Lancet Countdown Indicator 1.1.2. The “days of heatwave exposure” are measured as “person-days.” This unit encompasses both the number of people exposed and the length of exposure. One person-day represents one person being exposed to one day of a heatwave. In this way, if 10 people were each exposed to 10 days of a heatwave, there would be a total of 100 person-days of heatwave exposure in this population.