In cities, the number of human deaths caused by heat waves is often the barometer of summer weather severity.

Yet mortality in urban areas is only a partial measure of the human toll of extreme hot weather. Now, a new study by researchers at the University of Wisconsin-Madison, Purdue University and the National Center for Climatic Research and appearing this week (June 22, 2011) in the journal Climatic Change, documents the medical conditions aggravated by hot weather, the age groups most affected, and forecasts an increase in hospital admissions in urban areas due to predicted climate change and accompanying weather extremes.

The study, which utilizes meteorological, air pollution and hospital admission data for the years 1989-2005 for the city of Milwaukee, is important because it documents the primary medical causes of heat-related hospital admission. The report also assesses potential future climate change and accompanying hot weather extremes and how those may affect vulnerable populations in the urban Midwestern United States.

"This is based on 17 years of hospital and acute care admissions in Milwaukee," explains Jonathan Patz, the senior author of the study and the director of UW-Madison's Global Health Institute. "We identified a number of types of illnesses that seem to be especially sensitive to hot temperatures."

Heat-sensitive illnesses and conditions identified by Patz's group include diabetes, urinary tract and renal diseases such as kidney stones, respiratory conditions, accidents and suicide attempts. Surprisingly, the study did not find an increase in the incidence of hospital admissions due to heart disease, but Patz and his colleagues speculate that acute episodes of heart disease may be more lethal and are therefore reflected in records of mortality. Mortality records were intentionally excluded from the current study.

Age groups most at risk for hospitalization during heat waves are the very young, five-years-of age and younger, and the very old, 85 years and older.

Importantly, the study identifies temperature thresholds that, when surpassed, tend to prompt increases in the incidence of particular conditions. For example, conditions such as diabetes and kidney disorders become more problematic, with hospital admissions for those conditions predicted to rise by 13 percent for every two degrees the mercury rises above 85 degrees Fahrenheit.

"Unlike previous studies, we identified threshold temperatures for several health outcomes beyond which disease rates will increase rapidly," according to Bo Li, a professor of statistics at Purdue University and the lead author of the new study.

The study also seeks to forecast increased incidence of heat-related diseases and conditions for the years 2059-2070 as predicted climate change is expected to increase the frequency and intensity of heat waves. In general, hospital admissions for heat sensitive conditions under one well-studied climate change scenario will increase by seven percent over time and hospitals and public health agencies will need to plan accordingly, notes Patz, who is also a professor in the Nelson Institute for Environmental Studies and in the Department of Population Health Sciences in the UW School of Medicine and Public Health.

The new study was supported by the U.S. Environmental Protection Agency under its STAR Grant Program.

University of Wisconsin-Madison

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