Nearly 25% of world population exposed to deadly city heat. A study done by Columbia University’s Earth Institute found that the number of person-days (the cumulative population exposed to cumulative heat in a given year for a particular place) soared from 40bn a year in 1983 to 119bn in 2016, representing a threefold increase. In 2016, 1.7 billion people were subjected to extreme heat conditions on multiple days.