**2. Homelessness, mental health and substance use disorders**

A practical definition of substance use is one of the most commonly described and identified health risks among people experiencing homelessness [25]. Homelessness is defined as the absence of a permanent home. Individuals and families may live on the streets, in a shelter, a single room occupancy facility, abandoned building or vehicle, or in any other unstable or non-permanent living arrangement (Section 330 of the Public Health Service Act (42U.S.C., 254b)). For more than a decade, studies reporting on the experiences of homeless persons have examined the association between living arrangement and substance use disorder treatment program characteristics, because housing instability is known to impact mental health and SUD prevention and intervention efforts [17, 26]. Substance use among older adults, specifically who are homeless, is associated with decreased mobility of physical and mental health capacity [25] and early onset morbidity and mortality [27]. For these reasons, more attention on older adults as a special population of concerns for mental health prevention and intervention is warranted.
