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A number of fires involving polychlorinated biphenyl (PCB)-containing transformers and capacitors have occurred in the United States. PCB fires generate by-products such as polychlorinated dibenzofurans (PCDFs) and polychlorinated dibenzodioxins (PCDDs) and, when the transformer is in a building, contaminate the interior. Considerable concern exists over the potential human health effects associated with exposure by inhabitants to residual levels of PCBs, PCDFs and PCDDs. Office workers, for example, may be exposed to these chlorinated compounds via inhalation of contaminated particulates and vapors, dermal contact with contaminated surfaces, and incidental ingestion of dusts. A wide range of re-entry or cleanup levels have been developed for PCDDs and PCBs to protect workers who re-occupy a building following a PCB fire. Re-entry criteria have been used by property owners and regulatory agencies to determine whether the building is safe to re-occupy or to determine the extent of needed remediation. This paper presents a mass balance approach to deriving risk-based re-entry surface and air criteria for PCBs and PCDD/PCDFs. These criteria were based on a lifetime risk level of 10(-5), recent toxicological data on PCDDs and PCBs, and plausible exposure scenarios. Our analysis suggests that 125 ng/m2 2,3,7,8-TCDD TEQ for surfaces and 10 pg/m3 for air are acceptable. Based on Aroclor 1260, risk-based re-entry criteria for PCBs on surfaces and in air were 750 micrograms/m2 and 0.1 microgram/m3, respectively. In comparison to most previous guidelines, these risk-based criteria are less stringent, but can still be considered conservative. The surface criteria are 5 to up to 125 fold higher than previous guidelines. Air criteria range up to 5 times higher than criteria used at past PCB fire sites. Air concentrations associated with these were modeled and were negligible. For PCBs in air, the NIOSH guideline of 1 microgram/m3 is also appropriate for occupational settings.