A baseline audiogram is the reference hearing test of a worker exposed to workplace noise, against which later audiograms are compared to detect a shift in hearing. Under US federal OSHA, the comparison with the baseline is what reveals a standard threshold shift.
What US federal OSHA requires
The OSHA noise standard, 29 CFR 1910.95, requires a hearing conservation program when exposures equal or exceed an 8-hour time-weighted average (TWA) of 85 decibels on the A scale (§1910.95(c)(1)). Audiometric testing must be made available to all employees exposed at that level (§1910.95(g)(1)), at no cost to them (§1910.95(g)(2)). For the baseline itself:
- Timing. Unless mobile test vans are used, a valid baseline must be established within 6 months of a worker’s first exposure at or above the action level, an 8-hour TWA of 85 decibels (§1910.95(c)(2), (g)(5)(i)).
- Mobile test vans. Where vans are used, the deadline is 1 year, and workers must wear hearing protectors for any period beyond six months after first exposure until the baseline is obtained (§1910.95(g)(5)(ii)).
- Quiet period. Testing must be preceded by at least 14 hours without exposure to workplace noise, and hearing protectors may substitute for this (§1910.95(g)(5)(iii)).
- Who tests. A licensed or certified audiologist, otolaryngologist or other physician, or a technician certified by the Council of Accreditation in Occupational Hearing Conservation or who has demonstrated competence, working under an audiologist, otolaryngologist or physician (§1910.95(g)(3)).
How the baseline is used
At least annually after the baseline, a new audiogram must be obtained for each worker exposed at or above an 8-hour TWA of 85 decibels (§1910.95(g)(6)). Each annual audiogram is compared with the baseline to decide whether it is valid and whether a standard threshold shift has occurred (§1910.95(g)(7)(i)).
The baseline can change. An annual audiogram may replace it when the evaluating audiologist, otolaryngologist or physician judges that a shift is persistent, or that hearing shows significant improvement over the baseline (§1910.95(g)(9)). Audiometric test records must be kept for the duration of the affected worker’s employment (§1910.95(m)(3)(ii)). A work-related shift can also become a recordable case on the OSHA 300 Log under 29 CFR 1904.10.
Great Britain
The Control of Noise at Work Regulations 2005 require employers to place employees under suitable health surveillance, including testing of their hearing, where the risk assessment indicates a risk to their health (regulation 9(1)), and to keep a health record for each one (regulation 9(2)). The regulation itself does not mention a baseline. HSE guidance says surveillance would ideally start before people are exposed to noise, for new starters or those changing jobs, to give a baseline, followed by checks usually annually for the first two years of employment and then at three-yearly intervals.
Why it matters for employers
Shifts in hearing are measured against the baseline. For multi-site employers, the risk sits at the start: the baseline is triggered by a worker’s first exposure at or above the action level and must be in place within 6 months (1 year with mobile test vans), so new starters and people moving into noisy jobs are the moments to watch. In MyC’s health surveillance software, surveillance is scheduled per worker from their exposure group, with due dates and expiry alerts.
Related terms
- Standard threshold shift (STS): A standard threshold shift (STS) is a hearing change of 10 dB or more, averaged at 2,000, 3,000 and 4,000 Hz in either ear, against the baseline audiogram.
- Health surveillance: Health surveillance is a program of repeated health assessments for workers still exposed to a hazard after controls, to identify ill health caused by work.
- OSHA 300 Log: The OSHA 300 Log is the form on which US employers covered by 29 CFR Part 1904 record each work-related injury and illness that meets OSHA's recording criteria.