In environments with prolonged occupational noise exposure, monitoring worker hearing health is crucial. An audiogram, or hearing test, provides a snapshot of an individual’s hearing sensitivity. For regulatory purposes, a specific, measurable change in these snapshots is used to monitor progress and trigger preventative actions. This change is known as a Standard Threshold Shift (STS), representing a significant alteration in hearing ability from a worker’s established reference point.
Defining the Standard Threshold Shift Criteria
A Standard Threshold Shift is defined as a change in hearing relative to the initial baseline audiogram. This measurable change acts as an early warning signal, indicating that an employee’s hearing may be deteriorating due to workplace noise. The criteria specify an average shift of 10 decibels (dB) or more at the frequencies of 2000, 3000, and 4000 hertz (Hz) in either ear.
This definition is not a clinical diagnosis of hearing loss but a statistical trigger for compliance and intervention in occupational settings. The focus on these mid-to-high frequencies is because they are often the first to show damage from noise exposure. Identifying this shift signals the need for prompt protective measures to prevent further hearing damage.
Establishing the Audiometric Baseline
The Standard Threshold Shift relies on comparison to an established reference point: the baseline audiogram. This initial hearing test is the standard against which all subsequent annual tests are measured throughout an employee’s career in a noise-exposed role. The test must be administered within six months of an employee’s first exposure to noise at or above a time-weighted average of 85 decibels.
This initial test captures the employee’s hearing levels before occupational noise has had a significant effect. To ensure accuracy, employees must avoid high noise levels for at least 14 hours prior to the test or wear appropriate hearing protection. If a persistent change in hearing is later confirmed, the most recent annual audiogram may be adopted as a revised baseline.
Actions Following an STS Identification
Once an annual audiogram indicates a Standard Threshold Shift, mandatory follow-up steps are required to protect the employee’s hearing. The employee must be notified in writing of the finding within 21 calendar days of the determination, initiating a process focused on prevention and correction.
The first action involves evaluating and improving hearing protection. Employees who already use protective devices must be refitted and retrained on their proper use and care. If the employee had previously refused to wear protection, they must now be fitted with and required to wear it.
A follow-up medical evaluation is required, often including a retest within 30 days. This retest helps determine if the shift was a temporary threshold shift (TTS), perhaps caused by a non-work-related factor, or if it represents a permanent change. The goal is to rapidly assess the situation and implement controls to prevent the hearing loss from progressing further.
The Difference Between STS and Recordable Hearing Loss
While an STS triggers internal workplace action, it is distinct from a “Recordable Hearing Loss,” which requires reporting to regulatory bodies. The STS is the average 10 dB shift compared to the baseline and serves as an early warning.
A hearing loss becomes recordable only if two conditions are met: the employee must have experienced an STS, and the employee’s total hearing level must reach 25 decibels or more above audiometric zero. This 25 dB level is averaged across the same 2000, 3000, and 4000 Hz frequencies.
This two-part criterion means that not every Standard Threshold Shift must be recorded on the official log of work-related injuries and illnesses. An STS can trigger internal intervention without meeting the 25 dB total loss level necessary for mandatory regulatory reporting. When determining if an STS has occurred, the employer may apply an age correction to the results, but this allowance is not permitted when evaluating the total 25 dB level for recordability.