Noise from earphones can damage the coating of nerve cells

Researchers from the University of Leicester have discovered that noise from earphones has the potential to be as damaging to our hearing as that emitted by jet engines. The team found that if the volume is turned up too high, the noise can damage the coating of nerve cells in the ears, leading to temporary deafness.

It is well known that loud noise can damage a person’s hearing. Noises louder than 110dB, for example, can lead to temporary deafness and tinnitus. However, this study, which has been published in the Proceedings of the National Academy of Sciences, represents the first instance in which the damage caused to these cells has been observed.

I spoke to Dr Martine Hamann from Leicester’s Department of Cell Physiology and Pharmacology who led the study, to find out more about the potentially damaging effects of earphones operating at high volumes. I began by asking Dr Hamann, whose research was funded by the Wellcome Trust, MediSearch, GlaxoSmithKline (GSK) and the Royal Society, how she and her colleagues set about observing the underlying cell damage caused by high noise levels.

"We looked at both morphological and functional damage caused to the tissue," she explained. "We studied tissue from animals that had been exposed to loud sounds some time ago, and we investigated whether or not damage had been caused at the cellular level. We discovered that in some cases, loud noise causes damage to the coating of nerve cells. We also found that after two to three months, this coating can reform; not necessarily to its original level but to 70 or 80 per cent of its full capacity."

Electrical signals travel from the ears to the brain via nerve cells that possess a coating known as the myelin sheath, and this coating facilitates the flow of these signals. When the myelin sheath is damaged, therefore, the electrical signals are disrupted and so not all of the information reaches the brain.

"Damage to the myelin sheath can occur after two to three hours of exposure to loud noise," said Dr Hamann. "This is something that might happen regularly, for example, when people listen to music on their MP3 players. Such devices can be played at volumes of up to 110dB and it’s not unusual for people to listen to music very loudly. The damage that is caused depends on the noise level and the length of the exposure to this sound."

I asked Dr Hamann to explain why she and her colleagues had compared the noise produced by earphones with that generated by jet engines. As she explained, the damage caused by loud noises depends on a number of variables.

"We tend to quantify how damaging a sound is according to its intensity and its duration," said Dr Hamann. "If you are exposed to a noise for a long time, the damage to your hearing will be greater than if you are only exposed for a short time. Likewise, if you are exposed to a loud sound, the damage will be greater than if you are exposed to a sound of lower intensity. The point that we are making is that if you use earphones for two hours at an intensity of 110dB, it can be as damaging as a shorter exposure to the noise generated by a jet engine."

Some might argue that if people are using earphones unsafely, manufacturers should act to eliminate the possibility of their products causing damage to consumers’ hearing. I asked Dr Hamann whether there is anything that earphone and MP3 manufacturers can do in order to tackle this problem.

"I guess that manufacturers could introduce maximum limits for sound intensity that are below unsafe levels," she answered. "I believe that in France, some companies have regulated their products so that volumes do not exceed 95dB. Whilst this is not currently happening in the United Kindgom, it is conceivable that manufacturers might introduce some kind of measure to eliminate the possibility of sounds reaching damaging levels. However, I think that the most effective tool will be that of education. People need to be made aware of the damage that they might be causing to their hearing.

"When it comes to earphone use, the best precaution is to keep the sound volume at a safe level. For loud noises that originate from elsewhere, it is important for people to protect their ears. In either case, people can protect themselves by ensuring that they are only exposed to loud noises for short periods of time. The frequency of these occurrences also plays a role. For example, if you go clubbing every week, there is a greater chance that your hearing will be damaged than if you only go once in a while."

I concluded our interview by asking Dr Hamann how her findings might facilitate the development of appropriate cures for hearing loss.

"If we can understand how the cell coating becomes thinner, we might be able to provide a means of preventing this from happening," she explained. "A more molecular approach such as this could enable us to target the coating in order to prevent it from becoming damaged. If we can improve our understanding of the mechanisms involved in this process, we can begin to develop more effective methods of preventing hearing loss."