Int. J. Environ. Res. Public Health 2009, 6, 1633-1648; doi:10.3390/ijerph6051633 OPEN ACCESS International Journal of Environmental Research and Public Health ISSN 1660-4601 www.mdpi.com/journal/ijerph Article Who Is Exposed to Secondhand Smoke? Self-Reported and Serum Cotinine Measured Exposure in the U.S., 1999-2006 Wendy Max *, Hai-Yen Sung and Yanling Shi Institute for Health & Aging, University of California, San Francisco, 3333 California Street, Suite 340, San Francisco, CA 94118, USA; E-Mails: hai-yen.sung@ucsf.edu (H.Y.S.); yanling.shi@ucsf.edu (Y.S.) * Author to whom correspondence should be addressed; E-Mail: Wendy.max@ucsf.edu; Tel.: +1-415 476-8023; Fax: +1-415 476-3915 Received: 18 April 2009 / Accepted: 7 May 2009 / Published: 14 May 2009 Abstract: This study presents self-reported and serum cotinine measures of exposure to secondhand smoke (SHS) for nonsmoking children, adolescents, and adults. Estimates are disaggregated by time periods and sociodemographic characteristics based on analyses of the 1999-2006 National Health and Nutrition Examination Survey. Self-reported exposure rates are found to be highest for children, followed by adolescents and adults. Important differences in exposure are found by socioeconomic characteristics. Using serum cotinine to measure exposure yields much higher prevalence rates than self-reports. Rates of SHS exposure remain high, but cotinine levels are declining for most groups. Keywords: environmental tobacco smoke; secondhand smoke (SHS); passive smoking; serum cotinine; self-reported exposure 1. Introduction Secondhand smoke (SHS) exposure, also known as passive smoking or exposure to environmental tobacco smoke, is known to be associated with a number of health effects, including respiratory illness, cancer and heart disease. Exposure has been shown to have an impact on children, adolescents and adults. These negative health impacts have been recently documented in several substantial reports which summarize the literature linking SHS exposure to respiratory effects in children and adult