Groundwater contamination associated with two former industrial facilities in Denver, Colorado, has led to concerns about vapor intrusion into residences adjacent to the facilities. 1,1,1‐Trichloroethane (1,1,1‐TCA), 1,1‐dichloroethene (1,1‐DCE), and trichloroethene (TCE) are the main contaminants of concern in groundwater, with trace levels of 1,2‐dichloroethane (1,2‐DCA) present at one of the sites. Indoor air monitoring programs have been ongoing at these two sites since 1998 and recent results have suggested that background, indoor source, 1,2‐DCA has been increasing in the frequency of detection, and median and maximum concentration over the past several years. A lines of evidence evaluation was undertaken for both sites in order to document the predominance of indoor sources of 1,2‐DCA. Evidence utilized included spatial evaluation of 1,2‐DCA in indoor air; comparison of 1,2‐DCA concentrations in mitigated and unmitigated homes; a phone survey to evaluate the potential for smoking to contribute to indoor air 1,2‐DCA levels; evaluation of mitigation system effluent data; and an evaluation of volatile organic compound (VOC) ratios in groundwater and indoor air. The results of this evaluation indicated that smoking had no demonstrable influence on measured indoor air concentrations. In addition, it appears that consumer products have had a markedly increased influence on indoor air concentrations since 2005. Data from one of the industrial facilities at one of the sites also indicated that polyvinyl chloride (PVC) and vinyl composite floor adhesive used in a building remodel in 2005 apparently generated elevated levels of indoor 1,2‐DCA and vinyl chloride, which have been sustained up to the present time.