So far in this series, we’ve discussed how a diet rich in antioxidants can minimize or prevent toxic oxidative stress, and how oxidative stress leads to hearing loss through apoptosis. Now, we’ll concern ourselves with another way apoptosis and hearing damage may occur, and the devastating role noise exposure plays in this scenario.
A respirator isn’t the first thing that comes to mind when thinking about hearing protection.
But a respirator?
It turns out that many common chemicals are ototoxic (poisonous to the ears) and as damaging to hearing as noise. But exposure to noise and chemicals at the same time can be positively menacing. That’s because their effects are often synergistic rather than merely additive. In other words, the total damage is greater than the damage caused by the sum of the parts. For example, researchers studying the effects of noise on industrial workers found a higher incidence of hearing loss in workers exposed to lower levels of noise than workers in an area where the noise levels were higher.1 However, the workers exposed to the lower noise levels were also exposed to industrial solvents concurrently with the noise and that made all the difference! The noise and chemical exposure together accelerated the rate of apoptosis and hearing loss.
When it comes to chemical exposure and hearing loss, the worse culprits seem to be organic solvents such as benzene, toluene, xylene, and styrene. These solvents can be found around the home as well as in the factory in cleaners, paints and coatings, and adhesives. Nearly all of them are synergistic with noise exposure. Not only do they contribute to toxic oxidative stress in the cochlea, where hearing loss often first occurs, but they can also degrade the function of neural circuits in the brain’s auditory pathways. This neural damage can cause a type of hearing loss that even hearing aids can’t help. And the neural damage isn’t confined to hearing. Other sensory inputs (vision or smell, for example), cognitive function, and motor coordination may all be impaired to greater or lesser degree, both temporarily and permanently.
There is also evidence of hair cell apoptosis from phosphate-based chemicals used in farming, gardening, and industrialized agriculture. Some non-benzene carbon compounds, like carbon disulfide and carbon monoxide, also have a profound impact on hearing as do certain metals known to affect the nervous system. Included among these are the vapors and dusts of lead, mercury, manganese, and arsenic. One again, these are not rare or exotic materials confined only to industry. Some, especially lead, are often used—and overlooked as a hazard—by hobbyists and artists in their homes or studios, areas where the concepts of industrial safety are often not as rigorously applied.
Whereas hearing loss resulting from noise exposure is often confined to a narrow range of tones—usually the higher frequencies—damage from toxic chemicals can affect hearing anywhere across the frequency range. The inner ear also has the primary responsibility for helping us maintain our balance. Ototoxic compounds can affect all of the inner ear structures—not just the cochlea, but the balance organs too—putting a person at risk for not only hearing loss and tinnitus (ringing in the ears), but dizziness and vertigo (a spinning sensation) as well.
The point of all this is that wearing a respirator to minimize inhalation of chemicals, while a good thing in general, is as important as wearing hearing protection when it comes to preventing hearing loss. Because of the synergistic effect of many chemicals with noise, wearing a respirator and hearing protection together should be your first-line defense against hearing loss whenever chemicals are used—at work, or at home, whether in the garden, the garage, or the studio—even if noise levels don’t seem loud enough to cause hearing damage by themselves (e.g., < 80 dB SPL). The synergy with chemicals makes hearing loss more likely with lower levels of noise exposure.
In our next discussion we will address some other surprising ototoxins that are sometimes synergistic with noise and with each other, and make the wearing of hearing protection on the job, at home, and during recreation even more important.
1. Bergstrom, B. & Nystrom, B. (1986). Development of hearing loss during long-term exposure to occupational noise. A 20-year follow-up study. Scandinavian Audiology, 15, 227-234.