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When an employee’s annual audiogram shows a threshold shift, the first question is whether it was caused by occupational noise exposure or age-related hearing loss—or both. This distinction matters for OSHA recordkeeping, HPD program adjustments, and workers’ compensation. Noise-induced hearing loss (NIHL) and age-related hearing loss (presbycusis) produce distinct audiometric patterns. Understanding how to distinguish them is a core skill for any employer running a hearing conservation program under 1910.95.
Soundtrace audiometric testing includes baseline comparison, age correction options, and automatic STS flagging within its in-house and on-site hearing conservation platform.
Noise-induced hearing loss classically presents as a notch in the audiogram at 4,000 Hz, with partial recovery at 6,000 and 8,000 Hz. This notch occurs because the cochlear region tuned to 4 kHz is particularly vulnerable to noise damage due to its anatomical location and blood supply characteristics. Early NIHL may show only a slight 4 kHz dip; with continued exposure, the notch deepens and broadens into adjacent frequencies.
Primary NIHL signature: threshold elevated at 4 kHz, recovering at 6–8 kHz relative to 4 kHz
NIHL is typically bilateral and roughly symmetric; asymmetric loss suggests non-noise etiology
Early NIHL spares speech frequencies (500–2000 Hz); preserved speech discrimination
NIHL magnitude correlates with cumulative noise dose history
Age-related hearing loss typically produces a gradually sloping high-frequency loss without the 4 kHz notch recovery pattern. It affects both ears symmetrically and progresses gradually with age regardless of noise exposure. OSHA 1910.95, Appendix F provides age-correction tables that allow employers to adjust for expected aging when calculating whether an STS represents noise-induced change.
📈 4 kHz notch with recovery at 6–8 kHz
📈 Bilateral, symmetric
📈 Early sparing of low and mid frequencies
📈 Correlates with measured noise dose
📈 Plateaus without continued exposure
📉 Sloping high-frequency loss (no notch pattern)
📉 Bilateral, symmetric
📉 Progressively involves lower frequencies over time
📉 Correlates with age, not noise dose
📉 Continues regardless of exposure
OSHA permits but does not require employers to use age-correction values from Appendix F when determining whether an STS has occurred. Age correction reduces recordable STSs by factoring out expected threshold shift attributable to aging. NIOSH recommends against routine age correction on the grounds that it obscures noise-induced changes. If applied, the employer must document the decision and apply it consistently.
Age correction affects OSHA recordability of an STS—but not the obligation to refit and retrain. If the uncorrected shift meets the STS threshold (10 dB average at 2, 3, and 4 kHz), the employer must refit and retrain even if the age-corrected shift is below threshold. Recordability and the refit/retrain obligation are separate determinations.
When the audiogram pattern doesn’t clearly indicate NIHL or presbycusis, refer to an audiologist or physician before making program decisions
Regardless of etiology, confirmed STS requires HPD refitting and retraining under 1910.95(i)(4)
A deepening 4 kHz notch in a worker wearing adequate HPD suggests fit failure—move to quantitative fit testing and PAR verification
Asymmetric or unilateral threshold shifts have non-noise causes (acoustic neuroma, otitis media, medication); refer for medical evaluation
Distinguishing NIHL from presbycusis matters for recordability decisions, but the HPD program obligation is the same: an STS requires refit, retrain, and documentation regardless of etiology.
See: Hearing Protection & Fit Testing: The Complete Employer Guide
Soundtrace audiometric testing includes baseline comparison, STS detection, and age-correction options stored per employee in a cloud compliance platform.
Book a DemoGet a quote →Noise-induced hearing loss classically presents as a notch at 4,000 Hz with partial recovery at higher frequencies, bilateral symmetry, and correlation with noise dose. Age-related hearing loss (presbycusis) produces a gradually sloping high-frequency loss without the notch recovery pattern and progresses with age regardless of noise exposure. Both can occur simultaneously in older workers with noise exposure history.
OSHA permits employers to apply age-correction values from Appendix F of 1910.95 when calculating whether an employee has experienced an STS. Age correction factors out threshold shift attributable to expected aging, reducing recordable STSs. However, age correction affects only OSHA recordability, not the obligation to refit and retrain: if the uncorrected shift meets the STS threshold, the employer must still refit and retrain.
A 4 kHz notch is the characteristic pattern of noise-induced hearing loss, reflecting elevated thresholds at 4,000 Hz with partial recovery at 6,000 and 8,000 Hz. A deepening 4 kHz notch in a worker wearing hearing protection suggests HPD fit failure and should trigger quantitative fit testing and PAR verification of the employee’s current device.