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Effects of Heat Waves on Mortality: Effect Modification and Confounding by Air Pollutants

Analitis, Antonisa; Michelozzi, Paolab; D’Ippoliti, Danielab; de’Donato, Francescab; Menne, Bettinac; Matthies, Franziskac; Atkinson, Richard W.d; Iñiguez, Carmene,f,g; Basagaña, Xavierf,h,i; Schneider, Alexandraj; Lefranc, Agnèsk; Paldy, Annal; Bisanti, Luigim; Katsouyanni, Kleaa

doi: 10.1097/EDE.0b013e31828ac01b
Climate Change

Background: Heat waves and air pollution are both associated with increased mortality. Their joint effects are less well understood.

Methods: We explored the role of air pollution in modifying the effects of heat waves on mortality, within the EuroHEAT project. Daily mortality, meteorologic, and air pollution data from nine European cities for the years 1990–2004 were assembled. We defined heat waves by taking both intensity and duration into account. The city-specific effects of heat wave episodes were estimated using generalized estimating equation models, adjusting for potential confounders with and without inclusion of air pollutants (particles, ozone, nitrogen dioxide, sulphur dioxide, carbon monoxide). To investigate effect modification, we introduced an interaction term between heat waves and each single pollutant in the models. Random effects meta-analysis was used to summarize the city-specific results.

Results: The increase in the number of daily deaths during heat wave episodes was 54% higher on high ozone days compared with low, among people age 75–84 years. The heat wave effect on high PM10 days was increased by 36% and 106% in the 75–84 year and 85+ year age groups, respectively. A similar pattern was observed for effects on cardiovascular mortality. Effect modification was less evident for respiratory mortality, although the heat wave effect itself was greater for this cause of death. The heat wave effect was smaller (15–30%) after adjustment for ozone or PM10.

Conclusions: The heat wave effect on mortality was larger during high ozone or high PM10 days. When assessing the effect of heat waves on mortality, lack of adjustment for ozone and especially PM10 overestimates effect parameters. This bias has implications for public health policy.

Supplemental Digital Content is available in the text.

From the aDepartment of Hygiene, Epidemiology and Medical Statistics, Medical School, University of Athens, Athens, Greece; bDepartment of Epidemiology, Regional Health Authority, Rome, Italy; cWHO Regional Office for Europe, Bonn, Germany; dDivision of Population Health Sciences and Education and MRC-HPA Centre for Environment and Health, St George’s, University of London, London, United Kingdom; eCenter for Public Health Research, CSISP, Conselleria de Sanitat, Valencia, Spain; fCIBER de Epidemiología y Salud Pública, CIBERESP, Barcelona, Spain; gUniversity of Valencia, Spain; hCentre for Research in Environmental Epidemiology (CREAL), Barcelona, Spain; iIMIM (Hospital del Mar Research Institute), Barcelona, Spain; jHelmholtz Zentrum München, German Research Center for Environmental Health, Institute of Epidemiology II, Munich, Germany; kDepartment of Environmental Health, French Institute for Public Health Surveillance, Paris, France; lNational Institute of Environmental Health, Budapest, Hungary; and mEpidemiology Unit, Local Health Authority, Milan, Italy.

EuroHEAT Project was cofunded by the European Commission DG SANCO (grant agreement no. 2004322).

Supplemental digital content is available through direct URL citations in the HTML and PDF versions of this article ( This content is not peer-reviewed or copy-edited; it is the sole responsibility of the author.

Correspondence: Antonis Analitis, Department of Hygiene, Epidemiology and Medical Statistics, Medical School, University of Athens, 75 Mikras Asias Street, 115 27 Athens, Greece. E-mail:

© 2014 by Lippincott Williams & Wilkins, Inc