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The world’s tropics could become uninhabitable if global warming exceeds 1.5ºC (2.7°F) above pre-Industrial levels, a new study published Monday in Nature warns.

The study finds that at or above 1.5°C, the combination of high heat and humidity called wet-bulb temperature (TW) will exceed 95°F TW in the 3,000-mile band of the planet around the Equator. Because sweat cannot evaporate and the human body cannot cool itself down above 95°F (35°C) TW, prolonged exposure to those conditions can be fatal – and potential health risks can occur even at 1°C increase.

The tropics are home to 3.3 billion people – about 40% of the world’s population – a proportion that is expected to grow to reach about 50% by 2050.