Exposure to lead, mercury, styrene, and toluene and hearing impairment: evaluation of dose-response relationships, regulations, and controls.
Level 3 - non-randomized controlled study
Systematic review of observational epidemiological studies.
PubMed 33275083 · doi:10.1080/15459624.2020.1842428
What was done
A systematic review of Ovid Medline and Ovid Embase databases identified 86 epidemiological studies investigating dose-response relationships between exposure to lead, mercury, toluene, or styrene and hearing impairment. Authors extracted No Observed Adverse Effect Levels (NOAELs) and Lowest Observed Adverse Effect Levels (LOAELs) where possible, and conducted cross-jurisdictional comparisons of occupational health and safety regulations across the United States, United Kingdom, Canada, and Australia.
What was found
Increased risks of hearing loss were reported in 75% of lead studies, 74% of styrene studies, and 77% of toluene studies. However, 70% of toluene studies and 16% of styrene studies evaluated combined exposure to solvent mixtures or solvent mixtures with noise rather than isolated chemical exposure. Based on five studies, blood lead NOAELs ranged from 1–1.99 µg/dL to 2.148–2.822 µg/dL, and LOAELs ranged from 2 µg/dL up to 2.823–26.507 µg/dL. Mercury-specific numerical thresholds were not reported in the abstract. Regulatory review found no specific enforceable occupational limits for ototoxic chemicals in the US, Canada, or Australia outside of general duty clauses.
Why it matters
This review highlights that occupational auditory damage occurs at chemical exposure levels currently considered low or unregulated, supporting a recommended biological exposure index of 2 µg/dL for blood lead and lower noise thresholds (80 dB(A) for 8 hours) when chemical co-exposures occur.
Limits
The abstract provides no quantitative data or threshold ranges for mercury. Confounding from simultaneous occupational noise and mixed solvent exposures was common, particularly for toluene and styrene, making single-agent causality difficult to establish from the summarized literature.
Cited by
- supports Heavy metals such as lead and mercury are toxic to neurons in the ear as well as other neurons in the body.