It is no surprise that most work-related hearing loss is caused by noise
exposure, and that genetics and age can also be contributors. What may not be
as well known is that some chemical exposures can pose a potential risk to
hearing. Both animal experiments and human studies suggest that certain
chemical exposures may cause "ototoxic" effects (damage the hearing and balance
functions of the ear). In general, the exposure concentrations that cause these
effects are considered high. However, exposure to some of these chemicals and
noise at the same time can significantly increase the risk of developing
ototoxic effects.

What are ototoxins?

Ototoxins are chemicals which can damage hearing and can cause mild to severe
hearing loss, tinnitus (ringing in the ears), or deafness. An ototoxin can be
ingested, absorbed, or inhaled into the body. Once in the bloodstream, the
ototoxin is circulated to the ear and absorbed by the auditory nerve, damaging
the nerve and causing hearing loss. Ototoxins can also cause hearing loss by
damaging the cochlear hair cells (as happens in hearing loss caused by noise).

Effects of chemical exposure on hearing

Ototoxic chemicals can cause hearing loss on their own, however when combined
with noise exposure, the effects can be even more severe. Organic solvents are
the most commonly identified chemicals, but others may also be involved (e.g.
metals and chemical asphyxiants). The hearing frequencies affected by solvent
exposure are different than those affected by noise. Research suggests that
solvents may interact synergistically with noise. Even when noise and chemicals
are at permissible exposure levels, the impact of a combined exposure can do
more damage than a higher exposure to either hazard alone.

Some chemicals associated with hearing loss

• Benzene


• Carbon disulfide


• Carbon monoxide


• Ethylbenzene


• Hydrogen cyanide


• Lead


• Mercury


• n-Hexane


• Solvent mixtures


• Styrene


• Trichloroethylene


• Toluene


• Xylene

Organic solvents are widely used: in automotive and aviation fuels; in plastics
industries; as thinners for paints, lacquers and dyes; in the manufacture of
detergents, medicines, perfumes, fabric and paper coatings, printing inks,
spray surface coatings; and in insect repellents.

Activities where noise and chemical hazards can potentially combine include:

• boat building


• construction


• firefighting


• fueling vehicles and aircraft


• furniture making


• manufacturing of metal, leather and petroleum products


• painting


• printing


• weapons firing

Challenges

It may be difficult to determine the ototoxic effects of chemicals,
particularly organic solvents, in exposed workers. Workers are usually exposed
to a mixture of solvents with various compositions and concentrations, making
it difficult to isolate exactly which chemical, and how much exposure to that
chemical is causing damage. Also the industrial environments in which there
tend to be exposures to both chemicals and high levels of noise make it
difficult to differentiate the solvent effect from noise-induced hearing loss.

Although there is no firm guidance on the lowest occupational exposure limits
for solvents in relation to their effect on hearing, the current occupational
exposure limits as well as hearing conservation programs for solvent-exposed
workers may not be adequate.

How to protect workers

• Conduct a hazard assessment as the first step in a hearing loss prevention
  program to learn if and what hazardous exposures exist in the workplace.


• Remove the source of hazardous exposures from the workplace (the most
  effective way to prevent hearing disorders from noise or chemical exposure, but
  may not be possible).


• Substitute ototoxins with less hazardous chemicals.


• Take steps to minimize potential ototoxin exposures through inhalation,
  ingestion, and/or skin absorption.


• Minimize exposure to these chemicals through process changes, ventilation,
  and/or skin or respiratory protection.


• Reduce noise levels through engineering or administrative controls.


• Wear hearing protection when exposed to noise, or when exposed to ototoxins -
  even when noise levels are below the threshold - to prevent the combined
  effects of noise and solvent exposure.


• Start a hearing conservation program for workers at lower levels of noise
  exposure than is required by occupational health and safety legislation.


• Include workers exposed to chemicals in hearing conservation programs,
  whether or not they are exposed to noise. These programs should consider the
  possible combined effects of exposure to both solvents and noise.

Further Information

AplusA-online.de - Source: Canadian Centre for Occupational Health and Safety