What Does
Permanent Threshold Shift Mean?
The term “permanent threshold shift” (PTS) refers to a permanent reduction of the sensitivity of the human ear, decreasing the ability of the ear to detect sound. In this context, the “threshold” is the minimum decibel level at which an individual is able to detect sound. As the human ear is more sensitive to some frequencies than others, an individual’s hearing threshold is frequency-specific.
Permanent threshold shifts are colloquially be referred to as “hearing loss”. When they occur due to excessive noise exposure, they are referred to as Noise-Induced Permanent Threshold Shifts. Occupational exposure to excessive noise is one of the main causes of PTS.
The higher the hearing threshold, the louder a noise must be for an individual to detect it. As such, hearing loss is described as an increased or positive threshold shift, while improvements in hearing are referred to as decreased or negative shifts.
Safeopedia Explains Permanent Threshold Shift
Occupational PTS can be caused by either chronic or acute exposures to loud noises (acoustic stimulation). Noise exposure causes PTS by damaging the hair cells in the inner ear (cochlea) that are responsible for detecting and transmitting sound. This type of damage is referred to as “acoustic trauma”. The cochlea is most sensitive to damage from higher frequency noises.
As humans tend to naturally lose hearing over time, a PTS may not be considered a sign of acoustic trauma or excessive noise exposure unless the increase in hearing threshold reaches a certain level of significance. In the United States, OSHA’s standard for significance is a 10 decibel (dB) increase in hearing threshold, averaged across 2000, 3000, 4000 Hz frequency thresholds (29 CFR 1910.95). This 10-decibel decrease is referred to as a “standard threshold shift” (STS); OSHA requires any STS to be reported as a workplace injury.
Because PTS cannot be reversed, occupational safety authorities require employees to be provided with hearing protection that is sufficient to protect against long-term damage. OSHA permits exposures of 85 dB for 16 hours per day; for every 5 dB increase above 85 dB, the allowed exposure time is reduced by half (i.e., 8 hours for exposure to 90 dB, 4 hours for 95 dB). If employees are exposed to an 8-hour time-weighted average of at least 85 dB per-day, the employer must implement a hearing testing (audiometric testing) program to monitor employees for threshold shifts.
Not all threshold shifts are permanent; for example, the ear may temporarily reduce its sensitivity as a protective function. Establishing PTS normally requires multiple hearing tests to establish that a threshold shift is permanent.