Increased hospital admissions associated with extreme-heat exposure in King County, Washington, 1990-2010

Increased morbidity and mortality have been associated with extreme heat events, particularly in temperate climates. Few epidemiologic studies have considered the impact of extreme heat events on hospitalization rates in the Pacific Northwest region. This study quantifies the historic (May to September 1990-2010) heat-morbidity relationship in the most populous Pacific Northwest County, King County, Washington. A relative risk (RR) analysis was used to explore the association between heat and all non-traumatic hospitalizations on 99th percentile heat days, whereas a time series analysis using a piecewise linear model approximation was used to estimate the effect of heat intensity on hospitalizations, adjusted for temporal trends and day of the week. A non-statistically significant 2% [95%CI: 1.02 (0.98, 1.05)] increase in hospitalization risk, on a heat day vs. a non-heat day, was noted for allages and all non-traumatic causes. When considering the effect of heat intensity on admissions, we found a statistically significant 1.59% (95%CI: 0.9%, 2.29%) increase in admissions per degree increase in humidex above 37.4 degrees C. Admissions stratified by cause and age produced statistically significant results with both relative risk and time series analyses for nephritis and nephrotic syndromes, acute renal failure, and natural heat exposure hospitalizations. This study demonstrates that heat, expressed as humidex, is associated with increased hospital admissions. When stratified by age and cause of admission, the non-elderly age groups (<85 years) experience significant risk for nephritis and nephrotic syndromes, acute renal failure, natural heat exposure, chronic obstructive pulmonary disease, and asthma hospitalizations.