This study investigates the presence of thirdhand smoke pollution in low-income, multi-unit buildings in San Diego County. Thirdhand smoke (THS) is the lingering, toxic residue left behind from second-hand smoke that can mix with other indoor contaminants to create additional carcinogenic chemicals. Multi-unit housing increases the likelihood of shared airspace, which can contribute to THS contamination in the homes of nonsmokers; additionally, rental turnover rates of low income buildings may contribute to THS in nonsmoking homes. Twelve low income buildings were identified in San Diego County and 6% of each building's apartments were recruited for the study, resulting in 109 visits. Surface wipes were collected in each home, while participants were asked to complete a survey, regarding smoking habits and policies. Building managers were also interviewed. Surface wipe samples were analyzed for nicotine levels through isotope dilution LC-MS/MS. Homes where residents reported smoking indoors had a geometric mean (95% confidence interval) nicotine level of 144.4 µg/m² (41.3-498.6 µg/m²), smoking outdoors 7.6 µg/m² (3.8-14.4 µg/m²), and non-smoking 3.1 µg/m² (2.0-4.5 µg/m²). Of the non-smokers (never smokers) homes, 28.4 percent were higher than 3.1 µg/m² (the lower bound of smokers' homes in a previous study by the SDSU group) and almost 80% were higher than our previous 0.14 µg/m² cut-off for non-smoking homes. Extraordinarily high values (2586.2 & 3926.2 µg/m² ) were reported in homes with former smoking occupants, including one that had not been smoked in for nine years. These participants reported heavy indoor smoking for seven and twelve years respectively, but discontinued tobacco use due to serious smoking-related health complications. These results demonstrate THS, as measured by nicotine contamination, is persistent in the indoor environment, and can remain at extraordinarily high levels numerous years after residents have discontinued smoking indoors or altogether. Based on these results, occupants of low-income housing have higher levels of THS contamination than reported in previous studies. The most vulnerable occupants are infants and children, due to hand to mouth behaviors and developing organs. This study is part of a larger study investigating remediation procedures to potentially reduce THS exposure.