Chances are the first images you saw after the Boston Marathon bombings in April came to you via social media. Within seconds of the attack, graphic, bloody pictures flooded the Internet.

A new study investigates how prolonged exposure to these kinds of images affects people's mental health.

Psychologist Roxane Cohen Silver of the University of California, Irvine began studying the impact of media coverage after violent events in the wake of the 9/11 terrorist attacks. Her previous studies focused only on traditional media such as TV news. This new study, published in the Proceedings of the National Academy of Sciences, is the first to also to take into account social media, online news stories or posted videos.

Questioning several thousand individuals across the U.S. shortly after the marathon bombings, Silver found that those exposed to coverage in any form for more than six hours a day were more likely to develop "acute stress response."

They felt vigilant, on edge, anxious or had intrusive thoughts, Silver said.

"In fact, those people were experiencing more acute stress than were individuals who had been at the marathon themselves, or had family members there," she said. "With each increasing hour of bombing-related media exposure, we saw an increase in acute stress response."

The power of these pictures and news reports is underestimated, Silver said.

"I think people are just not aware of the potential for the psychological damages of these graphic images being seen over and over again," she said.

When tragedies happen, Silver said, sometimes it's best to tune out.

"Do I really need to see these pictures over and over again? Can I step away from the Internet, can I step away from the television?" she said.

Acute stress response can lead to post-traumatic stress disorder in some cases, Silver said, but usually goes away on its own after some time.