The Role of Ambient Ozone in Epidemiologic Studies of Heat-Related Mortality

Colleen E. Reid,¹ Jonathan M. Snowden,^2,3 Caitlin Kontgis,⁴ and Ira B. Tager²

¹Department of Environmental Health Sciences, and ²Division of Epidemiology, School of Public Health, University of California, Berkeley, Berkeley, California, USA; ³Department of Obstetrics and Gynecology, Oregon Health and Science University, Portland, Oregon, USA; ⁴Department of Geography, Center for Sustainability and the Global Environment, University of Wisconsin–Madison, Madison, Wisconsin, USA

Abstract

Background: A large and growing literature investigating the role of extreme heat on mortality has conceptualized the role of ambient ozone in various ways, sometimes treating it as a confounder, sometimes as an effect modifier, and sometimes as a co-exposure. Thus, there is a lack of consensus about the roles that temperature and ozone together play in causing mortality.

Objectives: We applied directed acyclic graphs (DAGs) to the topic of heat-related mortality to graphically represent the subject matter behind the research questions and to provide insight on the analytical options available.

Discussion: On the basis of the subject matter encoded in the graphs, we assert that the role of ozone in studies of temperature and mortality is a causal intermediate that is affected by temperature and that can also affect mortality, rather than a confounder.

Conclusions: We discuss possible questions of interest implied by this causal structure and propose areas of future work to further clarify the role of air pollutants in epidemiologic studies of extreme temperature.

Key words: causality, confounding variables, epidemiology, extreme heat, mortality, ozone.

Environ Health Perspect 120:1627–1630 (2012). http://dx.doi.org/10.1289/ehp.1205251 [Online 16 August 2012]

Address correspondence to C.E. Reid, 50 University Hall, Berkeley, CA 94720-7360 USA. Telephone: (510) 642-9530. Fax: (510) 642-5815. E-mail: creid@berkeley.edu

This work was partially supported by a U.S. Environmental Protection Agency Science to Achieve Results (STAR) fellowship (FP917200010).

The authors declare they have no actual or potential competing financial interests.

Received 23 March 2012; Accepted 16 August 2012; Online 16 August 2012.