Permanent Hearing Loss From Noise May Depend on Time of Day!

Research out of Sweden have discovered that noise-induced hearing loss sustained at night is more likely to become permanent than if it occurred during the day... at least in mice.

Apparently, there are certain genes found to cycle in the cochlea of mice in a pattern that followed the hours of the day. By measuring the auditory nerve activity, they found that mice exposed to moderate noise levels during the night suffered from permanent hearing loss while mice exposed to similar noise levels during the day did not. The ability to heal after noise-induced hearing loss (temporary threshold shift) was linked to the circadian rhythm due to levels of BDNF (brain derived neurotrophic factor).

Specifically, concentration of BDNF increases in the cochlea during the day which protected hearing from permanent damage whereas levels were absent at night. Furthermore, researchers were able to artificially increase BDNF levels at night when normally absent and confer hearing protection with noise exposure.

What does this mean for humans?

IF human ears also have a biologic clock similar to mice, than researchers MIGHT be able to develop a specific way to reverse (or prevent) noise-induced hearing loss. Currently, the only way to treat acute noise-induced hearing loss is high doses of steroids whether orally or by injection directly into the ear.

Also, humans should probably avoid listening to loud music at night (i.e. concerts, night-clubs, restaurants, etc) which elevates risk of permanent hearing loss (compared to during the day).

Reference:
TrkB-Mediated Protection against Circadian Sensitivity to Noise Trauma in the Murine Cochlea. Current Biology, 27 February 2014. 10.1016/j.cub.2014.01.047

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Hearing Protection and NRR (Noise Reduction Rating)

For all hearing protection devices from ear plugs to ear muffs, there is a NRR (Noise Reduction Rating) score that informs consumers how much of a decrease in noise exposure to expect with use. For example, if an ear plug has a NRR score of 30, use of the device would be expected to reduce a 90 decibel sound by 30 decibels resulting in a noise exposure of 60 decibels.

If two hearing protection devices are used, an ear plug inside the ear canal followed by ear muffs over the ear, the ultimate noise exposure would be an additional 5 to 10 decibels greater than the higher rated NRR of the two devices.

How is NRR determined anyways? This single number rating is determined by the Environmental Protection Agency as specified under 40 CFR (Code of Federal Regulations) Part 211, Product Noise Labeling, Subpart B - Hearing Protective Devices. The raw data compiled from test subjects is than plugged into a formula which than produces the NRR:

Given that, there are MANY different types of hearing protection devices on the market. Regardless of style, most devices offer a NRR between 20-30. However, as a general rule, ear plugs typically offer a higher NRR than ear muffs. Among high NRR earplugs, disposables offer higher NRR than reusables. BUT... ear muffs typically block sound more consistently than ear plugs which require some degree of molding and fit for optimal effectiveness. Using both obviously is better than using just one.

Also, keep in mind that unless hearing protection devices are correctly fit and used properly, the attenuation suggested by NRR may not over-estimated. Indeed, lay-use of such devices without training typically result in sound attenuation far below what NRR may suggest.

The highest rated NRR disposable ear plugs are around 33:


The highest rated NRR ear muffs are around 30:

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Dr. Chang Quoted in Washington Post Regarding Hearing Damage from Noise

The Washington Post on April 8, 2013, published a story about how loud noises can lead to hearing loss, even in young teenagers.

Dr. Chang was liberally quoted throughout this informative article.

"...That’s according to Christopher Chang, an ear, nose and throat doctor at Fauquier ENT Consultants in Warrenton, who sees patients every day with hearing-related issues. “What he’s hearing is way too loud, because it’s concentrated directly into the ear itself,” he says of my son, adding that the anatomy of the ear magnifies sound as it travels through the ear canal." 

"And when someone turns the music up louder? Well, in short spurts, that’s okay. Loudness is only one part of the equation. How long you’re exposed to loud noises also affects your risk. “The biggest concern is that teens and 20-somethings are using music players so regularly,” Chang says. 

"Cheap earplugs that you can buy at the drugstore offer some protection, Chang says. In addition, there are specialized ear plugs, such as ones for hunters that amplify low-level sounds and block out gunshots. Musicians’ earplugs reduce noise with higher fidelity than the muffling effect that regular plugs have: You want to hear the music that you and your orchestra mates are playing, after all."

Read the entire article here.

Source:
Hearing loss may be caused by cities’ noises and music that’s played too loud. Washington Post 4/8/2013.

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