant productivity loss. Most of the time in the hot summer months, the urban heat island effect exceeds 3 °C as data to be presented below will show.

This means that workplace productivity across South Asia will be significantly reduced in the summer months as people are working in temperatures at which one can expect 10% – 90% productivity loss.

Effect of night temperature on sleep quality

Interestingly, Day et al (2019) did not mention sleep quality. Sleep quality data from numerous empirical research reports indicates that the neutral temperature for humans sleeping on a mattress without clothing is about 27 °C (neither too hot nor too cold). At that temperature, metabolic heat produced inside the body is balanced by heat lost to the environment.

With clothing and thin bed covers, comfort temperatures range from 19 °C to about 25 °C (see, for example Lan et al., 2016; Lan et al., 2017). A ceiling fan can reduce the apparent temperature by 2 °C (Nicol, 2004), indicating a maximum temperature for sleeping at 27 °C. Above that temperature, the human body will experience sleep loss because there is insufficient cooling to disperse heat from body metabolism and maintain a sufficient reduction in core temperature for normal sleep. The absolute maximum temperature for adequate sleep quality, sleeping with a fan on a string mattress without clothing, is 31 °C.

The following graph shows typical night-time bedroom temperatures (based on actual measurements) superimposed on a similar graph of temperature and humidity showing the physiological limit for sleeping. The data points reflect actual weather conditions for a typical day in May (15th May 2022), during the hot, dry summer season and for a typical monsoon season day (30th June). It shows that most urban centres experience bedroom temperatures that significantly exceed physiological limits for sleeping.

In rural settings, people can sleep outside in walled compounds where they are exposed to the night sky, providing significant cooling. They sleep on wooden beds with woven string (charpai) which allows the entire body to lose heat rather than just the upper half when lying on a thick mattress. A clear sky at night has a radiant temperature well below zero, so it acts as an effective radiant heat absorber (though slightly less as the atmostpheric CO2 concentration rises).

Bamboo and straw huts provide cooler conditions for sleeping (Zeke Tucker,

Outdoor bed (charpai) in an Indian home courtyard with a private handpump. (Raju Sharma,

In larger villages, towns, and cities, conditions are very different. Sleeping outside in walled compounds is usually impossible: only the very wealthy can afford a reasonably large open courtyard in a city. In any case, the very wealthy prefer to sleep indoors with air-conditioners. Many wealthier people can retreat to cooler cellars in their homes.

Most urban dwellers have no freedom to move to cooler locations to sleep, particularly women and children because they rent small rooms which are part of larger buildings. In one of the images below, taken in an Indian city, there are a few beds visible outside homes for men, but not women and children who have to sleep indoors.

Indian city streets characterized by 2 – 4 storey buildings. The lower floors are used for commercial activity so residential accommodation is in the higher and hotter levels. Photo: Annie Spratt @

Since the 1980s, climate warming has caused earlier and longer periods of extreme heat. However, a much larger temperature increase in urban centres has occurred, known as the “urban heat island effect”, increasing the temperature by up to 17 °C relative to surrounding open countryside. This effect is particularly strong at night. (see image below) (Raj et al., 2020).

Heat Island Effects in May 2022: NASA Ecostress (Source: NASA, Indian Express). This satellite measures radiation temperature, which only approximates air temperature at night. The difference between urban and open-country temperatures is as great as 17 °C. May 5th was one of the coolest days until the first monsoon storm arrived on May 23. The official recorded air temperature in the Delhi region at the time this image was collected was 28 °C, however the external temperature of buildings was about 35 °C.

Inside the buildings, the heavy reinforced concrete slabs and brickwork retain heat gained during the day. Outside, the temperature is typically 40 – 46 °C during the day and 30 – 35 °C at night. The roof and walls exposed to the sun rise to about 50 °C or more during the day. This explains why indoor temperatures above ground level remain close to 40 °C.

From this data, we can conclude that healthy sleep is impossible without air-conditioning because indoor temperatures are about 10 °C higher than the upper limit for sleeping with a powerful electric fan, about 13 °C above the limit for sleeping without a fan.

Unfortunately, while we know that sleep quality decreases with temperatures above 25 °C, we know almost nothing about the effect of prolonged exposure to these temperatures. It is conceivable that sleep quality researchers (many based in Japan and Korea) are unaware that people are trying to sleep at temperatures above 40 °C. In my personal experience, even people who have air-conditioning which is intermittent (due to load shedding) suffer from poor sleep, becoming less able to handle mental and physical work and more irritable through exposure. It resembles a semi-permanent state of exhaustion.

It is remarkable to me that none of the ‘established’ explanations for slow economic development in the Global South mention the loss of sleep with high night-time temperatures, and only briefly mention the reduction in daytime work productivity at temperatures above 27 °C.

The obvious question is how can people survive in these conditions?

Most people do survive, somehow. What is noticeable, however, is that India seems to be developing a two speed economy. A small proportion of the population in cities like Delhi, Lahore and Bangalore (which has a much cooler climate) are doing well, buying cars and high-rise apartments for US $250,000 or more. These people can afford conventional air-conditioning at home and mostly enjoy air-conditioned workspaces and cars.

Living conditions for the other 97% (my own estimate) remain much as they were a couple of decades ago. Many survive the dry heat of April, May and June with water coolers and cope with ceiling fans through the monsoon heat. Wealthier families install their first air-conditioner in a formal sitting room, using it for social occasions to impress visitors. In extreme heat, the entire family move their bedding into that room and sleep together, and putting off the thought of how to pay the US$200 electricity bill that will arrive inevitably after a month or so. It is noticeable that none of the South Asian nations have yet been able to match China’s industrialization, economic development and rising prosperity and the reduction of productivity caused by months of extreme heat is a possible explanation for this difference. However, I am no economist so please consider this as speculation rather than evidence-based analysis.

A small working class enclave in the back streets of New Delhi, January 2023. Notice ladders for reaching upper level apartments and water coolers.

Air-Conditioning is Essential

The only conclusion from the data presented here is that air-conditioning is essential for healthy sleeping and productive work in South Asian cities, towns and villages during summer months, and many other countries as well. However, as others have pointed out, using conventional air-conditioning for billions of people who need cooling is not possible within sustainable limits (Campbell et al., 2018; Isaac & Van Vuuren, 2009).

Measures such as heat-reflective coatings on roofs and increasing shade in cities can help. But none can reduce temperatures by 10 – 15 °C which is necessary to avoid having to use air-conditioning.

Campbell and his colleagues (2018) suggested that we need a cooling solution with at least five times less energy and environmental impact, preferably less. In future posts I will explain why, at the moment, Coolzy seems to be the only commercially available and affordable solution: learn more here:


Campbell, I., Kalanki, A., & Sachar, S. (2018). Global Cooling Prize: Solving the Global Cooling Challenge – how to counter the climate threat from room air conditioners. Rocky Mountain Insititute. Retrieved December 1 from

Day, E., Fankhauser, S., Kingsmill, N., Costa, H., & Mavrogianni, A. (2019). Upholding labour productivity under climate change: an assessment of adaptation options. Climate Policy, 19(3), 367-385.

Isaac, M., & Van Vuuren, D. P. (2009). Modeling global residential sector energy demand for heating and air conditioning in the context of climate change. Energy Policy, 37(2), 507-521.

Lan, L., Lian, Z. W., & Lin, Y. B. (2016). Comfortably cool bedroom environment during the initial phase of the sleeping period delays the onset of sleep in summer. Building and Environment, 103, 36-43.

Lan, L., Tsuzuki, K., Liu, Y. F., & Lian, Z. W. (2017). Thermal environment and sleep quality: A review. Energy and Buildings, 149, 101-113.

Nicol, F. (2004). Adaptive thermal comfort standards in the hot–humid tropics. Energy and Buildings, 36, 628-637.

Raj, S., Paul, S. K., Chakraborty, A., & Kuttippurath, J. (2020). Anthropogenic forcing exacerbating the urban heat islands in India. Journal of Environmental Management, 257, 110006.

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