A Standard Threshold Shift (STS) is a formal, measurable change in a person’s hearing sensitivity, most often defined within the context of an occupational setting. This specific measurement is used to monitor the effectiveness of hearing conservation programs in workplaces where noise exposure is a concern. The purpose of identifying an STS is to flag a significant change in an employee’s hearing early enough to take corrective action and prevent further, more serious hearing loss. It functions as a regulatory trigger, indicating that an employee’s hearing has worsened compared to their initial test, signaling the need for immediate protective measures. This standardized definition helps employers track changes over time and address noise-induced hearing loss.
Defining the Standard Threshold Shift Criteria
The precise definition of a Standard Threshold Shift is a technical measurement established to ensure consistency in tracking hearing health. An STS is officially identified when an employee’s hearing threshold changes by an average of 10 decibels (dB) or more in either ear when compared to their baseline audiogram. This average is calculated across a specific set of frequencies: 2000, 3000, and 4000 Hertz (Hz). These frequencies are particularly sensitive to noise-induced damage, making them the most relevant markers for monitoring occupational hearing loss.
The 10 dB average shift represents a significant deterioration in the ability to hear those specific high-frequency tones, which are important for speech clarity. Because an STS is calculated as an average across three frequencies, it is possible for a large shift at one frequency to be balanced by a smaller shift at another. This established threshold serves as the minimum change that requires a formal response within a hearing conservation program.
Age Correction
A complicating factor in determining an STS is the consideration of age-related hearing loss, known as presbycusis, which naturally progresses over time. Federal guidelines permit the optional use of age correction tables to adjust the current audiogram results. The goal of this adjustment is to differentiate the portion of hearing loss that may be attributable to normal aging from the loss that is likely caused by workplace noise exposure.
The age correction procedure involves determining age-related loss values based on the employee’s age at the time of both the baseline and the current audiogram. Subtracting the age-related loss from the measured threshold shift can, in some cases, remove the 10 dB shift, meaning an STS may not be confirmed. However, some health professionals consider the non-use of age correction a more protective practice, as it ensures earlier intervention for any measured hearing change.
The Role of Baseline and Periodic Audiograms
Identifying a Standard Threshold Shift is entirely dependent on the comparison between two distinct hearing tests: the baseline and the periodic audiogram. The baseline audiogram is the initial hearing test, typically administered within the first six months of an employee’s exposure to hazardous noise levels. This test provides the individual’s best hearing threshold levels and serves as the permanent reference point against which all future hearing tests will be measured.
Once the baseline is established, employees are required to undergo periodic, usually annual, audiograms for continued monitoring. The results of these subsequent tests are directly compared to the original baseline to check for the 10 dB average shift that signals an STS. This yearly comparison is the mechanism by which employers track small changes over time, aiming to catch hearing deterioration before it becomes disabling.
If an annual audiogram suggests an STS, the employer is allowed to conduct a retest within 30 days of the initial determination. This retest is performed to rule out a temporary threshold shift (TTS), which is a temporary dip in hearing sensitivity that can occur after acute noise exposure and may recover over a short period. If the retest confirms the hearing loss is persistent, the shift is confirmed as an STS, indicating a more permanent change in hearing sensitivity.
A confirmed and persistent STS may lead to a revised baseline audiogram, where the new, worse hearing test becomes the new reference point for future comparisons. This revision prevents the same degree of hearing loss from triggering an STS notification every subsequent year. This change only occurs after the shift has been deemed persistent and reviewed by a qualified professional.
Mandatory Follow-Up Procedures
Once a Standard Threshold Shift is confirmed, mandatory administrative and protective actions must be initiated promptly. The employee must be formally notified of the STS determination in writing, a requirement that must be met within 21 days of the finding.
A primary step following notification is the immediate provision or improvement of hearing protection devices.
- Employees who were not using hearing protection must be fitted with appropriate devices and trained on their proper use and care.
- Employees already using hearing protection must be refitted, retrained, and provided with devices that offer a higher level of noise reduction if necessary.
If the STS is significant or the audiogram presents unusual results, the employer must ensure the employee is referred to an audiologist, otolaryngologist, or physician for a detailed clinical evaluation. This referral is necessary to determine if the hearing shift is truly work-related or if other medical issues are contributing to the hearing change.