Standard Threshold Shift
How exactly is an OSHA Standard Threshold Shift calculated?
For each ear, identify thresholds at 2000, 3000, and 4000 Hz for baseline and current audiograms. Calculate the difference at each frequency (current minus baseline). Average the three differences. If the average change is ≥10 dB in that ear, an STS is present. For example, baseline of -10 dB and current of 0 dB at all three frequencies yields a +10 dB average, constituting an STS even with better-than-normal hearing.
When can the baseline audiogram be revised, and how does that affect future STS detection?
Baseline revision is permitted when a persistent, confirmed STS is considered permanent. If STS occurs only in one ear, OSHA allows revision only in that ear. Once revised, future STS calculations use the new baseline, which may partially mask subsequent small deteriorations. Revisions must be documented and justified to maintain longitudinal surveillance quality.
How do age corrections interact with STS determination and recordability?
OSHA permits age correction using standardized tables to estimate expected hearing loss from aging. Age-corrected thresholds may reduce the apparent shift below 10 dB, preventing an STS determination. If an STS still exists after age correction, recordability rules (≥25 dB HL and work-related) apply. Jurisdictions may limit or disallow age correction; systems should allow configuration by jurisdiction.
A Standard Threshold Shift (STS) is a regulatory term defined by OSHA as a change in hearing threshold relative to the baseline audiogram of an average of 10 dB or more at 2000, 3000, and 4000 Hz in either ear. It is calculated separately for each ear and indicates a significant, likely permanent hearing change, triggering employer obligations under hearing conservation programs.
On a manufacturing shop floor, STS management is a core element of the Hearing Conservation Program (HCP). Workers exposed to noise (typically ≥85 dBA TWA) receive baseline and annual audiograms. When an STS is detected, employers must provide written notification within 21 days, evaluate and optimize hearing protection, review noise controls (e.g., enclosures, rotation), and arrange medical follow-up to determine work-relatedness. Supervisors use STS trends to identify failing controls or inconsistent PPE use. Workers with repeated STS may be reassigned to lower-noise tasks or provided custom protectors. Systems like SafeDesk automate flagging, notification, and recordability assessments.
Mismanaging baseline audiograms: Revising both ears' baselines when an STS occurs in only one ear, which is non-compliant and can mask progressive hearing loss.
Failing required notification and follow-up: Not providing written STS notification within 21 days, skipping re-tests within 30 days, or neglecting hearing protection and noise control evaluations.
Incorrect OSHA 300 Log recordability decisions: Recording all STS as recordable cases even when average thresholds are <25 dB HL or shift is non-work-related, or failing to record work-related STS meeting criteria.