If a work-related injury or illness results in a hearing loss, the employee is required to record, by checking the "hearing loss" column on the OSHA 300 Log, all cases in which an employee's hearing test (audiogram) revealed that a Standard Threshold Shift (STS) in hearing acuity had occurred. 29 C.F.R. § 1904.10(a). An STS is defined in 29 C.F.R. § 1904.10(b)(1) as "a change in hearing threshold, relative to the most recent audiogram for that employee, of an average of 10 decibels or more at 2000, 3000 and 4000 Hertz (Hz) in one or both ears."

The recordkeeping rule itself does not require the employer to test employee's hearing. However, OSHA's occupational noise standard (29 C.F.R. § 1910.95) requires employers in general industry to conduct periodic audiometric testing of employees when employees' noise exposures are equal to, or exceed, an 8-hour time-weighted average of 85dBA. Under the provisions of § 1910.95, if such testing reveals that an employee has sustained a hearing loss equal to an STS, the employer must take protective measures, including requiring the use of hearing protectors, to prevent further hearing loss.