Conventional air pollutants (PM10, CO, NOx) gradually increased from fall to winter during 2015 in Istanbul. Several air pollution episodes were observed during this period. This study was made in order to determine polycyclic aromatic hydrocarbon (PAH) levels, identify the sources of air pollution, and make toxicity assessment based on Benzo(a)pyrene equivalent concentrations. The sampling took 14 sequential days during winter. High-pressure weather conditions prevailed at the start of the sampling. The conditions were then changed to low-pressure condition towards the end of the sampling. Strong inversion was effective on the onset of the sampling. Strong inversion was effective at the onset of the sampling. A high-volume sampler was used to collect gas and particle phase samples. Total suspended particle concentrations were between 27 and 252gm(-3). Sixteen PAH species were investigated. Total (gas+particle) PAH concentrations were between 76.4 and 1280.3ngm(-3), with an average of 301.4ngm(-3). Individual PAH concentrations were between not detected (n.d.) and 99.2ngm(-3) in the gaseous phase, and between n.d. and 11.5ngm(-3) in the particle phase. Phenanthrene had the highest share among 16 PAH compounds. Benzo(a)pyrene was not detected in 8days. On the remaining days, its concentration ranged between 5.5 and 14.8ngm(-3) with an average of 3.7ngm(-3). Low-molecular-weight PAHs dominated gaseous phase; inversely, high-molecular-weight PAHs dominated particle phase. Possible sources were identified by diagnostic ratios. These ratios suggested that coal combustion and diesel vehicle exhaust emissions had a substantial impact on ambient air quality. Benzo(a)pyrene equivalencies were calculated for each PAH compound in order to make toxicity assessment. Total benzo(a)pyrene equivalencies ranged between 0.4 and 30.0ngm(-3) with an average of 7.2ngm(-3).