“Threshold Findings” in an Ecological Study : Epidemiology

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“Threshold Findings” in an Ecological Study

Correia, Andrew W.; Wang, Yun; Dominici, Francesca; Pope, C. Arden III; Dockery, Douglas W.; Ezzati, Majid

Author Information
Epidemiology 24(4):p 628, July 2013. | DOI: 10.1097/EDE.0b013e3182953de0

The authors respond:

We thank Dr. Morfeld for his interest in and comments1 regarding our article.2 We disagree, however, with the assertion that the results are uninformative about potential threshold effects. Our findings that continued reductions in air pollution were associated with improved life expectancies even during 2000–2007, and the finding that the association between increased life expectancy and reduced air pollution was not appreciably influenced by baseline pollution levels provides important evidence regarding threshold effects. We agree with Morfeld that these findings do not exclude the possibility of a threshold at very low levels of pollution. These results do suggest, however, that even at the relatively low levels of pollution in the United States during the time-period of the study, gains in air quality had public health benefit.

Andrew W. Correia

Yun Wang

Department of Biostatistics

Harvard School of Public Health

Boston, MA

Francesca Dominici

Department of Biostatistics

Harvard School of Public Health

Boston, MA

[email protected]

C. Arden Pope III

Department of Economics

Brigham Young University

Provo, UT

Douglas W. Dockery

Departments of Environmental Health and Epidemiology

Harvard School of Public Health

Boston, MA

Majid Ezzati

MRC-HPA Centre for Environment and Health and Department of Epidemiology and Biostatistics

Imperial College London

London, United Kingdom


1. Morfeld P. “Threshold findings” in an ecological study [letter]. Epidemiology. 2013;24:627–628
2. Correia AW, Pope CA 3rd, Dockery DW, Wang Y, Ezzati M, Dominici F. Effect of air pollution control on life expectancy in the United States: An analysis of 545 U.S. counties for the period from 2000 to 2007. Epidemiology. 2013;24:23–31
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