Headphones, In Ear Monitors (IEM's), Earbuds, it doesn't matter what you call them, the odds are they are used at some point during the church worship time.  They might be used by the sound board operator.  They might be used by worship team members.
 They might be very dangerous!

"Dangerous?  How can headphones be dangerous," you might be asking.

Before we get to the danger, let's talk types.

Headphones, used as the generic term, can cover your ears completely, can rest over your ears, can rest in your ears like the earbuds, or they can be stuffed into the ear canal like the IEM's.  The quality of the headphones, outside of the plastic that tends to hold them together, depends on the frequency of sound which they can reproduce.  Using the Shure SE-series in-ear monitors as an example, let's look at the audio frequency ranges of each model:

SE210: 25Hz - 18.5kHz
SE310: 22Hz - 19kHz
SE420: 20Hz - 19kHz
SE530: 18Hz - 19kHz

The numbers seem close, right? Let's look at what they mean...

The low number (25hz for the SE210's) indicates the amount of bass that can be reproduced.  The high number (18.5kHz) is the high notes.  Think of the keys of a piano.  Low keys are bass notes while high keys are in line with a soprano voice.  

What's the difference between bass frequencies at 25Hz versus 18Hz.  A lot and yet little.  The lower the bass frequency, the less we hear it and the more we feel it. This is especially true once we drop to the 20Hz frequency.  Bass players tell me they play by "feeling" the bass just as much as hearing it.  I've even hear of a bass player who had a bass amp placed under the floor of the church stage so they could feel it as well as hear it in the monitor.

On to the danger...

The most obvious danger of headphones is listening at an unsafe level which can cause temporary or perminent hearing loss.  This usually happens if the user willingly sets the volume level too high OR they use the headphones before checking the volume level and thus can get a "burst" of sound before ripping them off their head.

The less obvious is the one-in one-out method.  This occurs when the user wants to hear something via the headphones as well as something directly.  A good example is the IEM's.  A guitar player might want to hear the instruments in the IEM but also hear the congregation singing.  Seems like a simple solution.  Too bad it will cause hearing damage.  The reason is that as soon as you take one IEM out of one ear, you must significantly increase the sound to the other to make up for the loss of the same sound going to both ears.  

Overcome the one-in one-out method by using spot monitors or use microphones to capture the sound the user needs to hear.