The present work aimed to assess exposure of preschool children to polycyclic aromatic hydrocarbons (PAHs) by environmental monitoring (eighteen compounds in air) and biomonitoring (six urinary biomarkers of exposure (OH-PAHs)). The impact of preschool indoor air on excretion of urinary mono hydroxyl metabolites was also evaluated. Gaseous and particulate-bound PAHs were simultaneously collected indoors and outdoors in two Portuguese preschools. PAHs and OH-PAHs were quantified by high-performance liquid chromatography with fluorescence and photodiode array detection. Total air (gaseous + total suspended particles) levels of PAHs (Sigma PAHs) were higher indoors than outdoors. Gaseous phase (composed by >= 98% of 2-3 rings compounds) and particulate-bound PAHs (90-99% of 5-6 rings) accounted for 93-95% and 5-7% of Sigma PAHs in indoor air, respectively. Total (including probable/possible) carcinogenic PAHs represented 26-45% of Sigma PAHs; naphthalene and dibenz[a,h]anthracene were the strongest contributors. A similar distribution profile was observed between airborne PAHs and urinary OH-PAHs. Urinary 1-hydroxynaphthalene + 1-hydroxyacenaphthene represented more than 78% of Sigma OH-PAHs, being followed by 2-hydroxyfluorene, 1-hydroxypyrene, and 1-hydroxyphenanthrene. 3-hydroxybenzo[a]pyrene (PAH biomarker of carcinogenicity) was not detected. Results suggest that children had preschool indoor air as their major exposure source of naphthalene and acenaphthene, while no conclusion was reached regarding fluorene, phenanthrene and pyrene. (C) 2016 Elsevier B.V. All rights reserved.