Indoor and outdoor concentrations of polycyclic aromatic hydrocarbons (PAHs), oxygenated-PAHs (OPAHs), and nitro-PAHs (NPAHs) associated with PM_2.5 particles were monitored in a middle-school classroom from 8 to 22 March 2012 in Xi'an, China. The total PAHs ranged from 49.6 to 140.0 ng/m³ in outdoors and 50.3 to 111.6 ng/m³ in indoors, while OPAHs and NPAHs showed averages of 19.1 and 16.4 ng/m³, 0.1039 and 0.0785 ng/m³ for outdoor and indoor air, respectively. Strong correlations were found between indoor (I) and outdoor (O), and the I/O ratios were < 1 for almost all the species, indicating that outdoor air was the dominant influence on indoor air. Diagnostic ratios and Positive Matrix Factorization (PMF) analysis indicated that biomass burning, coal combustion, and motor vehicle emissions were the main sources for PAHs (which accounted for 30%, 27.4% and 26%, respectively, by PMF), but, secondary particle formation was important for the OPAHs and NPAHs. Inhalation cancer risks associated with outdoor and indoor particles were 6.05 × 10^{- 5} and 5.44 × 10^{- 5}, respectively, and so higher than the cancer risk guideline of 10^{- 6}. Although the cancer risk of NPAHs is negligible for its lower concentrations, their potential for direct mutagenic effects should not be ignored.

PM_2.5 bound 19 PAHs, 3OPAHs, and 8NPAHs were monitored in a middle school in Xi'an.

Indoor pollutants were mostly influenced by outdoor air.

Biomass burning, coal combustion, and vehicle emission were main sources for PAHs.

Secondary formation was the main source for OPAHs and NPAHs.

Inhalation cancer risks assessment of PAHs showed there were potential health risks for students during sampling time.