Hundreds of millions of people living in 'informal settlements' in tropical areas are living in unhealthy temperatures – with some approaching the limits of human survivability.
Climate change will mean this gets even worse, researchers have warned.
Scientists from Monash University said their climate models were often based on data from weather stations in populated areas.
But they said this ignored people who lived in informal settlements, who are more likely to be affected by global warming.
The researchers used heat sensors in and around 100 houses in Makassar, a settlement in a tropical part of Indonesia.
They said the conditions in Makassar were likely typical for many such settlements in the tropics – areas that support approximately 370 million people in East and Southeast Asia alone.
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The researchers found that 80% of the sensors recorded temperatures during the rainy season that were higher than established health thresholds.
They also found that, in a few instances, the sensors recorded temperatures that are believed to represent the upper limit of human survivability.
They wrote: "Millions of people living in many parts of the world are already living under heat conditions that are harmful to their health... many such people engage in physical labor for work. Doing so in extreme heat can be fatal.
"Perhaps most alarming is the near certainty that conditions in such places are going to get worse as the planet continues to warm."
Last year was the hottest ever in Asia, with extreme weather displacing millions of people and causing billions in economic damage, according to data from the World Meteorological Organization (WMO).
The continent's mean temperature in 2020 rose 1.39C above the average over the preceding 40 years.
Heat extremes included a temperature of 38C at Verkhoyansk in Russian, provisionally the highest known temperature anywhere north of the Arctic Circle.
The soaring temperatures pose a direct danger to people, WMO officials warned.
WMO secretary-general Petteri Taalas said: "Weather and climate hazards, especially floods, storms, and droughts, had significant impacts in many countries of the region, affecting agriculture and food security, contributing to increased displacement and vulnerability of migrants, refugees, and displaced people, worsening health risks, and exacerbating environmental issues and losses of natural ecosystems.
"Combined, these impacts take a significant toll on long-term sustainable development, and progress toward the UN 2030 Agenda and Sustainable Development Goals, in particular."
In 2020, floods and storms affected approximately 50 million people in Asia and resulted in more than 5,000 fatalities.
Tropical cyclones, floods and droughts induced an estimated average annual loss of several hundred billion dollars, according to the Economic and Social Commission for Asia and the Pacific (ESCAP).
The average annual loss is expected to be as high as 7.9% of GDP for Tajikistan and 5.9% of GDP for Cambodia.
The worst-hit areas were affected by droughts.
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