Breast cancer (BC) is the most common cancer affecting women worldwide and has been associated with active tobacco smoking. Low levels of nicotine (Nic) and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), have been detected in cases of second-hand smoke (SHS). However, the correlation between SHS and BC risk remains controversial. In this study, we investigated whether the physiological SHS achievable dose of Nic and tobacco specific nitrosamine, NNK act together to induce breast carcinogenesis using an in vitro breast cell carcinogenesis model. Immortalized non-tumorigenic breast epithelial cell line, HBL-100 used for a time-course assay, was exposed to very low levels of either Nic or NNK, or both. The time-course assay consisted of 23 cycles of nitrosamines treatment. In each cycle, HBL-100 cells were exposed to 1pM of Nic and/or 100 femtM of NNK for 48 hours. Cells were passaged every 3 days and harvested after 10, 15, and 23 cycles. Our results demonstrated that the tumorigenicity of HBL-100, defined by soft agar colony forming, proliferation, migration and invasion abilities, was enhanced by co-exposure to physiologically SHS achievable doses of Nic and NNK. In addition, α9-nAChR signaling activation, which plays an important role in cellular proliferation and cell survival, was also observed. Importantly, an increase in stemness properties including the prevalence of CD44+/CD24− cells, increase Nanog expression and mammosphere-forming ability were also observed. Our results indicate that chronic and long term exposure to environmental tobacco smoke, may induce breast cell carcinogenesis even at extremely low doses. © 2018 Wiley Periodicals, Inc.