As the planet warms under the effects of human-caused climate change, prolonged periods of high temperatures are projected to become a significant public health challenge. The elderly are often more susceptible than others to the effects of temperature extremes, so the aging population worldwide may exacerbate this trend. To quantify the level of danger posed by particular weather conditions, researchers have developed a series of heat stress indicators (HSIs), the most commonly known of which is the NOAA Heat Index. This index and other HSIs measure how hot it feels when other factors, like relative humidity, are taken into account.