How can the world reduce deaths from extreme heat?

The world will need to adapt to increased temperatures. What can societies do to save lives?

Extreme heat has major impacts on human wellbeing: it makes it harder for kids to learn at school, reduces the productivity of outdoor workers, and puts pressure on healthcare systems. In the worst case, it kills.

This is already an issue - particularly for countries in the tropics - but will become even more critical as the world warms. This article is the third in my series on extreme heat. In my previous articles, I looked at how many die from extreme temperatures today and how climate change could affect this in the future. In many of the world's poorest countries, deaths are expected to increase if we don't invest more in adaptation.

Protecting people from extreme heat will require blending the old and the new. Technological solutions like air conditioning (AC) will be essential, but relying on them alone would be a mistake.

The availability and affordability of AC is - and will continue to be - highly unequal, leaving the poorest households unable to protect themselves. It's also not a solution for those who work outdoors in agriculture, construction, or as street sellers. This is the reality for most people in tropical countries, where heatwaves will be most extreme.

The goal, then, is to build communities and cities more resilient to heat through urban planning, communication, and emergency responses.

We can learn a lot from our ancestors, who learned how to build cities and design lifestyles that could cope with scorching summers and intense heat waves. That will not be enough in a warming world, but it's a starting point to build new solutions.

How to design cities that are resilient to heat

Go to the old parts of many cities, and you'll find yourself walking through narrow streets. This helps to keep them cool. The ground and the walls of the houses are only exposed to the sun for a short period of the day when the rays come from directly above. Wider streets are in direct sunlight for long periods, absorbing large amounts of heat. Cul-de-sacs also form heat barriers, so they're more common, too.

Seville in Spain is a perfect example of this. It's one of Europe's hottest cities and is often hit by extreme heat. Older parts of the city - stretching back to the Middle Ages - were designed with these natural cooling techniques in mind. It has small squares where people can find shade, communal fountains for people to keep cool, and trees and vegetation line the streets, where people can find shade. Newer parts weren't designed like this: they often have large, wide avenues that can reach baking temperatures in the summer.

Lifestyles in Seville have also been adapted to deal with the heat. People stay indoors until the evening; the city comes to life only then. Afternoon siestas are normal for rest and shelter.

Cities don't only have the problem of dealing with climate change. They also tend to have higher temperatures than surrounding rural areas.

This "Urban Heat Island" effect is caused by several factors: artificial surfaces such as steel, concrete, and bricks absorb heat effectively; cities often lack vegetation, which can cause cooling through evapotranspiration; and energy users such as cars, buses, home appliances, and cookers produce heat that adds to increased temperatures. City temperatures are often at least 1°C to 2°C warmer than rural areas, and this difference can be as large as 10°C.1

More and more people are moving to cities, particularly in some of the world's hottest and most vulnerable parts. As the chart shows, urban populations are growing rapidly in many countries across the hotter parts of the planet. Finding effective ways to keep cities cool is crucial to protect billions from extreme heat.

There's a lot that can be done. Here are some of the best design solutions:

• Narrow streets tend to reduce the amount of heat absorbed by reducing direct exposure to sunlight.
• Cul-de-sacs or closed streets tend to be cooler than wide-open ones.
• Trees and vegetation can effectively cool the environment through evapotranspiration. They also provide shade from direct sunlight. Research suggests that cities with around 30% of tree coverage can significantly reduce deaths during extreme heat events.2
• Painting roofs and materials white increases albedo - the amount of sunlight reflected to space - so that these surfaces absorb less heat.
• "Green roofs", which also reduce the amount of dark, heat-absorbing surfaces exposed to sunlight, are becoming popular in many modern cities.

It's not only cities in the tropics that need to invest in these solutions: cities at higher latitudes, like London or Paris, would also benefit. They were not designed to deal with heat, but they now must take this risk seriously. They could learn a lot from the design principles of other cities that have had to withstand scorching temperatures for centuries.

Building resilient public health responses to extreme heat

The problem in temperate cities like London or Paris is that local populations are not used to extreme heatwave events. Hot temperatures are rare, so basic adaptation behaviors - as simple as drinking more water - are not intuitive.

However, awareness of extreme heat's dangers is not limited to colder climates. Even in tropical or subtropical countries, people can fail to recognize when temperatures rise from extremely hot (which is common, even without climate change) to life-threatening. They may also not have a choice: they might be forced into the heat if they earn their income from working outdoors.

That's why public messaging and emergency alerts are crucial. They make people aware of the dangers of extreme heat and give them potentially life-saving advice on how to cope.

The city of Ahmedabad in India provides a good case study for how community-level responses can reduce heat mortality. It combines a color-coded alert system - escalating from yellow to orange to red based on temperature - with public health interventions to support people during extreme heat events.

On "yellow" days, fans are provided in public transport stations, drinking water is distributed, and local journalists are informed to help spread the word. On "red" days, public spaces such as temples and malls are used as air-conditioned refuges for the public. Those who work outdoors are encouraged to shift their hours to avoid extreme daytime heat. Extra health workers are on hand to deal with dehydration, heat stroke, and other illnesses.

It will be vital to make these interventions available to everyone, especially the poorest, who are less likely to have cool homes to recover. Some families might even need economic support to afford a few days of lost income if outdoor temperatures are too severe to work in.

This can't be achieved through individual behavior change alone. It needs society-level planning involving sectors from government to business owners. Emergency-level responses need cooperation; it's not just about protecting ourselves; it's about protecting each other.

Most households are expected to have air conditioning in the future, but some could be left behind

City design and public planning are essential to support the poorest and most vulnerable during extreme events.

But, especially as families get richer, air conditioning will be another way to protect people's health. The research is clear: air conditioning is very effective in reducing deaths from heat.

The US provides a good case study since it has gone from a country with no air conditioning to one where almost every household has it. Research by Alan Barreca and colleagues looked at heat mortality rates over the 20th century.3 They found that the mortality impact of very hot days fell by 75% - most of it post-1960 - when more households started to adopt air conditioning.

Studies in other countries - from Greece to Japan - estimate AC will significantly reduce heat-related deaths as the climate warms.4

More AC means more lives saved.

In today's world, there are large inequalities in the adoption of AC. Buying and running an air conditioner is expensive, so its use is strongly linked to income. Richer countries and richer households within countries have more AC units.

The chart below shows the share of households with AC in 2018 for several countries. Air conditioning is almost universal in Japan, the US, and South Korea. Compare that with Indonesia, South Africa, and India, where less than 10% of homes have one.

This inequality is also true within countries. A study by Filippo Pavanello and colleagues looked at the share of households in Brazil, Mexico, India, and Indonesia that had air conditioning and more basic fans across the income distribution.5

You can see the results in the chart below.

Households get standard fans - shown as the blue line - at much lower incomes. Most households have them at levels of just $5,000 per person. The adoption curves for AC - shown as the red line - are much slower: very few households have them at incomes below $5,000 per person, which is where rates start to rise more quickly.

The chart also shows the distribution of incomes in each country.6 Most people in India, for example, earn less than a few thousand dollars. This is below the point where AC adoption takes off, which explains why just 5% of Indian households have it.

This strong relationship between AC and income is also seen in studies of other countries. A paper by Lucas Davis and colleagues used data from 16 countries across all continents to predict AC adoption rates across populations.7 Rates were very low until household income hit around $10,000, after which they rose sharply. This was true from Argentina and Paraguay to Nigeria and Sierra Leone.

It's not just about whether households have AC or not. Energy costs money, so income also affects how and when they use it. Poorer households wait until it's much hotter before turning it on and keep it running for fewer hours.8

Since AC adoption is so strongly tied to income, we expect rates to surge as countries - especially those with lower incomes - get richer. In fact, the study by Paul Davis projected that income, not climate, will drive the biggest boom in demand for air conditioning. Income growth explained 85% of the rise. Remember, many countries in the tropics would be much more comfortable to live in with AC, even in a world without climate change, so the additional temperature rise doesn't have a huge impact on demand. Having AC during the peak of summer in India would make conditions much more comfortable, regardless of climate change.

The International Energy Agency expects the number of air conditioning units globally to triple by 2050, rising from 2 billion to 5.6 billion. Two-thirds of the world's households will have AC. This will have important implications for electricity demand, which I'll look at in more detail in a separate article.

Despite this huge surge in AC adoption, some people who need it will still be without it.

The study we looked at earlier in Brazil, Mexico, India, and Indonesia projected adoption rates in 2040 at different income levels. In India and Indonesia, most households in the top deciles had AC in 2040; those in the middle had rates above 80%, but in the lowest deciles, less than a third of households had AC. This mirrors projections from Lucas Davis and colleagues across a wider range of countries, including Pakistan, Nigeria, and Ghana.

These inequalities don't just affect mortality rates from heat. Richer households can also reap the well-being and economic benefits of higher productivity levels, educational attainment for kids, and better overall health. It could widen inequality even more.

What can be done to close this gap?

Faster economic growth - especially for the world's poorest - is an obvious but not particularly specific answer. The root of the problem is poverty, so anything that helps to reduce poverty rates is positive. Faster economic growth wouldn't just help to provide AC units for the poorest in 2050; it would help middle-income families in these countries get potentially life-saving AC earlier, in 2030 or 2040, rather than mid-century. Anything that accelerates adoption rates will help.

Another key solution is to make air conditioning cheaper. This has two components.

The cost of running an AC unit depends on the cost of electricity. Subsiding electricity - particularly during the hottest months - would make AC more affordable. In some countries, this could be an important step. But, many countries already subsidize electricity. India, for example, offers generous power subsidies for consumers (although there is plenty of debate about whether these are effective and genuinely support the poorest). Rethinking these subsidies to make sure they target the poorest households could help.

The other major barrier is up-front costs. Many households live on money day to day or week to week and don't have the savings to buy one even if they could afford to keep it running. The Center for Global Development estimates that a standard 3-star efficiency air conditioning unit costs around US$800.9 A more efficient one is almost $1000. The median income in India is around $460 per year, meaning an average AC unit is equivalent to around twice someone's income.10 The cheapest 3-star unit I could find from one of the most popular Indian brands was around $350. That's three-quarters of the average Indian's income for an entire year. Compare that to a ceiling fan, which is around $17, and you can see why they are much more popular.

One option is for governments to subsidize some of these costs, especially for the poorest households. Another is to drive down the costs of AC units. Given that air conditioning has been around for a long time, it is unlikely that incremental technological improvements will make them much cheaper. We will need new innovations that take different approaches to make them affordable. The Center for Global Development calls for new finance mechanisms to incentivize these efforts.

Air conditioning is slowly moving from a luxury product to a necessity. When combined with the intelligent design of cities - trees, vegetation, narrow streets, reflective roofs, and smart materials - and society-level responses to extreme events, countries can save many lives from heat and make conditions more comfortable in the hottest parts of the world at the same time.