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The Consumer Handbook on Tinnitus

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.