NIHL vs. Age-Related Hearing Loss: Audiogram Guide for Employers

Noise-induced hearing loss and age-related hearing loss (presbycusis) are the two most common forms of sensorineural hearing loss in working-age adults — and in many workers over 45 with long careers in noisy industries, both are present simultaneously. For employers, safety managers, and anyone reviewing occupational audiograms, the ability to distinguish them directly determines OSHA STS calculations, workers’ compensation apportionment, OSHA 300 Log recordability decisions, and the appropriate follow-up action for each affected worker.

Soundtrace provides audiometric surveillance with STS flagging and trend analysis designed to support the distinctions that matter for OSHA compliance and WC defense.

Noise-Induced Hearing Loss (NIHL)

Noise-induced hearing loss is a permanent sensorineural hearing loss caused by acoustic trauma to the cochlear hair cells. It develops from cumulative exposure to sound levels at or above 85 dBA over time, or from sudden high-intensity acoustic events. The result is cochlear hair cell death — and in humans, these cells do not regenerate.

Key characteristics of occupational NIHL:

• Always sensorineural (no conductive component)
• Almost always bilateral, though asymmetry is possible
• Begins at 3000–6000 Hz, most distinctively at 4000 Hz
• Does not progress after noise exposure stops
• Loss at 3000–6000 Hz always greater than at 500–2000 Hz
• High-frequency losses rarely exceed 75 dB HL; low-frequency losses rarely exceed 40 dB HL
• Characteristic audiometric finding: a notch at 4000 Hz with some recovery (better thresholds) at 8000 Hz

For a detailed explanation of NIHL stages and progression, see Noise-Induced Hearing Loss: Symptoms, Stages, and What Employers Need to Know.

Age-Related Hearing Loss (Presbycusis)

Presbycusis is the gradual sensorineural hearing loss associated with biological aging of the auditory system. It is the most common cause of acquired hearing loss in adults, measurable in most people from approximately age 50 onward.

Presbycusis results from multiple age-related processes:

• Sensory presbycusis: Progressive cochlear hair cell death, particularly in the high-frequency basal region, from cumulative oxidative stress
• Neural presbycusis: Loss of spiral ganglion neurons and auditory nerve fibers, reducing neural coding capacity
• Metabolic (strial) presbycusis: Reduced function of the stria vascularis, producing a flatter, more uniform hearing loss pattern

Key characteristics of presbycusis:

• Always sensorineural, bilateral and typically symmetric
• Smooth, gently sloping high-frequency loss progressively worsening from approximately 2000 Hz through 8000 Hz
• No recovery notch at 8000 Hz — the critical audiometric distinction from NIHL
• Progresses continuously throughout life regardless of noise exposure status
• Rate predictable enough that standardized age-correction tables exist (ISO 7029, OSHA Appendix F)

Audiogram Patterns: The Key Diagnostic Differences

The NIHL audiogram: notch-and-recovery

The hallmark of NIHL on a pure-tone audiogram is the 4000 Hz notch with recovery:

• Thresholds at 500–2000 Hz are normal or near-normal (0–25 dB HL)
• Threshold drops sharply at 3000–4000 Hz, reaching the worst point at 4000 Hz
• Threshold then recovers — improves — at 6000–8000 Hz
• In advanced NIHL, the notch widens but 4000 Hz typically remains the worst frequency

The presbycusis audiogram: smooth downward slope

• Thresholds at 500–1000 Hz typically normal or minimally affected
• Thresholds decline progressively and smoothly from approximately 2000 Hz onward
• 4000 Hz is worse than 2000 Hz; 6000 Hz worse than 4000 Hz; 8000 Hz worst of all
• No recovery at 8000 Hz — thresholds continue to worsen through the highest frequencies
• Smooth and gradual slope, not sharply notched

▶ The single most useful diagnostic feature: if a worker’s audiogram shows better hearing at 8000 Hz than at 4000 Hz, that is evidence of the NIHL notch pattern. If hearing at 8000 Hz is equal to or worse than at 4000 Hz, the pattern is consistent with presbycusis.

Side-by-Side Comparison

When Both Are Present: Mixed Loss

In workers over 50 with significant occupational noise exposure histories, NIHL and presbycusis frequently coexist. This is the norm for a substantial portion of the older industrial workforce. The two conditions are additive and their combined audiometric effect can be difficult to disentangle.

When both are present, the classic NIHL notch may be obscured because presbycusis adds high-frequency loss that “fills in” the recovery at 8000 Hz. The audiogram may show a broad, deep trough across 3000–8000 Hz rather than a clean notch-and-recovery. The audiometric history — the baseline and sequence of annual audiograms over time — provides far more diagnostic information than any single audiogram.

Key diagnostic considerations when both are present:

• A baseline audiogram obtained early in employment (before significant noise exposure) is the most valuable single record. A notch pattern at baseline that deepens with time, plus broadening loss in upper frequencies over decades, is consistent with combined NIHL and presbycusis.
• Rate of progression matters. NIHL produces greater loss in the first 10–15 years then slows; presbycusis accumulates at a roughly age-predicted rate. If loss is progressing faster than age-correction tables predict, noise is likely contributing.
• Asymmetric loss is a red flag for noise. Pure presbycusis is typically symmetric. Asymmetry suggests noise, particularly when the worse ear corresponds to the side more exposed to the noise source.

Age Correction Under OSHA: What It Is and When It Applies

Federal OSHA’s 29 CFR 1910.95, Appendix F, provides optional age-correction tables employers may use when calculating whether an STS has occurred. The tables specify expected age-related threshold increase at each frequency for males and females across age ranges.

The mechanics: when comparing the annual audiogram to baseline, if the worker is older at the annual than at baseline, the employer may subtract the predicted age-related threshold increase from the measured shift before calculating the 10 dB STS average. If the measured shift averages 12 dB but age correction attributes 5 dB to aging, the age-corrected STS is 7 dB — below the threshold. No STS obligation is triggered.

State Variation: Washington’s No-Age-Correction Rule

Federal OSHA permits age correction as an option. Washington State’s WISHA standard (WAC 296-817) does not permit age correction when determining whether an STS has occurred. Washington employers must compare raw audiometric data to baseline without any presbycusis adjustment — generating more STS flags in older workers than federal OSHA would for an identical workforce.

Washington does permit age correction when determining recordability on the WISHA 300 Log under WAC 296-27-01113 — but not for the STS determination itself. For Washington’s aging manufacturing, food processing, and wood products workforces, this is a material operational difference. See the Washington State / WISHA Hearing Conservation Guide.

Workers’ Compensation Apportionment

When a noise-exposed worker files an occupational hearing loss WC claim, the employer may seek to apportion loss between noise-caused (compensable) and age-related (non-compensable) components. States vary widely in how they handle this. Some apportion using ACOEM methodologies; others use last-injurious-exposure rules that assign full liability to the last noise-exposing employer regardless of age-related contribution.

For state-specific WC SOLs, apportionment rules, and filing requirements, see the 50-State Occupational Hearing Loss WC Guide.

OSHA 300 Log Recordability

Under 29 CFR 1904.10, an occupational hearing loss case is recordable when: (1) the hearing loss is work-related; and (2) the current audiogram shows a 25 dB or greater average hearing level at 2000, 3000, and 4000 Hz compared to audiometric zero — without age correction.

The work-relatedness determination is where the NIHL vs. presbycusis distinction applies directly. If a physician or audiologist determines hearing loss is due entirely to presbycusis with no occupational noise contribution, it is not work-related and not recordable. When noise exposure is present alongside threshold loss, OSHA generally presumes work-relatedness. The employer bears the burden of demonstrating the loss is not work-related to avoid recording it.

▶ Bottom line: For older workers in noisy industries, the noise-related vs. age-related distinction is a recurring compliance judgment that depends on audiometric history, noise exposure records, and medical review. Long-term, unbroken audiometric records from hire to termination have both compliance and WC defense value.

Frequently asked questions