Evidence supports active smoking as a major source of exposure to polycyclic aromatic hydrocarbons (PAH), compounds that are mutagenic and carcinogenic in humans. The influence of involuntary exposure to tobacco smoke on PAH exposure levels among nonsmokers, however, is unknown. This study evaluated the association between both active and involuntary tobacco smoke and biomarkers of PAH exposure in the general U.S. population. A cross-sectional analysis of 5,060 participants>or=6 years of age was done using data from the 1999-2002 National Health and Nutrition Examination Survey (NHANES). PAH exposure was measured by urinary concentrations of 23 monohydroxylated metabolites of nine PAH compounds. Tobacco smoke exposure was defined as no exposure, involuntary exposure, and active exposure by combining serum cotinine levels, smoking status, and presence of household smokers. PAH metabolite levels ranged from 33.9 ng/L for 9-hydroxyphenanthrene to 2,465.4 ng/L for 2-hydroxynaphthalene. After adjustment for age, sex, race/ethnicity, education, household income, and broiled/grilled food consumption, participants involuntarily and actively exposed to tobacco smoke had urinary metabolite concentrations that were increased by a factor of 1.1 to 1.4 and 1.5 to 6.9, respectively, compared with unexposed participants. Associations for involuntary smoking were stronger and statistically significant for 1-hydroxypyrene, 2-hydroxyfluorene, 3-hydroxyfluorene, 9-hydroxyfluorene, 1-hydroxyphenanthrene, 2-hydroxyphenanthrene, and 3-hydroxyphenanthrene compared with other metabolites. Involuntary exposure to tobacco smoke was associated with elevated urinary concentrations of most PAH metabolites in a representative sample of the U.S. population. Policy and educational efforts must continue to minimize PAH exposure through active and involuntary tobacco smoke exposure.