Sewer construction in Taiwan lags other developed nations, and the authorities are undertaking major sewerage system construction projects in several cities. In Kaohsiung City, sewerage networks pass through residential, commercial and industrial areas. The composition of sewage thus is highly complicated. Eight target monitoring positions are chosen to analyze the compounds and concentrations of hazardous air pollutants. Pollutant concentrations are used to evaluate the cancer risk increment based on inhalation intake for sewer workers under using exhaust ventilation and wearing personal gas filtering equipment. GC/MS analysis confirmed that concentrations of benzene and trichloromethane compounds in sewer air for all the monitoring positions exceeded the minimum risk levels (MRLs) of 0.009 ppm benzene and 0.1 ppm trichloromethane, and the maximum concentrations reached 148.4 and 327.3 ppm, respectively. The cancer risks of benzene and trichloromethane for workers without personal protection approached 2.77–3.98 × 10-3 and 29.74–42.70 × 10-3, respectively. Through ventilation for 15 minutes and the wearing of gas filtering equipment, the cancer risks for benzene and trichloromethane were reduced to 0.0003–0.0004 × 10-3 and 0.0029–0.0041 × 10-3, respectively. The authorities thus must order all workers to follow a strict code of practice for sewer entry before entering sewer systems. This code of practice should include a minimum time for general exhaust ventilation and the use of personal protection equipment.