The short answer to the question in the headline is an unequivocal "yes!" There are industrial chemicals in common use that are ototoxic (poisonous to the ears), meaning that they can damage hearing just as easily as industrial noise. However, simultaneous exposure to noise and ototoxic chemicals is particularly insidious because of their synergistic effect.
In order to illustrate a synergistic relationship, let's suppose a given noise exposure is responsible for a 10dB threshold shift. Let's also suppose that an ototoxic chemical, by itself, can induce a 10dB threshold shift. If exposure to the noise and ototoxin together results in a 20dB threshold shift, then we say the combined effect is additive. A synergistic effect, however, is one in which the combined impact of the noise and ototoxin results in a much greater total threshold shift than would be predicted by simply adding together the noiseand ototoxinrelated threshold shifts—perhaps a 35 or 40dB threshold shift may occur. Also, such a threshold shift often impacts a wider range of frequencies. Indeed, a synergistic effect is greater than the sum of its parts, and this observation has been borne out by research involving industrial workers.1
Even if noise levels alone don't seem loud enough to cause hearing damage (e.g.,80 dBA), the synergy with chemicals makes the risk of hearing loss more likely even with noise exposures below the action level. In any environment where chemicals are being used, wearing hearing protection is good practice when noise is present at levels just below the action level, and especially where the work activity requires the use of a respirator. Wearing a respirator is every bit as important as wearing hearing protection when it comes to preventing hearing loss from ototoxins.
Most ototoxins damage hearing through toxic oxidative stress, the same condition caused by prolonged exposure to high noise levels. Oxidative stress is an accumulation of reactive oxygen species (free radicals) brought about by excessive metabolic activity. This condition triggers a form of programmed cell death called apoptosis (the process is depicted in the accompanying graphic). While apoptosis occurs naturally as a part of healthy cell maintenance, a localized avalanche of cellular death—say, within the inner ear—causes extensive damage from which the body cannot properly recover.
In addition to hearing, the inner ear also has the primary responsibility for helping us keep our balance. Unlike industrial noise, ototoxic compounds can affect all of the inner ear structures, including the balance organs—not just the hearing part—putting a person at risk for falls from a general loss of balance, dizziness, or, in severe (but rare) instances, vertigo. It is also important to remember that any chemical that is ototoxic is also likely to be poisonous to the kidneys, because the inner ear and the kidneys arise from the same germ layer during embryonic development. While kidney damage is rare, it can result from a serious complication of chemical exposures and is often presaged by hearing difficulties resulting from the same causes.
As alluded to earlier, exposure to some industrial chemicals can cause hearing loss without additional exposure to noise. At the top of the list are mixtures containing organic solvents, including benzenebased and aliphatic hydrocarbons (e.g.,styrene and nhexane, respectively) and other organic compounds (e.g., carbon disulfide and trichloroethylene). Phosphatebased chemicals for farming, gardening, and industrialized agriculture, and asphyxiants, such as carbon monoxide and hydrogen cyanide, also can have a profound impact on hearing. So do certain metals, particularly lead, mercury, manganese, and arsenic, including their salts and oxides. In addition to triggering apoptosis through oxidative stress, heavy metals are neurotoxins that attack the nerves that convey signals from the ear to the brain, as well as damage neurons in the brain itself.
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