Even just a few days of solitary confinement may significantly increase inmates’ risk of death after serving their sentences.

New research from Christopher Wildeman, professor of policy analysis and management at Cornell University, analyzed the Danish prison system and found that 4.5% of former inmates who had spent time in solitary confinement – most for less than a week – died within five years of being released. That was 60% more than those who were not placed in solitary.

“That’s a significant increase in the risk of mortality,” Wildeman said. “We think it’s a pretty substantial effect.”

This study tracked everyone – nearly 14,000 people – who began and ended sentences in Danish prisons or jails over a five-year period between 2006 and 2011.

Of the nearly 1,700 inmates who experienced solitary confinement, the average total stay was nearly nine days, but half spent fewer than five days and two-thirds less than a week.

Understanding the effect of such short exposures is important, Wildeman said, because they are often avoidable.

“For somebody who is in disciplinary segregation for 72 hours because of a specific infraction, there’s almost always an alternative that a warden or guard could use that wouldn’t involve putting someone in solitary,” he said.

Alternatives might include a fine, loss of privileges or short increase in sentence length.

The researchers had access to a wealth of data about the former inmates from Danish government registers – not only age, gender, race, ethnicity and education levels, but also information about their family backgrounds, employment and housing histories and prior contacts with the criminal justice system.

That allowed the study to account for many factors that might have made someone more likely to end up in solitary confinement or to die after being released, while acknowledging some gaps in data concerning potential mental health or addiction issues.

Government registers also confirmed dates and causes of death within five years of release. The study determined that the higher death rates for those who had experienced solitary confinement – generally younger inmates serving longer sentences – was driven primarily by non-natural causes such as accidents, suicides and violence.

The finding that just a day or two in solitary confinement appears linked to a higher risk of death after release is somewhat surprising, Wildeman said. More research is needed to understand exactly what amount of exposure elevates mortality, he said.

The study was published in The Lancet Public Health.