With the upcoming transition to clean electric vehicles, the sources of volatile organic compounds (VOCs) in the ambient environment are rapidly changing and highly uncertain. Here, through systematic characterization of emissions from a typical apartment in a Chinese megacity (Shenzhen), we show that indoor environments contribute significantly to the levels of ambient (i.e., outdoor) VOCs. In particular, we observe that the majority of indoor VOCs originate from unoccupied spaces, demonstrating temperature-dependent release from indoor surface reservoirs. The total indoor-to-outdoor VOC emission rates varied from 53 to 2300 mg day