Hint #1: The people around you are shooting death stares your way.

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Yesterday, I rode a New York City train with a woman blasting music from her earbuds. I was standing a good two feet away from her, and I could clearly hear songs over the roar of the train. I felt a flash of commuter rage, especially because I found her music annoying. Then I calmed myself down with this evil thought: She's going to damage her hearing.

I've been similarly irked when I've sat in the Quiet Commute car of the suburban train I take to and from work, only to have to listen to pounding sounds streaming out of the earbuds of a fellow passenger. (Once, I sat next to a guy who mumbled to himself, although I wasn't sure that officially violated the rules of the quiet car.)

I wondered: Was my fellow commuter damaging her hearing? How do you know if your earbuds are too loud? For answers I hit up Eric Smouha, MD, director of otology and neurology at Mount Sinai Hospital in New York City.

First, the sound food chain

"When it comes to music exposure, earbuds are worse for hearing than over-the-ear-headphones which are worse than speakers," says Dr. Smouha. It's for the reason you think: Earbuds put the source of sound close to your ear drums. Sounds above 85 decibels can damage hearing, although when and how much depends on three criteria: How loud, how long, and how close. So-called personal stereos can go up to 105 decibels, notes this Interactive Sound Ruler from the National Institutes of Health.

OK, so how do you know if your music is playing at a healthy level?

Dr. Smouha's rule of thumb: "Keep the volume at no more than one half to two-thirds of the maximum." Also: If you can't hear someone standing nearby you shouting (say, because your music is loud), then your music is too loud. The NIH website's Noisy Planet section notes that scientists recommend no more than 15 minutes of unprotected exposure to sounds that are above 100 decibels. Additional warning signs your music is too loud: The noise hurts your ears, you develop a buzzing or ringing sound in your ears (even temporarily), you don't hear as well as you normally do until several hours after you get away from the noise, and your fellow commuters are shooting death glares your way.

And how do you know if someone else is truly playing music too loud, or you're just being an overly grumpy commuter?

"If it's so loud that someone can hear the lyrics, it's too loud," says Dr. Smouha. However, it's best to avoid yelling at a fellow commuter blaring music, because shouting is 90 decibels.

And what are the odds that a person who is blasting her iDevice already has hearing loss?

"It's hard to say. But repeated exposure to loud music means repeated chinks in the armor."

Fellow commuters who enjoy the sound of silence, take heart.