Every day of our lives, we face being exposed to loud noises — particularly those of us who live in busy cities.
These are not normally loud enough to harm our hearing. But if we are consistently exposed to sounds that break a certain noise threshold, it may, in time, cause some amount of hearing loss.
The unit used to measured sound intensity is decibels, and the Centers for Disease Control and Prevention (CDC) offer examples of which types of sounds are harmless and which may endanger hearing, based on decibel level.
Normal conversation or soft background noises — such as the humming of an air conditioning unit — amount to about 60 decibels. Louder noises that you may find annoying — such as the sound of the washing machine running — amount to 70 decibels. The noise of city traffic could rise to about 80–85 decibels.
Exposure to noises above 85 decibels (but under 120 decibels) over a long period of time can harm the hearing. Such noises could be music listened to at maximum volume using headphones, sitting close to the speakers at a music concert or at the movies, and working with power tools.
Extremely loud noises over 120 decibels can cause immediate hearing loss.
Recently, researchers from Ohio State University in Columbus have found that young adults with minor hearing loss display changes in brain activity that are normally only seen in old age.
"Hearing loss, even minor deficits, can take a toll in young people — they're using cognitive resources that could be preserved until much later in life," notes lead researcher Yune Lee.
"Most concerning, this early hearing loss could pave the way for dementia."
The team's findings were recently published in the journal eNeuro.
Brain activity characteristic of aging
Initially, Lee and his colleagues had set out to complete a different kind of project. They recruited 35 participants aged between 18 and 41 who agreed to undergo functional MRI scans while listening to sentences of varying complexities.
The researchers were interested in monitoring and comparing brain activity when a listener had to process messages with a simple structure versus more complicated sentences that likely involved a different kind of cognitive effort.
Yet their study took a different turn as they noted something surprising about some of their young adult participants.
Some of the volunteers turned out to have subtle hearing impairments when tested at baseline, but Lee and team did not think much of this, as the deficits were not evident enough to warrant eliminating those participants from the study.
Yet after conducting the fMRI scans, they noticed that the participants with subtle hearing problems actually processed the messages they heard differently from their peers. And not just that, but their brain activity in this context was similar to that of aging listeners.
Specifically, healthy young adults with no hearing problems only use the left hemisphere of the brain to process heard information. But the participants with minor hearing impairments actually showed activity in both the left and the right hemispheres of their brains.
In the case of the latter, the right frontal cortex became active — something normally seen only in older people.
"This isn't about the ear — it's about the brain, the cognitive process, and it shouldn't be happening until people are at least older than 50," Lee explains.
'Twice as likely to have dementia'
The study authors explain that, normally, healthy young adults only use the left brain hemisphere when engaged in language comprehension tasks. As people age, however, they start to engage the right frontal part of the brain too, as they put more effort into processing spoken language.
"But in our study," says Lee, "young people with mild hearing decline were already experiencing this phenomenon."
"Their brains already know that the perception of sound is not what it used to be and the right side starts compensating for the left," he adds.
It is hard to say how this might impact these individuals later in life, but Lee and team worry that the hearing issues may only worsen, affecting comprehension. And, this can hasten the development of neurodegenerative conditions such as dementia.
"Previous research," says Lee, "shows that people with mild hearing loss are twice as likely to have dementia. And those with moderate to severe hearing loss have three to five times the risk."
"We can't be sure," he continues, "but we suspect that what happens is you put so much effort into listening you drain your cognitive resources, and that has a negative effect on your thinking and memory and that can eventually lead to dementia."
For these reasons, Lee advises young adults to look after their hearing more carefully and avoid exposing themselves to situations that are likely to have undesired consequences.
"Letting [hearing loss] happen early in your life," Lee cautions, "could be like spending your retirement money when you're in your 30s. You're going to need that down the road."