Occupational noise exposure represents a significant health challenge in many industrial environments. The term “Standard Threshold Shift” (STS) is a specific technical designation used within hearing conservation programs to flag a change in hearing sensitivity. It serves as an early warning that a worker’s hearing may be deteriorating, necessitating immediate action to protect against further loss. This formal designation ensures consistent identification and follow-up across workplaces.
Defining Standard Threshold Shift
The Standard Threshold Shift is a precise, regulatory term used to identify a change in an employee’s hearing ability compared to their initial hearing test results. It acts as an indicator that a measurable change has occurred, but does not automatically signify permanent hearing loss. The Occupational Safety and Health Administration (OSHA) uses this designation to mandate specific protective and educational actions within a workplace hearing conservation program.
The trigger for an STS is a specific average change in hearing sensitivity, measured in decibels (dB), relative to the employee’s baseline test. This measurement indicates that a worker now requires a louder sound to perceive a tone than they did during their initial audiogram. The shift is considered an early sign of potential noise-induced hearing damage, which is often reversible if caught early.
Establishing the Baseline Measurement
Monitoring hearing health depends on establishing an accurate initial measurement known as the baseline audiogram. This first valid hearing test provides the reference point against which all future annual tests will be compared to detect any shifts. The baseline audiogram records the softest sounds, or hearing thresholds, an employee can perceive at various frequencies in each ear.
Federal regulations require the baseline test be completed within six months of an employee’s first exposure to noise at or above the action level (typically an 8-hour time-weighted average of 85 decibels). To ensure the most accurate reading, the employee must be free from workplace noise for at least 14 hours immediately preceding the test. This required “quiet period” prevents temporary shifts in hearing sensitivity from skewing the permanent baseline measurement.
Calculating and Confirming the Hearing Shift
The calculation for an STS is highly specific and focuses on the frequencies most susceptible to noise damage. An STS is formally identified when an employee’s hearing threshold has deteriorated by an average of 10 decibels or more in either ear. This average is calculated across the speech frequencies of 2000, 3000, and 4000 Hertz (Hz) when comparing the annual audiogram to the initial baseline audiogram.
The initial finding is not immediately confirmed as a persistent shift, as it may represent a Temporary Threshold Shift (TTS) caused by recent noise exposure or other transient factors. If an annual audiogram indicates an STS, the employer has the option to conduct a retest within 30 days to confirm the finding. If the shift remains apparent on the retest, it is officially considered a confirmed Standard Threshold Shift, triggering mandatory follow-up actions.
Required Employer and Employee Follow-Up
Once a persistent STS is confirmed, specific regulatory and protective measures must be implemented immediately. The employer is required to notify the affected employee of the confirmed shift in writing, generally within 21 days of the determination. This notification is an opportunity to discuss the results and the importance of hearing protection.
For employees not currently using hearing protection, the employer must provide appropriate devices and ensure they are properly fitted and trained in their use and care. If the employee is already using hearing protection, they must be refitted and potentially provided with a device offering greater noise reduction to prevent further deterioration. An audiologist, otolaryngologist, or physician must review the audiogram to determine if a referral for a comprehensive clinical evaluation is necessary.
A confirmed STS may also trigger a recordable work-related injury, but only if an additional criterion is met. The case becomes recordable on the OSHA 300 Log if the employee’s total hearing level, including the shift, averages 25 decibels or more above audiometric zero at the same 2000, 3000, and 4000 Hz frequencies.