Audiometric testing and hearing loss records

Noise assessments, controls and protectors are all aimed at one outcome: workers' hearing stays the same over their working lives. Audiometric testing is how a program finds out whether that is happening. A baseline test records a worker's hearing before significant exposure, and periodic tests compare later results with it. Early, small changes, long before the worker notices anything, trigger follow-up that can prevent a permanent handicap. Taken together, the results of many workers show whether the program as a whole is working. This lesson covers how audiometric testing is done, how results are interpreted, what follow-up is needed and how hearing loss is recorded and compensated.

What an audiometric test involves

An occupational audiometric test is a pure-tone air-conduction hearing test. The worker sits in a quiet booth or room wearing headphones or insert earphones, and presses a button whenever a tone is heard. The tester, or an automated audiometer, presents tones at each test frequency, usually 500, 1,000, 2,000, 3,000, 4,000, 6,000 and 8,000 Hz, varying the level to find the quietest level the worker can hear about half the time. The results are recorded in dB HL for each ear and plotted on an audiogram, as described in Lesson 1. The test usually takes 10 to 20 minutes and should be combined with a short questionnaire about ear health, noise exposure at work and outside work, and protector use, and with a brief counselling session.

The quality of the test depends on several things. The room must be quiet enough not to mask the quietest tones, typically checked against maximum permissible background levels in each octave band. The audiometer must be calibrated: a daily functional check, a periodic acoustic calibration and an exhaustive calibration at longer intervals. The tester must be trained and must give consistent instructions. And the worker should have had a period without loud noise before the test, especially before the baseline, so that a temporary threshold shift does not distort the result.

The baseline audiogram

The baseline is the reference against which every later test is compared, so it must be good. A baseline taken right after a noisy shift may show a temporary threshold shift, making later results look better than they are and hiding real damage. A baseline taken with poor instructions or in a noisy room may show worse thresholds than the worker really has. Many programs take the baseline on a worker's first day, before any exposure, and some repeat it within a few weeks to confirm it. Pre-existing hearing loss found at baseline should be documented, explained to the worker and considered in protector selection.

Detecting a significant change

Each periodic test is compared with the baseline, and sometimes with the previous test, to detect changes. Some variation is normal: a test-retest difference of 5 dB at a single frequency is common. A program therefore uses defined criteria for a significant change, which triggers follow-up.

Canadian jurisdictions and programs use various definitions. Some provincial regulations define an abnormal shift, such as a defined change at one or more frequencies, and require notification and follow-up; others leave the definition to the professional reviewing the tests. Many Canadian programs also use the OSHA standard threshold shift, described below, as a benchmark because it is well defined and widely used in research. Whatever definition is used, it should be written into the program and applied consistently.

freqs = [500, 1000, 2000, 3000, 4000, 6000, 8000]

def shifts(baseline, current):
    return {f: c - b for f, b, c in zip(freqs, baseline, current)}

def average_shift_234(baseline, current):
    d = shifts(baseline, current)
    return (d[2000] + d[3000] + d[4000]) / 3

def hearing_level_234(current):
    c = dict(zip(freqs, current))
    return (c[2000] + c[3000] + c[4000]) / 3

worker = {
    "left":  {"baseline": [5, 5, 10, 10, 15, 15, 10], "annual": [5, 10, 15, 25, 35, 25, 15]},
    "right": {"baseline": [5, 5, 5, 10, 10, 15, 10],  "annual": [5, 5, 10, 15, 20, 20, 10]},
}
for ear, r in worker.items():
    avg = average_shift_234(r["baseline"], r["annual"])
    level = hearing_level_234(r["annual"])
    single = max(shifts(r["baseline"], r["annual"]).values())
    print(f"{ear:>5}: average shift at 2-4 kHz {avg:4.1f} dB, "
          f"largest single shift {single} dB, current average level {level:4.1f} dB HL")
    print(f"       10 dB average-shift criterion met: {avg >= 10}; "
          f"25 dB average level reached: {level >= 25}")

 left: average shift at 2-4 kHz 13.3 dB, largest single shift 20 dB, current average level 25.0 dB HL
       10 dB average-shift criterion met: True; 25 dB average level reached: True
right: average shift at 2-4 kHz  6.7 dB, largest single shift 10 dB, current average level 15.0 dB HL
       10 dB average-shift criterion met: False; 25 dB average level reached: False

In this example, the left ear shows an average shift of about 13 dB at 2,000 to 4,000 Hz and a current average hearing level of 25 dB HL, so it meets both the 10 dB shift criterion and the 25 dB level used in US recordkeeping. The right ear shows a smaller change that does not meet the shift criterion but should still be watched. No age adjustment has been applied; doing so would reduce the calculated shift somewhat for an older worker.

Follow-up after a significant shift

A significant shift is an early warning. It should lead to a set of actions aimed at stopping further loss.

Recording and reporting hearing loss

In Canada, there is no national equivalent of the US injury and illness log. Employers must report occupational illnesses to the provincial workers' compensation board when a worker loses time, receives health care or files a claim, and some provinces have specific reporting rules for hearing tests. In British Columbia, employers submit hearing test results to WorkSafeBC, which uses the data to monitor hearing loss across industries. Employers should keep audiometric records for the duration of employment and longer where provincial rules or a collective agreement require it, under the control of the health professional, with access for the worker.

Workers' compensation for noise-induced hearing loss

Every province and territory compensates occupational noise-induced hearing loss, but eligibility rules differ. Boards typically require evidence of prolonged exposure to hazardous noise, an audiogram pattern consistent with noise damage, and a minimum degree of hearing loss before a permanent impairment award is made; most will also pay for hearing aids even when the loss is below the threshold for a pension. WSIB in Ontario and WorkSafeBC in British Columbia each have detailed policies, and noise-induced hearing loss is among the most common occupational disease claims in several provinces. Good baseline and periodic records help both the worker and the employer, because they show when and where hearing changed.

Confidentiality and counselling

Audiograms are confidential health records. The worker should receive a copy of each result and an explanation in plain language. The employer needs to know that testing was done, whether a significant shift occurred and what actions are needed, but not the detailed audiogram. Committees should see aggregate statistics only.

Counselling at the time of the test is one of the most effective parts of the program. A short conversation, showing the worker their own audiogram trend, explaining what any change means, checking protector fit and asking about leisure noise, connects the abstract risk to the individual. Workers who see their own notch beginning are much more likely to protect their hearing consistently.