CHAPTER
SIX
Interaction between Noise and Chemicals
Found in the Workplace
Thais
C. Morata, Ph.D.
As discussed in earlier
chapters, several factors have been
studied to try to understand why the
prevalence and degree of noise-induced
hearing loss can vary so much within
a group and among groups. Some of the
factors studied include variations in
exposure, age, gender, race, and general
health indicators, such as blood pressure
and use of certain medications. The
focus of the present chapter will be
on the ototoxicity (the toxic effects
on hearing), industrial chemicals, and
their interaction with noise.
Hearing loss apart
from noise can occur after ingestion
of certain drugs due to their effects
on the auditory system or brain. The
ototoxicity of therapeutic drugs has
been recognized since the 19th century.
The first reports associated the intake
of certain drugs such as quinine and
acetylsalicylic acid (ASA) with temporary
hearing loss as well as dizziness and
tinnitus. In the 1940s, permanent damage
to the cochlea (sensory end organ for
hearing) was reported in several patients
treated with the newly discovered drug
for treatment of tuberculosis, the aminoglycoside
antibiotic streptomycin. Today there
are many well-known ototoxic drugs used
in clinical situations. Most of them
(antibiotics, chemotherapeutics, diuretics
and anti-malaria drugs) are used despite
these negative side effects in order
to treat other serious, sometimes life-threatening
conditions.
By comparison, only
recently the ototoxicity of chemicals
found in the environment from contaminants
in air, food or water, and in the workplace,
became a concern for audiologists and
other healthcare professionals. Initially,
there were just isolated reports following
acute intoxications, poisonings, and
observations that hearing losses were
more common and sometimes more severe
in work settings where chemical exposures
occurred. Other studies on the neurotoxicity
of chemicals indicated that chemicals
were also damaging more central portions
of the auditory system. Following these
reports, other research laboratories
started investigating the ototoxic properties
of chemical agents and identified ototoxic
properties in a few classes of industrial
chemicals: metals, solvents, asphyxiants,
pesticides, and polychlorinated biphenyls
(PCBs).
Studies conducted
with experimental animals have shown
that some toxicants can reach the inner
ear through the blood stream. They were
found in inner ear fluids (endolymph
and perilymph, discussed in more detail
in Chapter 2) and have caused damage
to some of the inner ear structures
and have impaired functions. Some of
these chemicals are also damaging to
the nerves. The onset, site, mechanism
and extent of ototoxic damage of these
toxicants vary according to risk factors
that include: type of chemical, interactions,
exposure level and duration of administration,
as is the case with ototoxic therapeutic
drugs such as cisplatin (used in chemotherapy)
and aminoglycoside antibiotics.
