In Southern Ontario, climate change caused an increase in frequency and intensity of heat waves which can cause heat stress. A heat wave is a 3-day period with temperatures exceeding 32°C daily. Heat stress is a physiological response to environmental heat which can result in discomfort, morbidity and mortality. The level of discomfort caused by heat stress can be estimated by measures known as Heat Stress Indices (HSIs). Maps visualizing the distribution of heat stress in Southern Ontario were produced by geostatistical kriging. Higher levels of heat stress during heat waves were predicted in the south, particularly urrounding major metropolitan areas. Subsequent research utilized these maps to predict the HSI at point locations. Using Quasilikelihood Poisson regression, it was estimated that a 1.03 (p<0.001) times higher on-farm dairy cow mortality rate occurs for every one unit increase in HSI. Using Poisson regression, it was estimated that rural hospitals have 1.11 (p<0.001) times higher admissions during a heat wave than ontrol period. These findings will improve policy for heat wave reparedness in rural Southern Ontario.