Heat-related illnesses and deaths are significant public health problems. Extreme heat is the No.1 deadliest form of weather on average in 1990-2019 in the US according to the National Weather Service. Measurements and forecasts made at regional weather stations are a common data source of Heatwave Early Warning Systems. However, regional weather stations provide inaccurate estimates of the heat index that people experience in different microclimates. Introducing a direct measurement of heat index experienced by individuals via wearable sensors will allow more accurate exposure assessment and identification of factors associated with dangerous exposures to extreme heat. The goal of this dissertation is to characterize the individually experienced heat index exposure via wearable sensors in an urban and a rural location in summer in a southern part of the United States. In the first study, 51 outdoor workers in Birmingham, Alabama wore a small thermometer attached to their shoe. Their occupational Wet Bulb Globe Temperatures (WBGT), a comprehensive heat exposure index, was estimated from either temperature from the shoe thermometers or nearby weather stations. In the second and third studies, 88 urban participants and 89 rural participants completed a seven-day intervention where they performed normal activity on Days 1-2 and spent an additional 30 minutes outdoors daily on Days 3-7. Participants wore a small thermometer attached to the shoe and a pedometer at their waist to track steps. Neighborhood hygrometers/thermometers were deployed close to participants' homes to measure neighborhood level heat indexes. In the fourth study, we conducted a phone survey including 101 participants in the same urban and rural locations to examine how their heat-health behaviors changed due to COVID-19 and high profiles of police brutality cases in Summer 2020 compared to previous summers. The results demonstrated that (1) a wearable thermometer on the shoe was a feasible way to measure individually experienced temperatures; (2) among outdoor workers, WBGT from shoe thermometer temperatures estimated more hours in dangerous exposure categories and recommended more protective work-rest schedules compared to WBGT from weather station temperatures; (3) neighborhood level heat indexes improved the prediction of individually experienced heat indexes compared to weather station data alone; (4) rural participants experienced higher heat index exposures than urban participants, after accounting for ambient conditions; (5) spending a small amount of additional time outdoors was a feasible and effective intervention where participants walked more steps and had lower individually experienced heat indexes during the intervention days compared to baseline days; (6) a significantly lower percent of participants reported they would use public cooling centers in Summer 2020 compared to previous summers. Taken together, the results of these studies identified methods for more accurate heat exposure assessment and its application in monitoring heat-safety while promoting physical activity via time spent outdoors in the summer. Future work could incorporate physiological response monitoring linked to simultaneous individually experienced heat exposure to further characterize exposure-response relationships across different populations. Additionally, a longer intervention and more advanced wearable devices such Fitbit, Apple Watches could be used to monitor sustainability of the intervention and intervention benefits beyond short term increases in physical activity, respectively. / Doctor of Philosophy / Extreme high temperatures/humidity can bring dangerous adverse effects in people. Extreme heat is on average the deadliest form of weather in 1990-2019 in the US estimated by National Weather Service. Heatwave Early Warning Systems are introduced to closely monitor extreme heat events, estimate the magnitude of health consequences due to extreme heat, send warning messages to vulnerable populations, and trigger response plans to reduce the dangerous health effects of heat. Heatwave Early Warning Systems generally rely on the measurement and forecasts from regional weather stations. However, the temperature/humidity measurements made at weather stations can be different from the temperature/humidity people experience. People can live far away from weather stations and they move through indoor and outdoor locations, where weather station measurements will not represent temperatures experienced, particularly in climate-controlled indoor settings. Therefore, we recruited participants in an urban and a rural location and had each participant wear a small thermometer clipped to their shoe to directly measure the temperature they experienced as they went about their normal activities. In the first study, 51 outdoor workers wore this small thermometer on their shoe at work. We calculated a comprehensive heat exposure index from either the shoe thermometer temperatures or nearby weather station temperatures. In the second and third studies, 88 urban participants and 89 rural participants completed a seven-day intervention where they performed normal activities on Days 1-2 and spent an additional 30 minutes outdoors daily on Days 3-7. Participants wore the small thermometer clipped to the shoe and a pedometer at their waist to track how many steps they walked. We placed temperature/humidity sensors close to participants' homes to take measurements at a neighborhood level. In the fourth study, we conducted a phone survey including 101 participants in the same urban and rural locations to examine whether they had different cooling methods due to the COVID-19 pandemic and high profiles of police brutality cases in Summer 2020 compared to previous summers. The results demonstrated that (1) a small thermometer clipped on the shoe was a feasible way to measure temperatures at the individual level; (3) among outdoor workers, the comprehensive heat exposure index using temperatures from the shoe thermometers estimated more hours when outdoor workers were at a risk of dangerous exposure to extreme heat, and it recommended more rest time for workers to cool off compared to using weather station temperatures alone; (3) neighborhood level temperature/humidity was more representative of the temperatures recorded from thermometers on the shoe compared to nearby weather stations; (4) rural participants experienced higher temperature/humidity than urban participants, even when their nearby weather station temperature measurements were the same; (5) spending a small amount of additional time outdoors is a feasible and effective intervention where participants walked more steps and experienced lower temperature/humidity during the intervention days compared to baseline days; (6) a smaller number of participants reported they would use public cooling centers/spaces (e.g., air-conditioned library, air-conditioned churches, waterparks) to cool down due to fear of contracting COVID-19 and safety concerns brought by the high profiles of police brutality cases in Summer 2020 compared to previous summers. Taken together, the results of these studies showed that the wearable thermometers clipped on the shoe could provide more accurate assessment of temperatures experienced by participants compared to weather stations. This method could be used in future outdoor time interventions to monitor and ensure participants safely spend time outdoors while minimizing the risk of heat-related illness. In future work, more advanced sensors (e.g., Fitbit, Apple Watch) can be worn by participants to measure physiological responses across different temperatures experienced. Additionally, a longer intervention time can be used to test if participants would continue to spend additional time outdoors.