Globally, it is estimated that over 150 million people experience homelessness . In the United States (U.S.) alone, over half a million people experience homelessness on a given night . Homelessness is associated with significant health disparities. All-cause mortality rates among people experiencing homelessness in the U.S. are 4.5 to 9.6-times higher than the general population  with higher incidence of drug overdose, substance use disorders, cancer, and heart disease driving disparities in mortality rates [3,4,5]. Tobacco use, particularly combustible cigarette smoking, also plays a significant role in widening health disparities between people experiencing homelessness and the general population. During a time when combustible tobacco use is at an all-time low in the U.S. general population, tobacco use remains common among people experiencing homelessness, with approximately three-quarters reporting current smoking . However, less is known regarding electronic cigarette (e-cigarette) use among this population and the role it might play in widening or narrowing disparities. E-cigarette use is increasingly common among the general population in the U.S., with close to 7 million adult users . Pod-based e-cigarettes, such as JUUL, entered the U.S. market in 2015 and have only accelerated the proliferation and popularity of e-cigarettes .
While current data suggest that e-cigarettes may be less harmful than combustible cigarettes , e-cigarettes are not harmless. Studies have shown e-cigarettes can deliver harmful and potentially harmful constituents including heavy metals, volatile organic compounds, and low levels of polycyclic aromatic hydrocarbons  and most contain highly-addictive nicotine [10, 11]. Dual use of combustible cigarettes and e-cigarette may be particularly harmful, especially to cardiovascular and respiratory health [9, 12, 13], with preliminary data showing potential associations between dual use and the pathophysiological respiratory changes associated with chronic obstructive pulmonary disease [12, 14] and cardiovascular disease . If rates of dual use among the people experiencing homelessness are high, this could be contributing to widening health disparities among this vulnerable population. Conversely, if people experiencing homelessness are fully transitioning from combustible smoking to less harmful e-cigarette use, this pattern of use may reduce health disparities. Thus, continued monitoring and investigation is warranted.
Few studies have investigated e-cigarette use among adults experiencing homelessness [15,16,17,18]. The limited data that do exist indicate rates of current e-cigarette use among homeless smokers of 12 to 51% [6, 15,16,17,18] with the highest estimates from samples limited to youth and young adult smokers [16, 17]. However, all existing data are from relatively small (< 500) samples, were collected in 2016 or earlier (and thus may not capture the full influx of pod devices onto the market which now account for over 70% of the retail e-cigarette market) , and do not report on changes in e-cigarette use rates over time. Further, these studies included only current tobacco users, which is problematic because it does not allow for estimates of rates of e-cigarette use patterns among all people experiencing homelessness (e.g., there is no data on the rate of exclusive e-cigarette use in this population).
There is minimal existing data on correlates of e-cigarette use among those experiencing homelessness. A 2014 survey of adults experiencing homelessness who smoke in Boston  found that subsistence difficulties (i.e., problems finding shelter, food, clothing, a place to wash/go to the bathroom) and younger age were associated with e-cigarette use. Similarly, a 2017–2018 survey of youth/young adults experiencing homelessness aged 13–25 in Los Angeles county  found that being around others who use e-cigarettes, ease of accessibility compared to combustible cigarettes, identifying as lesbian, gay, bisexual, transgender, queer, or asexual (LGBTQA), and symptoms of depression were related to higher likelihood of e-cigarette use. A better understanding of e-cigarette use correlates would allow for more targeted prevention and treatment efforts.
The current study aims, in an exploratory fashion, to understand 1) trends of e-cigarette and dual e-cigarette and combustible cigarette use among adults experiencing homeless from 2015 to 2018, 2) the correlates of e-cigarette use among participants in 2018, and 3) frequency of chronic health conditions by use trajectory (i.e., non-smokers, combustible cigarette smokers, e-cigarette users, and dual users) in the 2018 sample. The study uses a large, state-wide survey of adults experiencing homelessness in Minnesota to explore these questions.