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Kim Knowlton,^1,2 Miriam Rotkin-Ellman,³ Galatea King,⁴ Helene G. Margolis,^4,5 Daniel Smith,⁴ Gina Solomon,^3,6,7Roger Trent,⁸ and Paul English⁴

¹Health and Environment Program, Natural Resources Defense Council, New York, New York, USA; ²Environmental Health Sciences Department, Mailman School of Public Health, Columbia University, New York, New York, USA; ³Health and Environment Program, Natural Resources Defense Council, San Francisco, California, USA; ⁴California Department of Public Health, Environmental Health Investigations Branch, Richmond, California, USA; ⁵Department of Internal Medicine, School of Medicine, University of California Davis, Sacramento, California, USA; ⁶Pediatric Environmental Health Specialty Unit, California Poison Control System San Francisco, University of California San Francisco, San Francisco, California, USA; ⁷School of Medicine, University of California San Francisco, San Francisco, California, USA; ⁸California Department of Public Health, Epidemiology and Prevention for Injury Control Branch, Sacramento, California, USA

Abstract
Background: Climate models project that heat waves will increase in frequency and severity. Despite many studies of mortality from heat waves, few studies have examined morbidity.

Objectives: In this study we investigated whether any age or race/ethnicity groups experienced increased hospitalizations and emergency department (ED) visits overall or for selected illnesses during the 2006 California heat wave.

Methods: We aggregated county-level hospitalizations and ED visits for all causes and for 10 cause groups into six geographic regions of California. We calculated excess morbidity and rate ratios (RRs) during the heat wave (15 July to 1 August 2006) and compared these data with those of a reference period (8–14 July and 12–22 August 2006) .

Results: During the heat wave, 16,166 excess ED visits and 1,182 excess hospitalizations occurred statewide. ED visits for heat-related causes increased across the state [RR = 6.30 ; 95% confidence interval (CI) , 5.67–7.01], especially in the Central Coast region, which includes San Francisco. Children (0–4 years of age) and the elderly (≥ 65 years of age) were at greatest risk. ED visits also showed significant increases for acute renal failure, cardiovascular diseases, diabetes, electrolyte imbalance, and nephritis. We observed significantly elevated RRs for hospitalizations for heat-related illnesses (RR = 10.15 ; 95% CI, 7.79–13.43) , acute renal failure, electrolyte imbalance, and nephritis.

Conclusions: The 2006 California heat wave had a substantial effect on morbidity, including regions with relatively modest temperatures. This suggests that population acclimatization and adaptive capacity influenced risk. By better understanding these impacts and population vulnerabilities, local communities can improve heat wave preparedness to cope with a globally warming future.

Key words: climate change, electrolyte imbalance, emergency department, global warming, heat stroke, heat wave, hospitalization, morbidity, renal failure, temperature. Environ Health Perspect 117:61–67 (2009) . doi:10.1289/ehp.11594 available via http://dx.doi.org/ [Online 22 August 2008]

Address correspondence to K. Knowlton, Natural Resources Defense Council, 40 West 20th St., New York, NY 10011-4231 USA. Telephone: (212) 727-2700. Fax: (212) 727-1773. E-mail: kknowlton@nrdc.org

We thank E. Roberts and S. Hoshiko of the California Department of Public Health, Environmental Health Investigations Branch, who provided invaluable comments on this analysis. We also thank C. Horn and A. Horn for their support of Natural Resources Defense Council’s Global Warming and Health Project.

This work was supported in part by Centers for Disease Control and Prevention cooperative agreement U38/EH000186-01.

K.K., M.R.-E., and G.S. are employed by the Natural Resources Defense Council, a nonprofit environmental advocacy organization. The remaining authors declare they have no competing financial interests.

Received 21 April 2008 ; accepted 22 August 2008.