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.

Why do most hot countries remain poor?

In my first post in this thread, Pakistan is Never Boring, I introduced the key role that engineers have in economic development. In this series, I will explain how my research journey has led me to an understanding on what seems to be preventing economic and social development in countries like Pakistan and how engineers might remove most of the impediments. Pakistan is one of many countries experiencing an extremely hot climate, possibly the hottest on the planet, for several months every year. It also has cold winter months too.

Have you ever wondered why hot countries tend to be less prosperous, with some notable exceptions?  

Think of India, Bangladesh, Pakistan, Indonesia with more than a quarter of the world’s population. Then think of countries in Africa such as the Democratic Republic of Congo, Cameroon, Nigeria, Ghana, North and South Sudan, Kenya, Tanzania, Zimbabwe and many others.

There are also some cold countries that are poor too. Russia today is a relatively poor country.

Of course, measurable economic wealth is not necessarily related to happiness, but it certainly helps with health and education.

The image shows a world map showing GDP per person (There are some  white gaps in the map where no data is available for countries such as North Korea, Afghanistan and Somalia.) Here’s the data:

Map of the world showing countries with low and high per capita GDP. Poorer countries (low GDP) are clustered either side of the equator.

Among the wealthier hot countries, the exceptions, almost all rely on petroleum exports, including Venezuela, Saudi Arabia, Kuwait, Qatar, Bahrain, Brunei and the United Arab Emirates.

Collectively, the world’s poorest countries are often referred to as “the Global South”, in contrast to the wealthier northern hemisphere countries concentrated in North America and Europe.

Few question predictions that we will collectively fail to achieve the ambitious Sustainable Development Goals agreed by the UN in 2015. We will need to come to terms with the reasons to have any possibility of sustaining human life on earth. While some cite the Ukraine conflict and the Covid pandemic as reasons, I think one of the main reasons is climate, as I shall explain in this post and in more detail in later posts.

In this post I will explain the “conventional” explanations, mainly from economists and geographers. From my research I can now see additional powerful explanations – I will write about those in the next post.

Several notable economists have examined this apparent correlation between climate and economics. In 1976, the World Bank published Andrew Kamarck’s commissioned study on the relationship between climate and economic development. He reviewed many of the earlier explanations, such as a study by Maeterlinck who observed that bees stop producing and storing honey when taken to places where there is a continuous year-round food supply. He and others argued that people living in lush tropical climates where food could be plucked from plants or rivers at any time discouraged economic activities such as saving and investment. Others suggested that construction materials such as wood decay faster in the tropics because of humidity and termites, discouraging their use and limiting construction ambitions. Some even suggested that certain religions discouraged ideas of improving one’s living conditions.

Kamarck’s analysis pointed to many contributing factors.

First, human physiological thermodynamic limits impose muscle power limitations. Muscles convert energy stored as adenosine triphosphate (ATP) which is made from glucose which the body makes from food and stored fat: carbohydrates, proteins and fats. Muscles convert ATP to mechanical energy. However, just like any heat engine, most of the energy emerges as waste heat. Hot climates slow the rate at which our bodies can release heat to the environment, limiting muscle effort.

Next, he drew attention to weather variability, particularly the intensity of tropical storms that can devastate crops and buildings, and extended droughts. Winters kill so many microbes, viruses, pests and parasites in temperature latitudes, but in countries with continuous warmth these can thrive. He pointed out that soils in hot countries often lack humus and organic matter in comparison to temperate soils. Across much of Australia, for example, the heat from summer sunlight distils oils and wax from decaying leaves that form a water-repellent coating on soil particles so summer rains fail to penetrate below ground. However, there are also large tropical regions with some of the most fertile soils on the planet, particularly across South and South-East Asia.

Karmarck remarked how sgricultural pests can thrive in perennially warm climates. Locusts emerge in vast swarms after heavy rains, and trypanosomiasis parasites from tsetse flies strike cattle and other domesticated animals, humans too.

Disease is also an obvious factor affecting people: malaria parasites, bilharzia worm infections, dengue, cholera and yellow fever strike in the tropics far more than temperate climates.

Since Kamarck’s work, partly motivated by his analysis, tropical diseases and parasites have been at least mostly controlled, often by eradicating the insects that so often bring them to humans. Plant breeding programs and more recent genetic engineering techniques have helped produce agricultural crops that resist disease and parasites far better, boosting production many times. Improved farming methods, irrigation and fertilizes have boosted food production many times.

Yet, the relationship between climate and productivity remains nearly half a century after his report.

Two highly influential and popular books appeared around the turn of the millennium. Jared Diamond wrote “Guns, Germs and Steel” suggesting that these three factors explained the ascendance of European civilization in the last few centuries. He also suggested that geographic regions extending across a limited range of latitude encouraged successful plants to dominate the landscape, whereas in the Americas, plants that thrived in temperature North America could not spread to South America. David Landes wrote “The Wealth and Poverty of Nations” presenting his explanations on why some countries are wealthy and others poor. Landes, an economist, extended Kamarck’s findings, suggesting that organizational innovations such as individual property rights and companies that enabled people to share profits and risks collectively helped propel enterprise and innovation, resulting in wealth generation in Europe and North America.

At the same time, Andrew Mellinger, Jeffrey Sachs and John Gallup published a detailed quantitative analysis and the relationship between climate and economic development. They added another critical factor – water navigability – being an enabler of trade and access to industrial supplies.

I was fortunate to meet Sachs in 2003 at a meeting arranged by the Harvard Bureau for Economic Research on skilled migration. He is still one of the world’s preeminent authorities on economic development and poverty alleviation.

Among other findings, they calculated that, on average, GDP per square kilometre was eighteen times higher in temperate regions close to sea-going transport than tropical regions far from transport links. However, climate was still the dominant factor.

While socially organised insect control measures and modern health science can deal with disease and improved infrastructure, irrigation and agriculture can reduce the impact of tropical storms and droughts, the overriding factors that remain are temperature and humidity.

And that’s why Singapore’s founding prime minister, Lee Kuan Yew, attributed his nation’s success to the invention of air-conditioning, the ability to remove climate from the economic equation. He is not the only one to have noticed this connection. Robert Gordon, another economist, highlighted the significance of air-conditioning, eliminating climatic impediments, enabling large productivity gains across southern and eastern regions of the USA in the 20th century.

So, could air-conditioning bring Singapore’s prosperity to the Global South?

In principle, yes, of course. However, burning fossil fuels to produce electricity for air-conditioning and leaking refrigerants that are far more potent in promoting global warming than CO2 make this impossible today without literally cooking the planet. Further, across most of the Global South, only about 3% of the population can afford the cost of electricity to run their air-conditioners all the time.

So, if we are to eliminate the climate factor impeding social and economic development with air-conditioning, we need something different. Coolzy could be the solution we need for that.

In future blog posts, I will explain some of the recent findings from medical science and physiology that give a much clearer understanding on how climate affects economics and hence engineering too.

If we are to overcome the wealth disparity between the rich industrial, perhaps now post-industrial countries, and the Global South, and achieve the Sustainable Development Goals on a less ambitious schedule, there are many more influences we need to think about.

Further Reading

Diamond, J. M. (1999). Guns, Germs and Steel: The Fates of Human Societies. W. W. Norton & Company.

Diamond, J. M. (2005). Collapse: How Societies Choose to Fail or Succeed. Viking Penguin.

Gordon, R. J. (2012). Is US economic growth over?  Faltering innovation confronts the six headwinds. NBER. Retrieved September 25 from

Kamarck, A. M. (1976). The Tropics and Economic Development: A Provocative Inquiry into the Poverty of Nations. The Johns Hopkins University Press for the World Bank.

Landes, D. S. (1998). Wealth and poverty of nations. W W Norton and Co.

Mellinger, A. D., Sachs, J. D., & Gallup, J. L. (1999). Climate, water navigability, and economic development. Working Paper Series, 33.

Beyond Competencies

Is it possible that much of the engineering education research community, myself included, has misunderstood the notion of competency? With many others, I think, I was unaware of literature drawing attention to some of the mistakes that can easily be made when talking about competency. I conclude by suggesting a way forward, beyond ‘competencies’.

How did I reach this position?

Continue reading (15 mins)

What we know, and mostly don’t know about engineering practices

This is the script for my REES-AAEE-2021 Keynote. The video is here, and the powerpoint slides are available on request if you would like to use them for education purposes.

For a sustainable future, we need large productivity improvements. Engineers are critical contributors, but we need deeper understandings of engineering practices and how education influences them to make the necessary improvements. Without this, education reform arguments are fragile at best.

Read the Script of the presentation (30 mins)

Winners of the Global Cooling Prize announced

I rarely stay up late to watch serious TV. However, this announcement, three years in the making, was something that I just couldn’t miss.

At Close Comfort, we sincerely congratulate the Global Cooling Prize 2021 winners along with all the judges and participating teams! Everyone involved in the Prize helped develop new green technologies that can cool people around the world without warming or harming our planet.

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A Big Question

How are we going to adapt engineering education to prepare coming generations of engineers for climate warming and the need to protect people and infrastructure? How can we prepare them to re-engineer almost our entire civilisation to eliminate greenhouse and other harmful emissions in 25 years?

As you would know, I often write about engineers and engineering, and education issues. However, I have usually stopped short of specific recommendations, relying on my books and articles to convey ideas that educators can use.

Next week I am speaking at a panel discussion at Engineers Australia Perth on Wednesday March 31, 5:30 – 8pm. Register here to join in the discussion and contribute your ideas, or if you cannot join us then, reply to this post.

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Changing notions of comfort

I am so thankful I don’t have to work all the time in an air-conditioned office building. Especially since Covid-19, our entire Close Comfort team works part of the time at home. We’re happier and feel healthier too.
Of course, I have a Coolzy personal air conditioner with me. (Since 2021, Close Comfort portable air conditioners have been branded with the name Coolzy). Our team members each have at least one at home as well.
Lee Kuan Yew, honoured as Singapore’s founding father, loved to tell everyone how air conditioning enabled today’s Singapore by providing a comfortable working and sleeping environment. However, there’s a dark side that comes with 20th-century air conditioning systems.
It is well established that people who live most of the time in constant temperature air-conditioned buildings lose their natural thermal acclimatization. As a result, they only feel comfortable at about 23 °C.
Recently I hailed a Singapore cab and climbed into the shiny black refrigerator on wheels, feeling so glad I remembered to bring a cardigan tied around my shoulders. The driver exclaimed, “Ah, it’s so hot today, la!”
“What’s the temperature?” I asked.
“33, it’s really hot, la”.
“But, yesterday it was 32”.
“Yeah, 33, it’s so hot today, la!”

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Productivity isn’t everything, but…

No wonder Trump can easily still command rustbelt supporters. Stagnation in the US manufacturing industry is killing prospects for wage rises. Bureau of Labor Statistics data released two weeks ago shows that while productivity increased by about 3% annually from the 1980s till 2007, annual growth since has been only 0.4%. Most of that, and more, is needed for sustainability improvements like changing to clean energy.

Labor productivity depends on engineered tools, machines and materials, so engineers are the key people to restart productivity growth. While economics and labor saving solutions were the priority for engineers in the 1950s, as evidenced by the ASEE Grinter report, now that seems to have been forgotten. Our research is revealing that today’s engineers have limited understanding on how to generate commercial value.

Students need to learn the fundamental purpose of engineering. Distilled from our research on hundreds of engineers in several countries, that purpose is to enable people to be more productive.

“Engineers are people with technical knowledge and foresight who conceive, plan and organise delivery, operation and sustainment of artificial objects, processes and systems. These enable productivity improvements so people can do more with less effort, time, materials, energy, uncertainty, health risk and environmental disturbances.”

Sustainability depends on similar improvements.

As Paul Krugman wrote more than 30 years ago,

“Productivity isn’t everything, but in the long run it is almost everything. A country’s ability to improve its standard of living over time depends almost entirely on its ability to raise its output per worker.”

Economists are hoping that the digital economy will restore productivity growth. It might. But in a world where information supply is exponentially increasing, its value must be exponentially decreasing.

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