Syndromic surveillance data from ED visits were collected and analyzed from Ohio's syndromic surveillance application, Epi-Center, for July 2011. Since the physical effects of HRI can vary greatly and affect multiple body systems, a specific classifier was created to query ED visits that were likely related to HRI and was defined as chief complaints referencing heat 'exhaustion or exposure', dehydration or hyperthermia. Measurements for weather variables included temperature, dew point, humidity, pressure and wind speed. The average daily values of these variables were calculated from seven geographically representative cities in Ohio and used as a surrogate for statewide data. These data were obtained from Weather Underground, which collects data from Automated Surface Observations System (ASOS) stations located at airports throughout the United States. These data were analyzed via time-series analyses and stratified by age group and gender. Correlation and linear regression analyses were performed, using SAS v 9.2 to determine which weather variables were the best predictors of HRI, as defined by ED chief complaint data.