"We were very surprised at the degree to which repeated media exposure was so strongly associated with acute stress symptoms," said E. Alison Holman, associate professor of nursing science at UC Irvine and the study's lead author. "We suspect that there's something about repeated exposure to violent images or sounds that keeps traumatic events alive and can prolong the stress response in vulnerable people. There is mounting evidence that live and video images of traumatic events can trigger flashbacks and encourage fear conditioning. If repeatedly viewing traumatic images reactivates fear or threat responses in the brain and promotes rumination, there could be serious health consequences."

The study challenges key assumptions about how people react to collective traumas, such as the idea that individuals must be directly exposed to an event to be at risk for stress-related disorders. It also raises questions about the latest edition of the Diagnostic & Statistical Manual of Mental Disorders (DSM-5), which specifically excludes media-based exposure as a potential trigger for trauma response among nonprofessionals.

"In our prior work, we found that early and repeated exposure to violent images from the terrorist attacks of Sept. 11 and the Iraq War may have led to an increase in physical and psychological ailments up to three years [later]," said Roxane Cohen Silver, professor of psychology & social behavior, medicine and public health at UC Irvine and the study's co-author. "Our new findings contribute to the growing body of research suggesting that there is no psychological benefit to repeated exposure to graphic images of horror."

Researchers surveyed a national sample of 4,675 adults two to four weeks after the 2013 Boston Marathon to assess acute stress responses to the bombings, the degree of direct exposure to the bombings, indirect exposure through media and prior exposure to other recent community-based traumas. People exposed to six or more hours per day of bombing-related media coverage were nine times more likely to report high acute stress than those with minimal media exposure (less than one hour daily). Symptoms of acute stress include intrusive thoughts, feeling on edge or hypervigilant, avoiding reminders of the event and feeling detached from it.

The study authors stress that they do not want to minimize the trauma of experiencing violent events in person but instead caution that repeated viewing of grisly images can do great emotional harm. Previous exposure to collective traumas (such as the 9/11 attacks or the Sandy Hook school shooting), a pre-existing mental health condition or a lifetime history of continual exposure to traumatic events places individuals at even greater risk of developing acute stress, according to Silver and Holman.

"When you repeatedly see images of a person with gruesome injuries after an event is over, it's like the event continues and has its own presence in your life," Holman said. "Prolonged media exposure can turn what was an acute experience into a chronic form of stress. People may not realize how stressful these media-based exposures are. Looking at these images over and over again is not productive and may be harmful."