持续高温对身体造成的生理负担模拟了耐力运动，因缺乏睡眠时的恢复而耗尽身体储备，并显著增加中风或心脏病发作等心血管风险。在南亚、东南亚和非洲快速增长的巨型城市中，这一威胁尤为严重，预计在未来数十年中，这些城市的城市人口将增加约 10 亿人。在这些地区，热带地区的高基准夜间气温因缺乏被动式降温或安装空调的低收入住宅而进一步加剧。例如，在印度阿达马巴德，2010 年的一场热浪导致至少 1,300 人死亡，这促使了一项白漆屋顶计划，但该计划至今仅在少数建筑物上实施。
快速的都市化借由砍伐树木、填平湿地和铺设土壤加剧了这一问题，将天然的温度调节器替换为在夜间辐射热量的混凝土和沥青。在印度班加罗尔，自 1970 年代以来，大约有 80% 的湖泊和蓄水池已经消失，导致 25% 的人口依赖运水车。虽然空调提供了缓解，但它们会向外部排放额外的热量，且印度约 90% 的人口无法负担空调费用。尽管库里奇巴、深圳、麦德林、首尔和新加坡等城市已成功整合绿色空间以降低温度，但拥有 3,000 万居民的德里等新兴巨型城市在应对这些环境威胁时面临著严峻的治理挑战。
Day-time highs, such as the scorching 48.2 degrees Celsius (119 Fahrenheit) recorded in Uttar Pradesh, India, are survivable for most, but the lack of night-time cooling, exemplified by Delhi's 32.5 degrees Celsius night-time low, is increasingly fatal. Data shows that in Japan, a 2023 study of 25 million deaths between 1973 and 2015 revealed that mortality was as much as 10% higher on hot nights, independent of day-time heat. In parts of Switzerland, night-time heatwaves raised the risk of death by as much as a third. Similar mortality risks associated with elevated night-time temperatures have been documented across the United States and 28 East Asian cities.
The physiological toll of sustained heat mimics that of endurance exercise, exhausting bodily reserves and increasing cardiovascular risks like stroke or heart attack due to lack of sleep-time recovery. This threat is particularly severe in the rapidly growing megacities of South Asia, Southeast Asia, and Africa, where urban populations are projected to increase by approximately 1 billion people in the coming decades. In these regions, high baseline overnight temperatures in the tropics are compounded by low-income housing lacking passive cooling or air conditioning. For instance, in Ahmedabad, India, a 2010 heatwave killed at least 1,300 people, prompting a cool-roof painting program that has still only been implemented on a minority of buildings.
Rapid urbanization exacerbates this issue by felling trees, filling wetlands, and paving soil, which replaces natural temperature regulators with concrete and asphalt that radiate heat at night. In Bengaluru, India, approximately 80% of lakes and water-tanks have disappeared since the 1970s, leaving 25% of the population dependent on water tankers. While air conditioners offer relief, they dump extra heat externally, and about 90% of India's population cannot afford them. Although cities like Curitiba, Shenzhen, Medellin, Seoul, and Singapore have successfully integrated green spaces to mitigate temperatures, emerging megacities like Delhi, with 30 million residents, face severe governance challenges in addressing these environmental threats.