Noise-induced hearing loss is increasingly recognized as a ubiquitous — and entirely preventable — health threat.
In the US, one in four adults show signs of noise-induced hearing loss, and hearing loss (from all causes) is now the third most common chronic health condition in America, just after diabetes and cancer.
But how exactly can sound waves damage a person’s ability to hear? To grasp that, we need to walk through the fascinating machinery of the ear.
We’re born with about 16,000 little hair cells in our ears that act as sound detectors. When sound waves pass into our ears, the hair cells send a signal to the brain, and then our brain decodes the sound, whether it comes from a barking dog or a truck that’s passing by.
But “the brain’s interpretation is only as good as the signal it receives from the ear,” said Ellen Pfeffer Lafargue, an audiologist with the Center for Hearing and Communication in New York. “When the [hair cells in the] ear have been damaged, the sound that the brain gets is distorted or the ear can’t detect sound.”
To understand how noise degrades hearing, Lafargue offered the analogy of a grassy meadow. When you’ve walked through a path in the field once or twice, the grass bounces back up again. When you walk the same path again and again, they wear down, even disappear.
The same thing happens with the ears’ hair cells. Like grass responding to a person’s feet, the hair cells bend when they’re exposed to loud sounds. They can bounce back during a recovery period — but when loud sounds have passed through the ears enough times, or at a high enough intensity, the hair cells get permanently destroyed. Along with them goes our ability to detect sound and send noise signals to the brain.
Noise has a cumulative effect on our ears — but even a single loud pop can damage our hearing
Sound is made by waves of air pressure, and decibels are the unit by which sound intensity is measured. They fall on a logarithmic, or nonlinear, scale, meaning the marks on the scale are based on orders of magnitude. Every six decibels is perceived by the ears as about a doubling of loudness. So 96 decibels feels a whole lot louder than 90.
The official government stance on safe noise levels comes from the National Institute for Occupational Safety and Health, which sets the standard for noise at work. It recommends limits of 85 decibels — a bit more than the loudness of a freight train 100 feet away — for eight hours a day, and that people exposed to anything louder use hearing protection. That’s because chronic noise exposures above this threshold have been shown to cause hearing loss over time.
But 85 decibels is not necessarily where hearing loss begins. Back in 1974, the Environmental Protection Agency suggested keeping noise levels around 70 decibels to optimally prevent noise-induced hearing loss. And the National Institutes of Health today suggests that sounds above 75 decibels can harm a person’s ability to hear.
What’s more, while being around loud sounds every day can certainly diminish your hearing over time, you don’t need sustained exposure to get permanent damage. A single pop of thundering noise — like a gunshot or an explosion greater than 120 decibels — is enough to do permanent damage. So exposure to sounds at that level takes even less time to diminish hearing.
That’s why the World Health Organization and National Institutes of Health suggest people shouldn’t be exposed to noise at or above 100 decibels for any more than 15 minutes. For noise at or above 110 decibels, these health authorities recommend no more than one minute.