Historic exploration into Pb ototoxicity involved high doses of Pb that caused frank toxicity. Of the thirteen in vivo studies, nine involved high doses of Pb. Eight of these caused animal death or weight loss due to Pb treatment, while one study used a high dose of Pb and euthanized animals two hours after treatment. Six of these eight studies established the strong possibility of alterations in the auditory system, and possible neural damage, following Pb exposure; however, these findings may have been confounded by systemic toxicity effects. Two studies noted no auditory effects (Wilpizeski and Vogel 1972, Fazakas et al. 2005), though these negative results could have resulted from very small sample size (i.e., Wilpizeski et al., 1974), or the use of uncommon testing procedures (i.e., the auditory cortical evoked potentials and electrocorticography used by Fazakas et al., 2005).

Studies displaying positive results for auditory dysfunction following high doses of Pb predominately used auditory brainstem response (ABR) and analysis of thresholds or waveforms. ABR can be performed in humans and rodents; five main waves are measured as neuronal signals pass from the cochlea to the auditory cortex in the brain. Alterations in the amplitude or latency of these waves can be signals of pathology. Hearing thresholds, interpreted as waveforms present, but diminishing in amplitude as the stimulus presented quiets, are another measure of hearing ability. In humans, thresholds from 0–19 dB are considered within the normal hearing range. Thresholds of 20 dB or more demonstrate different degrees of HL; thresholds of 20–34 dB show mild HL, 35–49 dB show moderate HL, 50–64 moderately severe, 65–79 dB show severe, 80–94 dB show profound HL, and 95 dB and over show complete HL (Vos et al. 2015). The World Health Organization defines HL as thresholds at or above 25 dB in one or both ears for pure-tone single frequency audiometry (World Health Organization 2018). Mean auditory thresholds vary by species and stimuli (particularly the frequency or pitch), but are similar to humans (Zheng et al. 1999).

Takahashi et al. (1984) found a significant increase in latency of N1 and a significant increase in P1-N1 amplitude after the sixth day of treatment. A significantly longer latency of N1 action potential was observed by Yamamura et al. (1984). Output voltage lowered under 20 dB in the highest Pb treatment group of Yamamura et al. (1987). Whole nerve action potentials were elevated to 25 dB after five weeks of treatment in results from Yamamura et al. (1989). Histopathology explored by Gozdzik-Zolnierkiewicz et al. (1969) showed axonal degradation and demyelination in the vestibulocochlear nerve (VIII cranial nerve), but no pathology in inner ear cells or in the spiral ganglion.

The study that used a terminal procedure explored the effects of Pb, both Pb acetate (PbAc) and tetra ethyl Pb on cochlear microphonic and compound action potentials thresholds. In guinea pigs, auditory nerve compound action potentials were disrupted due to administration of Pb acetate and increased thresholds by 5–10 dB at high and mid frequencies (Tuncel et al. 2002). Threshold shifts in the tertra ethyl Pb group were significantly greater than controls.

The four studies that administered lower doses of Pb employed different measures of auditory function and generally demonstrated ototoxicity. Nagymajtényi et al. (1996) used electrocorticography, but in contrast to the study with negative results in a high dose of Pb, found a significant decrease in electrocorticography index in the auditory centers, and a significant increase in mean frequencies for the study’s highest and longest treatment group. Liu et al. (2011) found changes in ABR wave I-V latencies in rats exposed to Pb acetate via gavage. Jamesdaniel et al. (2018) used ABR in mice to determine significant 8–12 dB shifts from baseline threshold at 4, 16, 24, and 32 kHz. In contrast, Carlson et al. (2018) did not find significant changes in threshold shifts or distortion product otoacoustic emissions (DPOAEs) after Pb treatment, and no outer hair cell loss in the cochlea.

The majority of animal studies included in this review are laboratory in vivo studies. However, one in vitro study of cochlear outer hair cells from adult guinea pigs is summarized in Table 1 and 2. This study showed alterations of neural signaling necessary for auditory processing following exposure to Pb. PbAc decreased outward potassium currents in adult pigmented guinea pigs outer hair cell explants (Liang et al. 2004).

Further in vitro studies on neurological tissues indicate possible mechanisms for Pb toxicity in the auditory system, though the following studies were not performed with auditory tissues. Nerve terminals from rat brains receiving Pb treatment impacted the crucial proton gradient of synaptic vesicles and decreased glutamate accumulation inside these synaptic vesicles (Borisova et al. 2011); this is related to auditory processing as glutamate is the primary neurotransmitter used in ascending auditory neurons (Raphael and Altschuler 2003).

The included in vivo studies explored a variety of exposures involving Pb. Several studies showed ototoxic effects; for example, acute low Pb doses of PbAc caused delayed auditory wave latencies in rats, which were alleviated through doses of copper (Liu et al. 2011). Phenyl-tert-butyl-nitrone administration alleviated threshold shifts from tetra ethyl Pb to some degree (Tuncel et al. 2002). Other studies found negative results. Fazakas et al. (2005) suggested that alcohol administration counteracted the effects due to Pb, but changes in electrocorticography in the auditory center due to a combination of Pb, mercury, and alcohol administration were not significantly different from a control group (Fazakas et al. 2005).

In vivo studies are necessary to understand the combination of physical and chemical hazards that have been explored only in two recent studies. Carlson et al. (2018) did not observe any significant changes in thresholds from combinations of Pb and cadmium; Pb, cadmium and noise; or Pb and noise. Jamesdaniel et al. (2018) observed greater threshold shifts to animals exposed to both Pb and noise compared to mice exposed to Pb alone suggesting potentiation.

In conclusion, the majority of animal studies identified in this review point to a neurological source of damage and support the ototoxic properties of Pb.

The largest area of concern and variation with epidemiologic study methodology was the varying quality and type of hearing examinations. Good hearing requires a functional outer, middle, and inner ear, and unimpeded transmission of neural signals from the vestibulocochlear nerve, through the brainstem, to the primary auditory cortex in the cerebral cortex of the brain. An important need for both toxicological and epidemiological studies is to examine the location and mechanism of hearing damage due to Pb exposure, and its interaction with noise. For example, while noise-induced damage typically occurs as cochlear hair cell death, the toxicological studies suggest damage due to Pb take place in the neural processing networks. Without this knowledge, the optimal measure (or measures) of hearing is unclear.

The twenty-four included human studies used a range of hearing tests, including DPOAEs, pure-tone audiometry, and brainstem auditory evoked response (BAER), each of which measures different aspects of the hearing system. DPOAEs, used in two human studies, are best used to assess cochlear outer hair cell function. Audiometry, used in fifteen studies summarized for this review, identifies the lowest level of subjectively detectable sound. Finally BAER, also called brainstem auditory evoked potentials (BAEP), used in eleven studies, measures neuronal transmission of action potentials to the auditory center of the brain. As Pb is a known neurotoxicant, further exploration into modification of auditory neural processing through BAER is likely worth investigation in Pb ototoxicity studies. Past studies have found that correlations between pure-tone audiometry and DPOAEs may be poor (Engdahl et al. 2013). Determining the best auditory function measures for epidemiological analysis will require a more complete understanding of mechanisms surrounding Pb ototoxicity.

Regardless of the test used, a quiet environment is necessary to ensure accurate hearing measurements. Four studies described the environment where tests occurred; background noise levels during these tests ranged from a “quiet area” (Counter and Buchanan 2002) to a sound-treated chamber (Park et al. 2010). The background levels of 30 (Hwang et al. 2009) and 50 dB (Wu et al. 2000) in two human studies are troubling, as normal thresholds are in the range of 0–20 dB. Prior to testing, tympanometry and visual inspection of the tympanic membrane to rule out ear infections or obstructions are needed. However, these tests were only described by one (Farahat et al. 1997) of the twenty-four studies.

Temporary threshold shifts due to noise exposure are another important consideration that could substantially alter the results of hearing tests and attenuate potential relationships between Pb, noise, and HL. Only two of the twenty-four human studies gave details on the timing of tests after the last exposure to high noise: fourteen (Wu et al. 2000) and sixteen (Chuang et al. 2007) hours of quiet, respectively.

Finally, due to the resilient nature of the hearing system (Pepler et al. 2014), it may be essential to rely on multiple tests to determine deficits in hearing function. To address this limitation, we recommend use of BAER to evaluate auditory neural processing, in combination with pure-tone audiometry to evaluate perception of sound.

Pb levels, measured in twenty-two of the twenty-four studies, were the most frequently used assessment technique. Other techniques included measurements of airborne concentrations of Pb (used in one study) and measurement of blood Pb in fingernails (used in one study). While blood lead levels (BLLs) may be representative of recent exposures, they do not account for the accumulation of Pb in the bones following chronic exposure. Blood Pb has a half-life of about 30 days (Roberts et al. 2001), depending on initial levels, but also represents an equilibrium of the body burden carried in the bone. Bone Pb has a half-life over decades or longer (Wilker et al. 2011) . Two epidemiological studies used time-weighted averaging approaches to address the critical issues of exposure timing and intensity (Hirata and Kosaka 1993, Bleecker et al. 2003). One study measured the duration of exposure and Pb concentrations in ambient air (Hirata and Kosaka 1993), and both studies investigated the gold standard: individual monitoring of past BLLs from workers’ five-year history. Park et al. (2010) took advantage of a recent development in measuring cumulative Pb exposure and used X-ray fluorescence to quantify Pb in bone. Use of this approach should increase the ability to accurately estimate metals exposure over long periods of time where BLLs are not especially meaningful.

Noise exposure is a major concern in occupational studies, since noise is a nearly ubiquitous occupational exposure. In some workplaces, Pb and noise exposure may co-occur, requiring an even more thoughtful exposure assessment to evaluate the exposures concurrently. Five (Farahat et al. 1997, Wu et al. 2000, Bleecker et al. 2003, Chuang et al. 2007, Hwang et al. 2009) of the twenty included occupational studies suggested HL in workers exposed to both noise and metals greater than expected for the amount of noise exposure alone. However, none of these studies provided sufficient information on the levels of occupational noise to which workers were exposed. Only one study investigated a link between noise exposure and heavy metals exposures for mining occupations (Saunders et al. 2013). The findings were inconclusive due to confounding issues of poor health as well as the large number of statistical analyses performed on multiple metals.

Quantification of environmental or leisure noise exposure is also critical. Of the twenty-four epidemiological studies included, two (Choi et al. 2012, Huh et al. 2016) included some adjustments and exploration into sources of noise that were non-occupational. Firearms create unique blast noise exposure, which is a threat to the entirety of the hearing system. There is also potential of additional Pb exposure from the use of firearms. Moreover, the use of guns is often correlated with high occupational noise exposure (Agrawal et al. 2010). Determination of leisure activities and community settings where loud noise exposures occur is essential. Communities feature multiple sources of noise exposure – e.g., transportation (Neitzel et al. 2012) – and all of these sources must be assessed to comprehensively evaluate the risk of HL with or without the presence of Pb.

Demographic information for participants was difficult to evaluate in a number of human studies. Every study listed participant age ranges and accounted for increased HL with age; however, this was often confounded by Pb exposure metrics that also increased with age or duration of employment. Given the correlation between these factors, more thoughtful analysis is needed. As HL levels have been found to vary by race (Sun et al. 2014, Hoffman et al. 2017) and income level (Chou et al. 2015), detailed information on participants characteristics is necessary for a clear understanding of possible confounders. Choi et al. (2012) had a great deal of information on their participants and detailed hearing outcomes as significantly different for non-Hisapnic whites and non-Hispanic blacks; this study also found borderline significant differences between BLL and threshold levels for Mexican-Americans and non-Hispanic whites.

In addition to these four sources of uncertainty in the identified studies described above, there are two major areas of potential confounding (protective factors and exposures to mixtures of ototoxicants) that need to be addressed more systematically. These are described below.

Accurately examining potential modification and confounding of Pb ototoxicity by noise exposures will allow for more protective health policies and procedures. Workers exposed to Pb have been shown to have higher levels of oxidative stress (Khan et al. 2008). These levels may have negative effects on their hearing ability. Encouragingly, changes to neurobehavioral performance have been shown to be reversible in Pb workers when exposures were reduced (Chuang et al. 2005). This may also be the case for negative effects of Pb on hearing.

Relevant health policy changes have recently occurred for environmental Pb exposures. In 2012, the CDC lowered the community action limit of Pb in the blood to 5 μg/dL (CDC 2012). Conversely, the Occupational Safety and Health Administration has not updated its BLL standards since 1978 (National Toxicology Program 2003). This agency requires workers whose BLL are at or above a 40 μg/dL limit to receive medical examinations. The medical provisions of 29 CFR § 1910.1025 could be amended to require a hearing examination to assess possible influence of Pb on auditory outcomes. This modification could assist in investigating the links of Pb and HL.

Publication of all studies exploring the ototoxic nature of Pb is needed, whether outcomes are positive or negative. While the results of the available literature suggest some consensus of ototoxic findings, it is worth considering that these results may have been influenced by publication bias and the relative absence of published studies with negative results (Song et al. 2014).