Sixteen years ago this morning, two hijacked planes felled the sky-tipped towers of the World Trade Center. A third crashed into the Pentagon; a fourth was downed in a Pennsylvania field before the men who commandeered it could reach their intended target and commit even greater harm.
In all, 3,000 people were murdered that horrific morning and thousands more were wounded. But the impact did not end there. Rather, 9/11 changed everyday existence for untold numbers of people—and in ways that have yet to be fully measured: from the long-term illnesses that plague many first-responders, to the economic cost of airport security delays, to the lasting psychological impacts of emotional trauma.
Consider the last of these. Five to eight weeks after 9/11, Sandro Galea and colleagues randomly interviewed a thousand New Yorkers and found evidence of post-traumatic stress disorder in 7.5% of the group. Based on that rate of prevalence, the researchers estimated, in a 2002 New England Journal of Medicine study, that some 67,000 Manhattanites who lived south of 110th Street (within 11 miles of the Towers) had some indication of PTSD during that time. (Careful readers of Brainstorm Health Daily will recognize Dr. Galea, a well-cited expert in this newsletter, as the dean of the Boston University School of Public Health.)
Additional studies have found similarly wide-scale effects from 9/11 and other terrorist attacks. But the diagnosis of PTSD didn’t get its start with the warfare of terror; it came to light after the terror of war—being added to the third edition of the American Psychiatric Association’s Diagnostic and Statistical Manual of Mental Disorders (DSM) in 1980, after it was clear that many Vietnam War veterans were suffering from a constellation of psychological troubles. “The post-Vietnam syndrome was said to be characterized by hyperalertness, sudden reliving of the combat experience, emotional numbing…and guilt over having survived when others had died,” wrote John Helzer and colleagues in a 1987 NEJM article.
Notably, these were the same symptoms experienced by many who had survived major natural disasters—particularly when those events had led to a great loss of life or acute financial pain. A substantial number of people whose lives were upended by Hurricanes Katrina and Sandy have suffered from—and, in some cases, continue to suffer from—PTSD. And, unfortunately, we will see the same outcome in a significant share of those who witnessed their loved ones be harmed, or their homes be destroyed, in Hurricanes Harvey and Irma.
There is increasing evidence, moreover, that this syndrome isn’t just psychological. Several research studies have found a strong link between PTSD and systemic inflammation and other immune system responses. A paper published a year ago in the Journal of Psychiatric Research found elevated levels of c-reactive protein, a blood-based marker of inflammation (which I wrote about two weeks ago) in survivors of the World Trade Center attack; the more traumatic the experience, the higher the CRP. The association between this biomarker and PTSD is so strong, in fact, that some have proposed diagnosing the disorder with a CRP blood test. (It may even be that high levels of systemic inflammation predispose people to this stress pathology.)
Sixteen years after 9/11, we are still discovering the full health impacts of that dark day. On a more hopeful front: We are also still learning how to heal.
This essay appears in today’s edition of the Fortune Brainstorm Health Daily. Get it delivered straight to your inbox.