Title: Observed Impacts of Environmental Conditions on Heat Illness Morbidity in the Military
Description: Heat stress illnesses, including heat stroke and heat exhaustion, represent a serious, persistent, and growing public health threat to military and civilian populations. Global climate change, due primarily to increased greenhouse gas concentrations in the atmosphere, has resulted in measured increases in temperature and humidity. Climate models project warming trends to continue in the future with very high confidence, resulting in more dangerous mean and extreme heat conditions. The associations between environmental heat indicators and observed adverse health outcomes have been increasingly studied for mortality endpoints and among elderly populations. This dissertation aimed to expand this investigation to heat illness morbidity outcomes among active-duty military servicemembers while assessing a range of heat indices. This is an assumed healthy, working-age population that is regularly exposed to outdoor heat in combination with high levels of exertion. Comparable civilian populations, in some respects, include athletes and outdoor laborers such as construction workers or farmers.

In Chapter 2, we assessed annual rates of ambulatory encounters, hospitalizations, and reportable events among active-duty soldiers at ten US Army installations from 1991 to 2018 to produce rate ratios for estimation of future climate change impacts. In this chapter, we identified positive long-term associations between annual heat indices and heat stress illness hospitalization and reportable event outcomes. Chapter 3 assessed incident active-duty US military heat stress illness cases at 24 installations between 1998 and 2019 on a daily-scale, resulting in odds ratio exposure-lag-response curves applicable to near-term risk assessment. The daily-scale relationships betweenheat indices and case-defined heat stress illnesses were non-linear, with increasing odds ratios starting from mild temperature ranges, and displayed short-term delayed effects. Finally, Chapter 4 described demographic and body composition risk factor trends in the US military from 1998 to 2019. In this assessment, we identified a null trend for body mass index among heat stress illness subjects over the study period.

Overall, our findings demonstrate a connection between temperature and humidity indicators and observed heat stress illness morbidity outcomes among multiple sets of indices and timescales. This dissertation highlights an urgent need for vigilant heat stress prevention and control measures to protect health and maintain performance in hot and humid environments.