In environments where employees are regularly exposed to significant noise levels, such as manufacturing plants or construction sites, monitoring hearing health is an important component of occupational safety. Regular hearing tests, known as audiograms, track an individual’s hearing sensitivity over time to prevent permanent damage. The results of these tests are compared against an initial measurement to identify any significant changes. A Standard Threshold Shift (STS) is a specific, measurable change in hearing ability that triggers mandatory follow-up actions under regulatory standards. Understanding this metric is fundamental to any comprehensive hearing loss prevention program, indicating when intervention is necessary.
Defining Standard Threshold Shift
A Standard Threshold Shift is defined as a change in an individual’s hearing threshold significant enough to warrant attention and action. This metric is primarily used within occupational health monitoring programs, such as those mandated by the Occupational Safety and Health Administration (OSHA). The STS represents a worsening of hearing ability relative to the employee’s initial baseline audiogram, signaling that hearing has decreased since their first test.
The occurrence of an STS suggests that the employee’s current hearing protection or noise exposure controls may be inadequate. It focuses on the degree of change, rather than the total amount of hearing loss. The presence of an STS forces an employer to re-evaluate the employee’s working conditions and institute protective measures to detect a measurable negative change in hearing that requires intervention.
How STS is Calculated and Identified
Identifying an STS begins with establishing a baseline audiogram, which is the employee’s initial hearing test taken before or shortly after their first noise exposure in the workplace. This baseline serves as the permanent reference point for all future comparisons. Subsequent annual audiograms are performed to monitor for any shifts in hearing ability.
The specific calculation involves comparing the current annual audiogram to the baseline test at three distinct frequencies: 2000 Hertz (Hz), 3000 Hz, and 4000 Hz. An STS is confirmed if the average change in hearing threshold at these three frequencies is 10 decibels (dB) or more in either ear.
Regulatory standards allow for an adjustment to be made to account for the expected effects of aging, known as presbycusis, using age correction tables. This adjustment helps differentiate between hearing loss caused by occupational noise exposure and natural age-related decline. If the annual audiogram indicates an STS, the employer may conduct a retest within 30 days to confirm the shift. If the retest does not confirm the shift, the retest results can be substituted for the annual audiogram.
Required Employer and Employee Responses
Once an STS has been confirmed, employers must take mandatory actions to protect the employee’s hearing and comply with regulations. The employee must be informed of the STS determination in writing within 21 days to ensure they are aware of their change in hearing status.
A primary action is the re-evaluation of hearing protection. Employees not previously using protection must be fitted with appropriate devices, trained on their use, and required to wear them. Those already using protection must be refitted, retrained, and provided with protectors that offer greater noise attenuation if necessary. For employees with an STS, hearing protection must reduce their noise exposure to an 8-hour time-weighted average of 85 dB or below.
The employer may also be required to refer the employee for a medical evaluation by an audiologist or physician. The employer must determine if the STS is work-related; unless a physician determines otherwise, it is treated as such. If the STS is work-related and the employee’s total hearing level is 25 dB or more above audiometric zero (averaged at 2000, 3000, and 4000 Hz), the case must be recorded on the OSHA 300 Log, which tracks workplace injuries and illnesses.