A cancer risk assessment for an office building contaminated with polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) was performed. Concentrations were obtained of polychlorinated biphenyls (PCBs), total PCDDs, total PCDFs, and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) (1746016) measured in the ambient air, airborne particulates, and on various surfaces in the Binghamton State Office Building, Binghamton, New York, which was contaminated with PCBs as a result of a transformer fire. Data were used to calculate the exposures experienced by persons working in the building 250 days per year for 30 years. The calculations were based on extrapolating data obtained in animal bioassays and from the no observed effect level determined in animal evaluations. It was assumed that the contaminant concentrations remained constant during the 30 year period (scenario-A), that they decreased by 99 percent over the 30 year period according to first order kinetics (scenario-B), or they decreased with a first order halflife of 1 year (scenario-C). The exposures were calculated as TCDD equivalent, PCB, 2,3,7,8-tetrachlorodibenzofuran (51207319) (TCDF), or TCDD concentrations that would produce a cancer risk of 1x10(-2). When expressed as TCDD equivalents, the air exposures leading to a cancer risk of 1x10(-2) for scenario-A, scenario-B, and scenario-C were 4.8x10(-7), 2.3x10(-6), and 1.0x10(-5) nanograms (ng) per cubic meter (m3), respectively. The corresponding surface exposures were 0.020, 0.010, and 0.44ng per square meter (m2), respectively. The PCB air concentrations that would result in a 1x10(-2) cancer risk were 5.1x10(-4), 2.4x10(-3), and 1.1x10(-2)ng/m3 according to scenario-A, scenario-B, and scenario-C, respectively. The corresponding surface concentrations were 21, 110, and 470ng/m2, respectively. The air TCDD exposures that would result in a cancer risk of 1x10(-2) were 2.8x10(-5), 1.3x10(-4), and 5.8x10(-4)ng/m3 for scenario-A, scenario-B, and scenario-C, respectively. The corresponding air TCDF exposures were 0.002, 0.0091, and 0.041ng/m3, respectively.