Hazardous noise exposure affects millions of workers and could lead to hearing loss if not mitigated. However, chemicals such as heavy metals and solvents commonly found in industrial workplaces may damage the peripheral and central auditory systems. While noise has traditionally been considered among the primary risk factors for hearing loss, industrial work environments are complex venues that may expose workers to multiple hearing stressors like noise and a variety of air contaminants. Previous investigations have focused mainly on the ototoxic effects of solvents. This review, however, will focus on the combined effects of metal and solvent ototoxicants and noise in a work environment.
HEAVY METAL EXPOSURE AND MECHANISMS OF ACTION
A suggestive association between lead exposure and hearing loss is not new. Specifically, a 2013 report probed the high levels of lead in Beethoven's bones at the time of his death (Laryngoscope. 2013 Nov;123(11):2854). Shrunken cochlear nerves consistent with axonal degeneration was reported during his autopsy. While Beethoven's physicians believed he had alcohol dependence, lead exposure was suspected to have been a result of consuming lead-contaminated wine (Laryngoscope. 2013).
Heavy metals such as lead, cadmium, and arsenic are ubiquitous materials found both naturally in the environment and as a result of man-made activities. In addition to industrial environments, several consumer products have historically contained heavy metals, such as pipes containing lead and copper that were used to deliver drinking water to homes, leaded gasoline from automobiles, food cans with lead and tin solder, lead-based paint in homes, lead-arsenate pesticides, and cigarettes, to cite a few (Agency for Toxic Substances & Disease, 2007; J Korean Med Sci. 2015;30(3):272). Many of these sources have been eliminated or are regulated. However, the persistent nature of chemicals like lead may result in continuous exposure of both adults and children in the general population via ambient air, food, drinking water, soil, and dust.
While aromatic solvent ototoxicity is suspected of leading to hair cell chemical poisoning and direct action on the organ of Corti cells, the mechanism of action for metals remains a mystery (Dis Mon. 2013 Apr;59(4):119). It has been hypothesized that heavy metals may be classified as neurotoxicants due to their suspected adverse effect on the peripheral and central auditory systems. High doses of lead have been reported to increase the auditory nerve action potential threshold, produce segmental demyelination and axonal degeneration of the cochlear nerve without affecting the cochlear structure, and cause dysfunctions of the eighth cranial nerve (Neurotoxicology. 1993;14(2-3):191).
OCCUPATIONAL METAL EXPOSURE AND THE AUDITORY SYSTEM
The effect of manganese, copper, zinc, arsenic, cadmium, lead, and noise exposure was investigated using pure-tone audiometry (PTA) from 500 to 8,000 Hertz (Hz) of 412 steelworkers (Sci Total Environ. 2009 Dec 15;408(1):43). In the blood samples examined, only lead was significantly correlated with hearing levels. Analysis showed lead levels greater than seven micrograms per deciliter (µg/dL) of blood was significantly associated with hearing loss at 3,000 through 8,000 Hz. The odds of having a hearing threshold ≥25 decibels hearing level (dB HL) among workers with at least 7 µg/dL of lead in their blood ranged from 3.06 to 6.26 (p<.05) after adjustment for age and noise level (Sci Total Environ. 2009). The Occupational Safety and Health Administration (OSHA) classifies 40 µg/dL as the threshold requiring an annual medical exam and 60 µg/dL as the threshold for worker removal from the lead environment.
In a study of blood lead levels and noise exposures of workers in a lead acid battery facility, an average blood lead level of 56.9 µg/dL and average noise exposure of 86 dBA were identified (Arch Environ Health. 2000 Mar-Apr;55(2):109). Analysis revealed a statistically significant correlation between personnel with high blood lead levels and decreased hearing ability (Arch Environ Health. 2000).
In an investigation of printing facility workers exposed to lead, the average hearing thresholds of workers at 2,000, 4,000, and 8,000 Hz were found to be signiﬁcantly worse (p<0.05) by 4.11 dB, 2.89 dB, and 4.18 dB, respectively, than the hearing thresholds of non-exposed workers (Environ Res. 1997;73(1-2):189). Lead body absorption and lead exposure duration were also important in the progression of hearing loss. Workers with blood lead levels ≥30 µg/dL and 10 years of exposure duration had 3.14 dB and 4.01 dB worse hearing, respectively, at 8,000 Hz (Environ Res. 1997).
Another investigation that used auditory brainstem response (ABR) found lead-exposed workers to have signiﬁcantly longer absolute wave latency than non-lead exposed workers (Neurotoxicology. 1992 Spring;13(1):207). The lead exposure group had an absolute wave V latency 0.18 ms (95% Confidence Interval [CI] 0.11–0.25) longer than non-exposed workers (p = 0.000) (Neurotoxicology. 1992).
A study on the effect of noise and cadmium fumes on the hearing of metallurgical workers found that these workers had worse hearing at 4,000 and 6,000 Hz compared with a group of workers exposed only to noise (Rev Bras Otorrinolaringol. 2002;68(4):488).
An investigation of the relationship between filling type (amalgam, porcelain, or gold) and hearing thresholds revealed a significant association between mercury amalgam filling and hearing thresholds at ultra-high frequencies of 8,000 (p = 0.039), 11,200 (p = 0.044), 12,500 (p = 0.005), 14,000 (p < 0.001), and 16,000 Hz (p = 0.010) (Int J Audiol. 2008 Dec;47(12):770). Hearing thresholds at 14,000 Hz showed the strongest correlation (p < 0.001), where each additional amalgam ﬁlling was associated with a 2.4 dB increase in hearing threshold (95% CI 1.3–3.5 dB).
COMBINED EFFECTS OF METAL, SOLVENT, AND NOISE TO HEARING
Co-exposure to heavy metals and noise in different Korean workplaces was studied to determine if hearing loss risk was modified in an environment with hazardous noise exposure (Plos ONE. 2014;9(5):1). The researchers reviewed the workers’ metal and noise exposures and PTA evaluations. Workers exposed to both metals and noise had 64 percent greater odds (Odds Ratio (OR) =1.64) of hearing loss at 500 to 6,000 Hz compared with those exposed only to noise (Plos ONE. 2014).
An investigation of hearing loss in a shipyard identified workers exposed to various combinations of metals, solvents, and noise (Am J Ind Med. 2017;60(3):227). Exposures to a combination of high metal concentrations (including lead, cadmium, and arsenic) and solvents (including toluene and xylene) without the influence of noise increased the odds of developing significantly reduced hearing at 2,000, 3,000, and 4,000 Hz by nearly 2.5 times when compared with workers with exposures <85 dBA (OR of 2.4, 95% CI [1.462, 3.944]). In another shipyard investigation, workers exposed to a combination of noise and high concentrations of lead, cadmium, arsenic, toluene, and xylene were found to have significantly worse hearing at 1,000 Hz and 2,000 to 4,000 Hz compared with a group of workers exposed only to high levels of noise (J Occup Environ Med. 2018 Jan;60(1):e55).
DISCUSSION AND FUTURE STUDIES
Ototoxicant investigations have used a variety of methods such as biological monitoring results to determine the chemical dose received by a worker. Hearing change measured using PTA suggests metals and solvents adversely affect one's sound detection abilities due to sensorineural hearing loss. Investigations that revealed longer ABRs suggest chemicals such as lead, cadmium, and mercury may adversely affect the central auditory pathways.
Today, more studies are being done to examine the effects of combined exposures to metals, solvents, and noise. However, because of the high complexity of identifying workers with all possible exposure combinations, the specific chemical concentrations when chemicals begin to elicit adverse audiological outcomes is not known. Previous studies have suggested that ototoxicants decrease hearing most in the middle and high frequencies. Future studies should focus on better defining the mechanism of action of heavy metals to determine which audiological test battery or combination of batteries is most effective in detecting adverse audiological conditions. Specifically, other audiological tests to supplement PTA should be investigated so adverse audiological outcomes beyond sensorineural hearing loss may be detected. Previous studies have also suggested that middle- and high-frequency hearing loss cases are most affected by ototoxicants. Future investigations should continue to explore the frequencies most affected by metal exposures to differenciate between chemical- and noise-induced hearing loss.
Occupational health practitioners such as audiologists, occupational medicine physicians, and industrial hygienists all have a role in assessing the potential adverse audiological outcomes in industrial work settings. Exposure evaluations of workplaces should consider the full spectrum of hearing stressors rather than just noise. Audiologists should consider heavy metal exposure when taking workers’ clinical history, and note other auditory signs beyond just sound detection difficulty when examining a patient with a history of heavy metal exposure. Audiological evaluations and behavioral techniques should also be used to assess the effects of metal exposure on the central auditory pathway.
Disclosure: The contents of this publication are the sole responsibility of the authors and do not necessarily reflect the views, opinions or policies of Uniformed Services University, the Department of Defense, or the Department of the Navy. Mention of trade names, commercial products, or organizations does not imply endorsement by the U.S. Government.