The number of pedestrians seriously injured or killed while wearing headphones has tripled since 2004, a new study found.

Researchers from Baltimore scoured U.S. news archives and research databases for pedestrian injuries and deaths involving headphones over the last seven years. They found 116 cases - most involving men younger than 30 - rising from 16 in the year ending in 2005 to 47 in the year ending in 2011.

"The risks posed by the use of these devices by drivers are well documented, but little is known about the association between headphone use and pedestrian injury," the authors reported in the journal Injury Prevention. "Although causal relationships cannot be proven, we speculate on implications for pedestrian safety."

With smart phones and MP3 players on the rise, more pedestrians are shutting out the sounds that keep them safe on the street. About 70 percent of the collisions reported were fatal, with more than half of the victims being struck by trains. In roughly one-third of the cases, horns or sirens sounded before the victim was hit, according to eyewitness reports.

"Two phenomena are likely contributors to the possible association between headphone use and pedestrian injury: distraction and sensory deprivation," the authors wrote.

Previous studies have found that pedestrians are less cautious and more likely to engage in "risky crossing behavior" when wearing headphones or talking on cell phones. And a 2010 study found pedestrians using cell phones took longer to cross the street, although the study failed to find the same link with MP3 players.

Another health hazard linked to headphones is hearing loss, which affects an estimated one in five people older than 12. Among children aged 12 to 19, hearing loss is up 30 percent since the late 1980s - a trend largely credited to high-volume sound blasted through headphones.