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National Football League teams are racing this season to secure the title of loudest outdoor stadium in the world.

The Seattle Seahawks, who boast that their fans caused a small earthquake after a 2011 touchdown, acclaimed their crowd’s record decibel level this September after an effort orchestrated by the fan group Volume 12.

Four weeks later, the Kansas City Chiefs – who are still unbeaten – topped the month-old Seattle record, in part because of a scream-a-thon organized by the fan group Terrorhead Returns.

“Be LOUD AND PROUD and blow my eardrums out!” one Chiefs fan wrote on Facebook.

The NFL encourages the din.

“Fans know they are going to a football game and not searching for a book at a library,” said Brian McCarthy, an NFL spokesman.

But all that noise can come with a serious cost. With peaks for touchdowns and troughs at timeouts, the average volume during an NFL game is probably in the mid-90-decibel range, said Elliott Berger, an acoustical engineer at 3M, which makes protective hearing devices.

At those levels, the National Institute for Occupational Safety and Health recommends limiting exposure to 60 minutes. The average NFL game lasts about three times that long. And Berger said someone screaming at close range could reach 120 decibels – as loud as an ambulance siren.

Fans accustomed to hollering may scoff at the warnings as nanny-state silliness. But to auditory experts, the danger is very real.

“People think it’s cool or funny or whatever, but there is increasing evidence that, if your ears are ringing, damage is happening,” said M. Charles Liberman, a professor of otology at Harvard Medical School and the director of a hearing research lab at the Massachusetts Eye and Ear Infirmary. “There’s something irreversible going on. It’s only going to worsen as you get older.”

Liberman’s research shows that, even if the immediate effects of noise overexposure subside – the ringing, the muffling, the feeling of pressure – ears do not really recover.

“There is a huge range of ear vulnerability,” Liberman said, with some people having “tough” ears and others having “tender” ears. “You don’t know till it happens to you.”

The potential damage includes not just partial deafness and ringing, but also less common auditory abnormalities such as hyperacusis, an intolerance to sound sometimes accompanied by ear pain.

Just about everyone inside a football stadium on game day – players, coaches and fans – acknowledges that the noise is overpowering.

In the Chiefs’ game against the Oakland Raiders at Arrowhead Stadium in October, the noise peaked at 137.5 decibels – louder than a jackhammer nearby. Other sports are following the NFL’s lead. On Friday night, the din from the home crowd at a Sacramento Kings basketball game reached 126 decibels to set a record for an indoor arena, according to Guinness World Records. The noise level at Arrowhead is the current world record for an outdoor stadium, according to Guinness.

Chiefs coach Andy Reid said he could “feel the ground shaking” and later heard ringing in his ear that was not covered by a headset.

Loud fan loyalty is encouraged by people like Joe Tafoya, a former NFL linebacker who does marketing for Volume 12, the fan group that promotes full-volume screaming. As Tafoya publicly rallied fans to topple the world record for loudest stadium, he heard from the Hearing, Speech and Deafness Center with concerns about the noise.

“I had no idea,” Tafoya said. “When we started this, it wasn’t something I had thought about.”