What Is a Significant Threshold Shift? How Australian Employers Must Respond
A significant threshold shift (STS) is the moment when abstract audiometric data becomes concrete workplace obligation. When testing reveals that a worker’s hearing has deteriorated by a meaningful amount compared to baseline, the employer is no longer observing hearing—the employer is acting. This article explains what STS is, how it’s defined, and exactly what the employer must do when one is detected.
For WHS managers, understanding STS is critical because failure to respond appropriately is a direct violation of workplace health and safety duties.
What a Threshold Shift Is
A threshold shift is any measurable change in hearing threshold from one test to the next.
Example:
- Baseline test (2024): 20 dB HL at 4000 Hz (normal hearing)
- Retest (2025): 28 dB HL at 4000 Hz (elevated threshold)
- Shift: 8 dB (threshold elevated by 8 dB)
This is a threshold shift, but not necessarily a significant one. Threshold shifts of 3–10 dB can occur naturally due to:
- Normal variability in test conditions
- Slight differences in how the worker responds on different days
- Minor ambient noise variations
- Normal day-to-day hearing fluctuation
To distinguish meaningful deterioration from natural noise, the standard defines “significant” threshold shift.
Temporary vs Permanent Threshold Shift
Before defining significance, distinguish between two types of shifts:
Temporary Threshold Shift (TTS)
What it is: A reversible shift that resolves within hours to days of quiet exposure
Cause: Recent noise exposure causing cochlear fatigue
Timeline: Develops during or immediately after noise exposure; resolves fully within 16–18 hours of quiet
Significance: Not permanent damage, but indicates the cochlea is being stressed. Repeated TTS episodes can eventually result in permanent threshold shift.
Testing implication: This is why the 14-hour quiet period before testing is required. If testing occurs while TTS is present, the audiogram appears worse than the worker’s true hearing and gives misleading results.
Permanent Threshold Shift (PTS)
What it is: An irreversible shift representing actual cochlear (inner ear) damage
Cause: Cumulative noise exposure, aging, ototoxic chemical exposure, or medical conditions
Timeline: Develops over weeks/months/years; does not recover with rest
Significance: Represents a real and lasting change in hearing. This is what concerns WHS law.
Detection: Confirmed by retesting after adequate quiet and comparison to baseline
AS/NZS 1269 Significant Threshold Shift Definition
In Australia, the standard for WHS occupational audiometry (AS/NZS 1269:2005 and referenced in Regulation 58) defines significant threshold shift as:
A change of 15 dB or more at any single frequency compared to baseline, OR a change of 10 dB or more at two or more adjacent frequencies.
How This Works in Practice
Example 1: Single-Frequency Shift
- Baseline at 4000 Hz: 20 dB HL
- Retest at 4000 Hz: 37 dB HL
- Shift: 17 dB (exceeds 15 dB threshold)
- Result: Significant threshold shift
Example 2: Multi-Frequency Shift
- Baseline: 3000 Hz = 18 dB HL, 4000 Hz = 20 dB HL
- Retest: 3000 Hz = 28 dB HL, 4000 Hz = 30 dB HL
- Shifts: 10 dB at 3000 Hz, 10 dB at 4000 Hz (two adjacent frequencies, both 10+ dB)
- Result: Significant threshold shift
Example 3: Small Shift (Not Significant)
- Baseline at 4000 Hz: 20 dB HL
- Retest at 4000 Hz: 28 dB HL
- Shift: 8 dB (below 15 dB threshold)
- Result: Threshold change noted, but not significant. Monitor closely.
Note on Different Standards
OSHA standard (USA): Different definition—10 dB average shift at 2000, 3000, 4000 Hz. Not applicable in Australia, but worth knowing if you work with US operations.
Australia (AS/NZS): 15 dB at any single frequency. More conservative than OSHA.
WHS Employer Obligations When STS Is Detected
When STS is detected, the employer has specific legal obligations under WHS law:
Obligation 1: Review and Improve Control Measures
Legal basis: WHS Regulation 38–39 (duty to manage occupational noise risk)
What this means: The employer must treat the STS as evidence that current noise controls are inadequate. The response is not punitive—it’s recognition that the engineering, administrative, or PPE controls need improvement.
Action steps:
- Review current noise control measures (engineering, administrative, hearing protection equipment)
- Conduct (or revisit) a noise risk assessment
- Identify why controls failed to prevent deterioration
- Implement improved controls: engineering first (reduce noise at source), then administrative or PPE
- Document the review and improvements in WHS records
Example response: STS detected in a factory operator. Investigation shows hearing protection compliance is only 60% (workers not consistently wearing plugs). Response: implement hearing protection training and compliance program, install automatic HPE dispensers, improve hearing protection fit-testing, consider noise engineering improvements to reduce reliance on PPE.
Obligation 2: Refer the Worker for Specialist Assessment
Legal basis: WHS Reg 58; employer duty of care
What this means: The employer must arrange for the worker to see an audiologist or ENT specialist for further evaluation. The specialist determines whether the shift represents work-related hearing loss or another cause.
Cost responsibility: The employer bears the cost of the specialist assessment as part of WHS obligations.
Process:
- Notify the worker of the STS finding and need for specialist referral
- Arrange referral to a local audiologist or ENT (can coordinate through occupational health service or GP)
- Provide worker with referral letter and their audiogram results
- Specialist conducts detailed assessment: full audiological testing, medical history, physical examination
- Specialist report confirms or clarifies the diagnosis and recommendations
Obligation 3: Record the Shift and Control Review
Legal basis: WHS recordkeeping; demonstrating due diligence
What to document:
- Date STS was detected
- Specific frequencies affected and degree of shift (in dB)
- Comparison to baseline (include audiogram copies if available)
- Date worker was notified
- Date specialist referral was arranged
- Summary of control review undertaken
- Improvements implemented (with dates)
- Specialist assessment results and recommendations (once received)
- Any follow-up monitoring plan
Why documentation matters: In the event of a workers’ compensation claim or regulator investigation, your documentation proves you took STS seriously and acted appropriately. Absence of documentation suggests negligence.
Can a Worker Continue Working After STS?
A critical question for WHS managers: “Must I remove the worker from noise exposure?”
Answer: Not automatically, but it depends.
Scenario A: STS with Adequate Controls in Place
If STS is detected, controls are reviewed and improved, and the specialist confirms the shift is not severe enough to require removal from work, the worker may continue in their role with enhanced controls.
Example: STS of 15 dB at 4000 Hz detected. Review reveals hearing protection compliance was inadequate. Employer implements intensive HPE training and fit-testing. Specialist assessment confirms early-stage NIHL but no severe loss. Worker continues in role with improved hearing protection and more frequent monitoring (annual instead of biennial testing).
Scenario B: STS with Inadequate Control Effectiveness
If STS is detected and controls are reviewed but found to be working (high compliance, modern equipment), deterioration may indicate the worker is unusually susceptible to noise damage. Options:
- Reassign the worker to a lower-noise role (if available)
- Intensify monitoring and HPE, with agreement from the worker
- Investigate other risk factors (ototoxic chemical exposure, hearing loss from other source)
Scenario C: Severe STS or Specialist Recommendation
If STS is severe (>25 dB shift) or the specialist recommends removal from noise exposure, the employer must modify the worker’s role or re-deploy to non-noisy work.
WHS obligation: The employer cannot require a worker to continue in a noisy role that is damaging their hearing and has been shown to cause deterioration.
Practical approach: Discuss with the worker. Many will prefer ongoing monitoring with better HPE rather than reassignment (especially if it means lower pay or less preferred work). But if hearing deterioration is clear, reassignment is the safer legal option.
Tracking Threshold Shifts Over Time
With multiple testing cycles, you’ll build a longitudinal record showing hearing trends:
- Stable thresholds across years: Controls are working; hearing is protected. Continue current program.
- Gradual decline: Hearing is slowly deteriorating despite controls. Investigate causes, improve controls, increase monitoring frequency.
- Sudden large shift: Major deterioration. Immediate investigation, specialist referral, and control review required.
- Shift in one worker only: May indicate asymmetric exposure, poor HPE compliance, or individual susceptibility. Targeted intervention.
- Widespread shifts across multiple workers: Systemic control failure (e.g., noise levels rising, HPE program broken, maintenance neglected). Major corrective action required.
This longitudinal view is one reason regular (annual or biennial) testing is valuable. One-off testing cannot show trends.
Frequently Asked Questions
Q1: Is a 10 dB shift significant?
Not according to AS/NZS 1269 (which requires 15 dB at one frequency). However, 10 dB shifts should be monitored closely, especially if occurring at multiple frequencies or in the same worker over multiple years.
Q2: What if a worker refuses specialist referral?
Document the refusal. The employer has fulfilled the obligation to arrange and refer; if the worker declines, note this in records. Continue monitoring with enhanced testing frequency.
Q3: Does STS mean the worker has NIHL?
No. STS means deterioration in hearing threshold compared to baseline. Whether this represents work-related NIHL or another cause is determined by specialist assessment. STS is a trigger for investigation, not a diagnosis.
Q4: How long after STS should the specialist referral happen?
Quickly. Best practice: within 2–4 weeks. The sooner specialist assessment occurs, the sooner recommendations for management are available.
Q5: Can we make the worker return to baseline testing?
No. Once an audiogram shifts, the previous baseline becomes the new reference. You can’t undo a shift. The next test will compare to the new baseline.
Q6: Does STS automatically mean a workers’ comp claim?
No. STS triggers investigation and control review. Whether the worker has a work-related condition requiring compensation is determined by specialist assessment and the worker’s history. Some STS will result in NIHL claims; some will not.
Q7: If we improve controls and retest, could hearing recover?
No. Permanent threshold shift (PTS) does not recover. Improved controls will prevent further deterioration, but won’t reverse past damage. Hearing aids or other management may be needed.
Q8: What if STS is detected but the worker says they feel fine?
Absence of subjective symptoms doesn’t negate objective test findings. Early hearing loss may not be noticeable to the worker. Proceed with control review and specialist referral.
