Extensive watching of news coverage of terrorism events can be harmful to mental health, a new study from the University of California, Irvine suggests.

A study of people watching news coverage in the week following the Boston Marathon bombing last spring found that people who were exposed to six or more hours of media coverage of the terrorist event showed more acute stress symptoms than people actually at the marathon. These acute stress symptoms increased in subjects with each additional hour of media they were exposed. The media included in the study were television, social media, radio, videos, print and audio.

After adjusting for factors that could have affected the results, those exposed to excessive media coverage typically reported feelings such as re-experiencing the tragedy and feeling stressed thinking about it.

"We were very surprised that 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."

Holman said that being exposed to six hours of media coverages is not atypical, considering the many ways available for people to consume news. In addition to watching several hours a television in the evenings, it's easy for people to get news while at work by streaming video online and regularly checking into social media sites.

The study's authors say the research suggests that people need not be directly exposed to a traumatic event to be at risk for stress-related disorders stemming from the event. The study serves as a challenge to the most recent 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 study co-author Roxane Cohen Silver, a professor of psychology and social behavior, medicine and public health at UC Irvine. "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."

To conduct their study, the researchers sampled more than 4,600 adults around the US within two to four weeks of 2013 Boston Marathon to assess the acute stress people felt in relation to the bombings, the degree of direct exposure they had to the bombings, and their indirect exposure to the event via the media.

People exposed to six hours or more of terror-related media coverage were nine times more likely to report high levels of acute stress than people with less than one hour per day of media exposure to the terror event.

"We do not believe that people who engage in such behavior are more likely to have a pre-existing mental health condition or necessarily have a predisposition for experiencing negative psychological responses - they're merely unaware of the impact of this media exposure," Silver said.

The researchers noted that they do not want to minimize the trauma of experiencing a violent event in person, but instead they want to caution people that repeatedly viewing news coverage of these events can do great emotional harm.

"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."

The study is published in the journal Proceedings of the National Academy of Sciences.