She said some diseases and treatments cause hearing loss, along with age, but she believes the hearing loss makes their physical and mental health worse.

Remensnyder has joined other audiologists and the American Academy of Audiology, the Hearing Loss Association of America and AARP to encourage regular screenings, use of hearing aids and installation of hearing loops in public places such as theaters, shops and churches. The loops magnetically transmit sounds to a wireless receiver in hearing aids, filtering outside noise.

"Patients come in here and tell me that they've stopped going to activities," she said. "We not only have to provide hearing aids but make sure venues are audible. It's an incredible issue, and if we don't get it addressed we'll be paying for it. Maybe we can defer the onset of dementia."

Other scientists, including Dr. George Gates, a hearing expert at the University of Washington in Seattle, say they're not so sure hearing loss causes dementia. Gates says his research shows that dementia may cause hearing loss, and he is an advocate for more testing.

"What we've found is long before anyone gets a diagnosis of dementia, their ability to hear noise is severely affected, so a hearing test may reveal people at risk of dementia long before any other test," said Gates. He added that there is no cure for dementia, but early interventions may slow the progression.

He agreed with Lin that improving hearing can stave off depression, which he said can lead to other mental and physical maladies. "There absolutely is a public health threat from hearing loss," he said.

Lin, also an assistant professor in the epidemiology department in Hopkins' Bloomberg School of Public Health, plans to continue looking at the link to physical and mental health.

He's studying the cognitive abilities in those over 50 with hearing loss. He's also planning a larger study that will test the cognitive and physical abilities of people with hearing loss over years. One set will get free hearing aids, intensive instruction on using them and loop systems in some public areas. The other set will just be observed (they can buy a hearing aid themselves if they want).

"No one would think of not treating their high blood pressure, but hearing loss is still perceived as not that bad for you," Lin said. "If 30 million have hearing loss and less than one in five people do anything about it, and studies show that hearing loss is associated with adverse outcomes, think of how huge a public health threat that is."

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