Introduction: The average temperature in Canada has risen 1.7°C between 1948-2016, increasing the frequency, severity and duration of extreme heat events. These events can exacerbate underlying health conditions, bringing patients to emergency departments (EDs). There is limited data associating sustained heat events to Canadian ED volumes and performance. This retrospective analysis assessed the impact of humidex and temperature on ED volume and length of stay (LOS). Methods: LOS is an indicator of ED overcrowding and system performance. The authors compared median and maximum LOS (hours) and patient volumes in both ambulatory and stretcher ED sections of two community hospitals (NDH, VH) in Montreal, QC to humidex and temperature during the summers of 2016-2018. Data were analyzed with one-way ANOVA and post hoc means analysis with Fisher LSD tests of a priori determined thresholds of mean three-day maximum humidex and temperature preceding ED presentation. Results: The mean maximum humidex and temperature values for the 2016-2018 summers in Montreal, QC were 30.4 and 26.1°C, respectively (n = 276 days). Elevated mean three-day maximum humidex was associated with increased ED volumes (F[3,88] = 4.2,p = 0.008) and median LOS (F[3,88] = 7.7,p = 0.0001) in the NDH. Mean three-day maximum humidex was associated with ED volumes (F[3,272) = 2.9,p = 0.03) but not with median and maximum LOS (p > 0.05) in the VH. Parallel comparisons with mean three-day maximum temperature similarly showed an association with increased ED volumes (F[3,88] = 5.0,p = 0.003) and increased duration of median LOS (F[3,88] = 3.5,p = 0.02) in the NDH. Mean three-day maximum temperature was associated with increased ED volumes (F[3,272] = 3.3,p = 0.02) but not with median and maximum LOS (p > 0.05) in the VH. Conclusion: Warming climates are associated with an increased number of ED presentations and longer median ED LOS. As heat events disproportionately impacted NDH, future investigations need to determine why these two hospitals were affected differently. This study provides local evidence that climate change can disrupt emergency services by increasing the demand for and delaying timely care. This is the first study that the authors are aware of that demonstrates these findings. Hospitals need to be climate ready. Heat waves often happen during times when summer bed closures and vacations already impact system capacity. EDs should dynamically adapt to meet community needs during periods of extreme heat.