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Chit Ming Wong,¹ Lin Yang,¹ Thuan Quoc Thach,¹ Patsy Yuen Kwan Chau,¹ King Pan Chan,¹ G. Neil Thomas,¹ Tai Hing Lam,¹ Tze Wai Wong,² Anthony J. Hedley,¹ and J.S. Malik Peiris³

¹Department of Community Medicine and School of Public Health, University of Hong Kong, Hong Kong Special Administration Region, People's Republic of China; ²Department of Family and Community Medicine, Chinese University of Hong Kong, Hong Kong Special Administration Region, People's Republic of China; ³Department of Microbiology, University of Hong Kong, Hong Kong Special Administration Region, People's Republic of China

Abstract
Background: Both influenza viruses and air pollutants have been well documented as major hazards to human health, but few epidemiologic studies have assessed effect modification of influenza on health effects of ambient air pollutants.

Objectives: We aimed to assess modifying effects of influenza on health effects of ambient air pollutants.

Methods: We applied Poisson regression to daily numbers of hospitalizations and mortality to develop core models after adjustment for potential time-varying confounding variables. We assessed modification of influenza by adding variables for concentrations of single ambient air pollutants and proportions of influenza-positive specimens (influenza intensity) and their cross-product terms.

Results: We found significant effect modification of influenza (p < 0.05) for effects of ozone. When influenza intensity is assumed to increase from 0% to 10%, the excess risks per 10-µg/m³ increase in concentration of O₃ increased 0.24% and 0.40% for hospitalization of respiratory disease in the all-ages group and ≥ 65 year age group, respectively ; 0.46% for hospitalization of acute respiratory disease in the all-ages group ; and 0.40% for hospitalization of chronic obstructive pulmonary disease in the ≥ 65 group. The estimated increases in the excess risks for mortality of respiratory disease and chronic obstructive pulmonary disease in the all-ages group were 0.59% and 1.05%, respectively. We found no significant modification of influenza on effects of other pollutants in most disease outcomes under study.

Conclusions: Influenza activity could be an effect modifier for the health effects of air pollutants particularly for O₃ and should be considered in the studies for short-term effects of air pollutants on health.

Key words: air pollution, cardiorespiratory disease, effect modification, hospitalization, influenza, mortality. Environ Health Perspect 117:248–253 (2009) . doi:10.1289/ehp.11605 available via http://dx.doi.org/ [Online 3 October 2008]

Address correspondence to A.J. Hedley, Department of Community Medicine and School of Public Health, University of Hong Kong, 5th Floor, William MW Mong Block, Faculty of Medicine Building, 21 Sassoon Rd., Hong Kong SAR, China. Telephone: 852-2819-9282. Fax: 852-2855-9528. E-mail: hrmrajh@hkucc.hku.hk

Supplemental Material is available online at http://www.ehponline.org/members/2008/11605/suppl.pdf

We thank the Hong Kong Environmental Protection Department, Census, and Statistics Department and Hospital Authority of Hong Kong for providing the data.

This study is part of the Public Health and Air Pollution in Asia project, funded by the Health Effects Institute in the United States (agreement 4713-RFIQ03-3/04-9-2) .

The authors declare they have no competing financial interests.

Received 3 April 2008 ; accepted 3 October 2008.

Correction

In Table 1, the values for ARD were incorrect in the manuscript originally published online. They have been corrected here.