Seventy-four per cent of the world's population will be exposed to deadly heat-waves by 2100, if carbon dioxide emissions continue to rise at current rates, a study warns. Even if emissions are aggressively reduced, the percentage of the world's human population affected is expected to reach 48 per cent, researchers said. "We are running out of choices for the future. For heat- waves, our options are now between bad or terrible," said Camilo Mora, associate professor at the University of Hawaii at Manoa in the US. "Many people around the world are already paying the ultimate price of heat-waves, and while models suggest that this is likely to continue, it could be much worse if emissions are not considerably reduced," said Mora, lead author of the study published in the journal Nature Climate Change. The human body can only function within a narrow range of core body temperatures around 37 degrees Celsius. Heat-waves pose a considerable risk to human life because hot weather, aggravated with high humidity, can raise body temperature, leading to life threatening conditions. A team of researchers led by Mora conducted an extensive review and found over 1,900 cases of locations worldwide where high ambient temperatures have killed people since 1980. By analysing the climatic conditions of 783 lethal heat episodes for which dates were obtained, researchers identified a threshold beyond which temperatures and humidities become deadly. The area of the planet where such a threshold is crossed for 20 or more days per year has been increasing and is projected to grow even with dramatic cuts in greenhouse gas emissions, researchers said. Currently, about 30 per cent of the world's human population is exposed to such deadly conditions each year. Some of the cities that have experienced lethal heat- waves included New York, Washington, Los Angeles, Chicago, Toronto, London, Beijing, Tokyo, Sydney and Sao Paulo. "Finding a threshold beyond which climatic conditions turn deadly is scientifically important yet frightening," said Farrah Powell, a UH Manoa graduate student and one of the co- authors in the study. Watch this also: For example, by 2100 New York is projected to have around 50 days with temperatures and humidities exceeding the threshold in which people have previously died, researchers said. That same year, the number of deadly days for Sydney will be 20, 30 for Los Angeles, and the entire summer for Orlando and Houston, they said. The study also found that the greatest risk to human life from deadly heat was projected for tropical areas. This is because the tropics are hot and humid year round, whereas for higher latitudes the risk of deadly heat is restricted to summer.