Millions of workers are exposed to potentially damaging levels of noise on a daily basis and, when uncontrolled, this noise can cause life-altering hearing damage. The same can be said about the chemical agents found in many workplaces. Significant research has been devoted to understanding the negative impact that ototoxic chemicals (known as ototoxicants) can have on one’s hearing and balance. As with unsafe noise levels, a well-designed hearing conservation program will protect workers against hearing health issues resulting from exposure to ototoxicants.
Even the slightest exposure to commonly found substances such as solvents or pesticides containing ototoxicants can cause damage to the inner ear and lead to hearing loss and balance issues. This damage can either be temporary or permanent. When workers are exposed to both elevated noise levels and ototoxicants without proper safeguards, the effects of exposure are significantly increased, and often synergistic.
Exposure to ototoxicants can happen through skin absorption, inhalation, and ingestion. Ototoxic damage can vary based on many factors, including the length of exposure over time, as well as the intensity and frequency with which the worker was exposed. Other factors, such as general health and age, can also impact the severity of symptoms. There are various susceptible areas of the human hearing and vestibular systems that are at risk when an individual is exposed to ototoxicants: the cochlea, the vestibular canals, the vestibular nerve, and the central auditory nervous system.
Ototoxic substances can attack the cochlea, the spiral-shaped hearing organ in the inner ear that picks up sound and plays a vital role in our auditory system. Exposure to ototoxicants such as toluene, xylene, and styrene can also damage the vestibulocochlear nerve, a nerve that both transmits sound and filters it for the brain to understand, as well as the auditory cortex, can also be damaged without proper protective measures.
In addition to hearing loss, damage to the vestibular portion of the inner ear can increase the risk of falls, cause general loss of balance and dizziness, and can even result in vertigo in severe cases. There is evidence to suggest that an expectant mother’s exposure to ototoxic chemicals can also damage the developing kidneys of an unborn child, as both the kidneys and the inner ear are formed from the same tissue layer during embryonic development.
Futhermore, unsafe exposure can trigger damage in the central auditory nervous system, which can lead to speech discrimination dysfunction (not being able to isolate voices from background noise), and a decrease in overall hearing clarity. Speech discrimination dysfunction is particularly troublesome, as it impedes an individual’s ability to hear and understand verbal instructions, especially in noisy environments–something which can have severe consequences when working with dangerous machinery and chemicals.
A study conducted by the University of Queensland in Australia concluded that people serving in the Australian Defence Force who were exposed to four or more ototoxicants were two to four times more likely to experience mild to severe tinnitus when compared to the average population (https://ohsonline.com/Articles/2013/06/01/NHCA-Conference-Highlights-Top-Hearing-Conservation-Trends.aspx). The full impact of ototoxicant exposure is not fully known; many workers who experience hearing loss due to exposure are often exposed to unsafe noise as well, making it difficult to isolate the exact root cause of the damage.
The requirement that workers wear proper hearing protection when unsafe noise levels are present is a critical element of any successful hearing conservation program; the use of a respirator is equally essential when workers are exposed to ototoxicants.. Wearing hearing protection and using audiometric testing to detect early signs of hearing loss is beneficial, even if workers are exposed below the action level and ototoxic chemicals below the PEL (Permissible Exposure Limit). For example, The US Army requires all employees exposed to any known or suspected ototoxicant to be included in a hearing conservation program when the ototoxicant exceeds 50 percent of the occupational exposure limit (https://armypubs.army.mil/epubs/DR_pubs/DR_a/pdf/web/p40_501.pdf).
Special care should be given to identify and locate all ototoxic health hazards found in the workplace, and clear training should be offered to workers. When possible, replacing hazardous chemicals with less toxic alternatives is an excellent way to reduce risk. Engineering controls such as isolation and enclosures to control exposure to noise and ototoxicants are proven risk-reduction strategies when these chemicals cannot be removed. Since many ototoxicants are absorbed through the skin, it is recommended that workers wear protective gloves, aprons, arm sleeves, and any other appropriate protection.
Historically, workplace-related hearing loss was associated solely with exposure to high levels of noise, especially over extended periods. By the 1980s, regulatory bodies such as OSHA in the U.S. (Occupational Safety and Health Administration) began to realize that exposure to ototoxic substances could also damage a worker’s hearing, especially when coupled with noise exposure. Since this time, we have gained a better understanding of the harm that can be caused by these toxic agents, and many safeguards have been introduced into the workplace to protect employees’ overall hearing health.