In recent years, cities around the world have seen extensive growth in unsheltered homelessness—or people sleeping in cars, parks, sidewalks, tents, abandoned buildings, or other makeshift shelters not meant for human habitation. In the United States, the number of people experiencing unsheltered homelessness increased by 28 percent between 2007 and 2015.

It is well documented that people experiencing homelessness have poorer health outcomes than the general population. However, less is known about how unsheltered people are faring compared to their sheltered counterparts living in public or private shelters, hotels, or other temporary arrangements.

To that end, Jessica Richards and Randall Kuhn of the University of California Los Angeles conducted a literature review to see what current research shows about unsheltered homelessness and health.¹ From an evaluation of 42 studies, here are their findings:

Being Unsheltered Is Tied to Poorer Health Outcomes

The authors found a consistent and strong association between unsheltered homelessness and high rates of chronic disease. For example, one study in Los Angeles found that unsheltered women had greater odds of being in fair or poor physical health and of having experienced pain in the last six months than sheltered homeless women. Another study conducted in Wales found that unsheltered individuals suffered from chronic conditions such as oral health diseases, orthopedic problems and arthritis, and vision impairment.

Being Unsheltered Is Often Paired With Serious Mental Illness

Schizophrenia and mood disorders, including major depression, are common diagnoses among unsheltered populations, according to studies from Brazil, Ethiopia, Ireland, Japan, and the United States. For example, in the Los Angeles study, unsheltered women had much greater odds of being in poor mental health than sheltered homeless women.

Being Unsheltered Makes Substance Use Worse

Unsheltered homeless populations are more likely to use substances than their sheltered peers, often a combination of alcohol, crack cocaine, heroin, and/or cannabis. For example, the Los Angeles study found that unsheltered women were more likely to have used alcohol or non-injecting drugs in the past six months than their sheltered counterparts. Along the same lines, a study in Harris County, Texas, found that sheltered young adults were less likely than their unsheltered counterparts to have used alcohol, marijuana, or synthetic marijuana in the previous month.

Being unsheltered also increases the risk of engaging in several risky behaviors. Among opioid-using veterans in New York City, being unsheltered was a significant predictor of engaging in behaviors that could lead to overdose. Among a cohort of out-of-treatment substance users, unsheltered people across race and sex (except for white women) were at a higher risk for HIV, according to a composite score that factored in drug use and sexual activity.

There is also evidence that substance use increases when people are unsheltered for longer periods of time. For example, among unsheltered adults in London, longer durations of unsheltered homelessness were accompanied by increased daily substance use, injection drug use, and drug dependency.

Being Unsheltered Is Linked to Chronic Homelessness, Which Can Exacerbate Serious Mental Illness and Substance Use

Chronically unsheltered individuals are more likely to have lifetime mental illness—defined as either a history of psychiatric hospitalization or current mental health counseling or treatment—than individuals who are not chronically unsheltered. Chronically unsheltered adults are also more likely to have a dual diagnosis of mental illness and substance use than those who aren’t chronically unsheltered. For example, unsheltered women in Los Angeles with recent substance use had much greater odds of poor mental health than sheltered women.

Being Unsheltered Is Linked to Low Health Care Use—Despite Significant Need

Unsheltered homelessness is associated with low rates of health care utilization. For example, older unsheltered adults in Oakland, California, were less likely to use primary care services than older homeless adults in other residential categories. In the Los Angeles study, women experiencing unsheltered homelessness were less likely than their sheltered counterparts to have seen a dentist or to have received either a Pap smear or a tuberculosis test in the past year. In another study, unsheltered homeless people who were current or former drug users were less likely to have sought out or received substance use treatment than sheltered homeless people.

Being Unsheltered Increases the Risk of Premature Death

These health disadvantages manifest in higher burdens of mortality. Unsheltered homeless populations have high rates of premature mortality. For example, mortality for unsheltered homeless people in Boston is nearly three times higher than for their sheltered counterparts, after adjusting for age.

Addressing the Health Needs of Unsheltered People Requires Further Research and Targeted Interventions, Researchers Say

Understanding the impact of unsheltered homelessness on health outcomes has important implications for the allocation of housing and health services, Richards and Kuhn write. The authors identify four main areas for additional research:

1. Examining the impact of unsheltered homelessness on injuries, communicable diseases, and sexual and reproductive health.
2. Exploring the relationship between the duration of sheltered and unsheltered homelessness and health outcomes, as well as the mediating mechanisms that lead to poor health among unsheltered populations.
3. Determining the specific causal mechanisms that drive poor health among unsheltered populations to improve the impact of street medicine and other street-based services.
4. Assessing the interaction of social inequality by race, gender, and sexuality with health outcomes among people experiencing unsheltered homelessness.

Despite these significant research gaps, immediate actions are needed to address the unique and severe health challenges of unsheltered populations, the researchers conclude.

This article was produced under a grant from the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD). The work of researchers from the NICHD-funded Population Dynamics Research Center at the University of California Los Angeles was highlighted.