Health experts say there are two major factors that affect hear over time, age and noise exposure, and one at least is completely preventable.

With the overwhelming popularity of personal music players, damage to hearing is increasingly becoming a concern.

So CTV set out to measure just how loud some MP3 can go, and how that compares to everyday noises.

Kemar is a mannequin that simulates the changes that occur to sound waves as they pass a human head. The mannequin contains a microphone connected to a computer that measures sound levels.

Kemar is used to test hearing aids developed and manufactured at Unitron, where Brad Stephenson is a senior audiologist.

He says most people don't realize how loud personal stereo player settings can be.

"You have it at the preferred level for a period of time and then you increase it because your hair cells are getting more and more swollen."

Stephenson is referring to the hair cells in the human ear that allow people to hear.

Music goes through the ear drum to the middle ear bones and then the inner ear, which is lined with tiny hair cells that can be damaged by loud music. That can result in hearing loss.

Acceptable noise levels are no more than 80 to 85 decibels (dB), which is equivalent to the sound of dial tone or city traffic heard from inside a car.

Of the MP3 players test by CTV, most had levels well over 100 decibels. That's equivalent to a chainsaw, car horn or jackhammer.

However those same players have the capability to be as loud as a concert and as Stephenson says "listening to this for an hour it would be way too loud."

With the help of Kemar, Stephenson tested the built in players in the iPhone and the Samsung Galaxy.

At full blast, both players reached levels well over 100 decibels, which means "We have people walking around like they are at a concert, and this concert is lasting for four hours, three hours, two hours."

A recent Health Canada study found MP3 players combined with earphones purchased separately can reach an output of 125 decibels, the equivalent of a thunderclap or jet plane from 30 metres away.

Coming up in part three: What MP3 player makers have to say about the concerns and and how you can safeguard your own hearing.