Surface dust samples were collected from the two important industrial cities of semi-arid Lanzhou and arid Urumqi in Northwest China. Eighteen polycyclic aromatic hydrocarbons (PAHs) in these samples were analyzed for their sources and human exposure health risks based on positive matrix factorization and Monte Carlo simulation. The total concentration of 18 PAHs (Sigma 18PAHs) in Lanzhou and Urumqi ranged from 0.652 to 62.1 and 0.0209 to 21.7 mg/kg, with the median of 2.48 and 1.17 mg/kg, attributing to 4- and 5- ring PAHs. Higher Sigma 18PAHs were found in the industrial areas of the two cities, spatially concentrating in the east and west of Lanzhou and the south and north of Urumqi. PAHs in Lanzhou and Urumqi originated from vehicle emissions (36.3 % and 20.7 %), petroleum products leakage (10.9 % and 18.2 %), vehicle emissions and biomass combustion (23.3 % and 30 %), and coal combustion (29.5 % and 31.1 %). The carcinogenic risk of adults and children exposed to PAHs in urban surface dust was 3.7 x 10(-6) and 4.8 x 10(-6) in Lanzhou as well as 1.1 x 10(-6) and 1.5 x 10(-6) in Urumqi, suggesting to be acceptable. The carcinogenic risk was primarily attributed to vehicle emissions (82.6 %) in Lanzhou, while a combination of vehicle emissions and biomass combustion (93.5 %) in Urumqi.