Alternative Explanations for Hearing Loss in an Industrial Shipyard
Occupational Hearing Loss
Noise is a well-studied risk factor for the development of hearing loss. However, recent evidence suggests exposures to chemicals commonly found in industrial environments may affect hearing alone or in combination with noise exposure. The term “ototoxic” is used to define any substance, including drugs or industrial chemicals, that are toxic to the auditory system (Nordic Expert Group. Arbete och Hälsa. 2010;44(4):1 http://bit.ly/2LxDnPQ). Noise exposure may damage the cochlea as a peripheral auditory system component; however, chemicals are believed to affect both the cochlea and central auditory system (Nordic Expert Group. 2010). Decreased hearing and sound clarity pose problems for people working in loud environments where accurate understanding of audible emergency warnings and co-worker instructions is necessary to minimize the risk of workplace accidents and injury.
OTOTOXICITY WEIGHT OF EVIDENCE
Previous researchers have sought to identify which industrial chemicals demonstrate ototoxin potential. In a review of animal and human studies, Vyskocil, et al., classified ethyl benzene, n-hexane, and xylenes as “possibly ototoxic substances” and lead, styrene, toluene, and trichloroethylene as “ototoxic substances” (Toxicol Ind Health. 2012 Oct;28(9):796). When evaluating the effect of exposure combinations, toluene and carbon monoxide worsen hearing loss when exposure occurs simultaneously with noise (Toxicol Ind Health. 2012). Campo, et al., found “good” or “fair” ototoxicity weight of evidence for chemicals such as toluene, ethyl benzene, xylene, lead, mercury, cadmium, arsenic, and halogenated hydrocarbons (EU-OSHA, 2009 http://bit.ly/2LuTmOj). The American Conference of Governmental Industrial Hygienists has recognized the concern for ototoxin exposure by recommending audiograms for workers exposed to ethyl benzene, styrene, toluene, and xylene in non-noise environments. The Occupational Safety and Health Administration (OSHA) and National Institute for Occupational Safety and Health (NIOSH) released a joint Safety and Health Information Bulletin to educate occupational health practitioners about the dangers of ototoxins at levels less than Permissible Exposure Limits (NIOSH, 2018 http://bit.ly/2Lv6uDf).
OTOTOXIN RISKS IN INDUSTRIES
The chemicals previously discussed may be found in a variety of industries, including manufacturing, mining, utilities, construction, and agriculture (NIOSH, 2018 http://bit.ly/2Lv6uDf). The shipyard sub-sector via occupational activities such as shipfitting, welding, woodworking, and facility maintenance is exposed to noise and ototoxin at varying concentrations. Specifically, ototoxin exposures may occur during equipment repair, metal machining, welding, coating/painting removal, coating/painting, and clean-up and handling of hazardous material.
The purpose of this study was to determine if occupational exposure to metals (consisting of lead, cadmium, and arsenic) and solvents (consisting of toluene and xylene) in addition to noise lead to more severe cases of hearing loss than exposure to noise alone among industrial shipyard workers.
Personnel employed by the Puget Sound Naval Shipyard (PSNS) who received reference and recurring pure tone audiometry (PTA) from 2004 to 2015 were included in this study. PSNS is responsible for the modernization and repair of U.S. Naval ships, with processes that include shipfitting, metal forging, welding, shipwrighting, fabric working, woodworking, and sheet metal fabrication. These activities expose workers to various ototoxins and hazardous noise.
IDENTIFYING HEARING IMPAIRMENT
Each employee's hearing loss was determined in decibels (dB) by subracting the decibel hearing level (dBHL) of the reference or first audiogram from the person's final audiogram. This change was determined for 500, 1,000, 2,000, 3,000, 4,000, and 6,000 Hertz (Hz) octave bands, and averaged across 2,000 to 4,000 Hz octave bands consistent with the OSHA Standard Threshold Shift (STS) criteria. Hearing change was also averaged across 500 to 6,000 Hz octave bands to determine the potential for broadband systemic toxicity from ototoxin exposures.
CHEMICAL AND NOISE EXPOSURES
Employee exposures were determined by measuring their inhalation exposures to the ototoxins (lead, cadmium, arsenic, toluene, and xylene) and personal noise exposures. Because of the complex combination of chemical and noise exposures, the study participants were classified as having “high exposure” to metals if their lead, cadmium, or arsenic exposure results exceeded respective OSHA action levels. Participants were classified to have “high exposure” to solvents if their toluene or xylene exposure exceeded 25 parts per million (ppm) and 3 ppm, respectively, consistent with previous investigations that found adverse audiological effects starting with these concentrations (Int J Occup Med Environ Health. 2007;20:309). Those with noise exposures ≥85 decibels “A” weighted (dBA) were considered to have “high exposure” to noise. Participants who did not meet these criteria were classified as having “low exposure” to noise and chemical stressors. Ultimately, the participants included in the investigation were classified into the following exposure groups:
- Low metals/Low solvents/High noise (reference group)
- High metals/High solvents/Low noise
- High metals/Low solvents/High noise
- High metals/High solvents/High noise
RESULTS AND DISCUSSION
Hearing loss in employees with chemical and noise exposures was assessed both at individual frequencies and across multiple frequencies collectively. At 1,000 Hz, hearing loss was significantly worse for workers exposed to high concentrations of metals (consisting of lead, cadmium, and arsenic) in combination with exposure to high noise levels compared to a group of workers with only high noise exposures (P=0.012). Hearing for workers with combined exposures to high concentrations of metals, solvents, and noise was significantly worse at 1,000 Hz (P=0.007), from 2,000 to 4,000 Hz (P=0.014), and across 500 to 6,000 Hz (P=0.014), compared with workers with only high noise exposures. Median hearing loss for workers with high metals/high solvents/high noise was 5 dB at 1,000 Hz, 3.3 dB across 2,000 to 4,000 Hz, and 2.1 dB across 500 to 6,000 Hz (Fig. 1). Median hearing loss in workers with high metals/low solvents/high noise was also 5 dB worse at 1,000 Hz compared with workers with low metals/low solvents/high noise exposure.
Noise-induced hearing loss typically affects the cochlear outer hair cells first between 2,000 and 4,000 Hz. However, metal and solvent ototoxins in the tissues appear to damage the outer hair cells in a more uniform manner by affecting the hair cells in the median portion of the basilar membrane around 1,000 Hz. Employees may not report noticing differences in their communication and sound detection abilities for losses ranging from 1.7 to 5 dB. However, if monitoring these hearing changes allows for earlier detection of hearing loss, occupational health professionals may be able to implement engineering, administrative, and personal protective equipment controls in environments with combinations of metals, solvents, and noise to prevent further hearing loss.
UNMET HEARING CONSERVATION NEEDS
Hearing loss resulting from the combination of metals, solvents, and noise exposure is a vital health concern. This investigation revealed that exposure concentrations less than maximum exposure limits for metals (lead, cadmium, and arsenic) and solvents (toluene and xylene), when combined with noise exposures, resulted in more hearing loss than noise exposure alone. Additional investigation is needed, particularly in determining what chemical concentrations are safe to the auditory system. However, the lack of ototoxin regulation suggests hearing conservation programs may not be taking chemical exposures into consideration. As a result, there may be numerous workers with unmet hearing conservation needs. Workplace interventions should target exposure reduction of multiple occupational stressors, not just noise, to reduce the risk of adverse workplace and social outcomes.
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