Exposure limits and the law

An exposure limit turns the biology of Lesson 1 and the physics of Lesson 2 into a rule a workplace can follow. It says how much noise a worker may receive in a day before the employer must act. The limit itself is only part of the law. Regulations also say how exposure is measured, which controls come first, when hearing protectors may be used, when hearing tests are needed and what training and records are required. This lesson explains how limits are built, compares the approaches used in Canada and the United States, and sets out the legal framework in your jurisdiction.

Exposure limits are not a line between safe and harmful. Even at 85 dBA over a working lifetime, a small percentage of workers will develop a measurable hearing loss. The limits reflect a judgement about acceptable risk, and good programs aim well below them.

The parts of a noise exposure limit

Every occupational noise limit has three parts.

The criterion level and the exchange rate together define the allowable time at any level. With an 85 dBA criterion and a 3 dB exchange rate, a worker may be exposed to 85 dBA for 8 hours, 88 dBA for 4 hours, 91 dBA for 2 hours, 94 dBA for 1 hour and 100 dBA for only 15 minutes. With a 90 dBA criterion and a 5 dB exchange rate, the times are 8 hours at 90, 4 hours at 95, 2 hours at 100, 1 hour at 105 and 15 minutes at 115.

In the formula, T is the allowable time, L is the A-weighted level, Lc is the criterion level and E is the exchange rate.

Why the exchange rate matters so much

The 3 dB exchange rate follows the equal-energy principle: a doubling of acoustic energy, which is a 3 dB increase, halves the allowable time, so the total energy received is the same. Most scientific bodies, including NIOSH, ACGIH and the International Organization for Standardization, support the 3 dB rule. The 5 dB rule dates from the 1960s and assumed that intermittent noise, with quiet breaks, allows the ear to recover. It is more permissive at high levels. At 100 dBA, a 3 dB rule with an 85 dBA criterion allows 15 minutes; a 5 dB rule with a 90 dBA criterion allows 2 hours, eight times longer.

def allowable_hours(level, criterion, exchange):
    return 8 / 2 ** ((level - criterion) / exchange)

def fmt(h):
    return f"{h:5.2f} h" if h >= 1 else f"{h * 60:5.1f} min"

rules = {"85 dBA / 3 dB": (85, 3), "87 dBA / 3 dB": (87, 3), "90 dBA / 5 dB": (90, 5)}
print("Level   " + "".join(f"{name:>16}" for name in rules))
for level in range(85, 116, 3):
    row = "".join(f"{fmt(allowable_hours(level, c, e)):>16}" for c, e in rules.values())
    print(f"{level} dBA" + row)

Level      85 dBA / 3 dB   87 dBA / 3 dB   90 dBA / 5 dB
85 dBA          8.00 h         12.70 h         16.00 h
88 dBA          4.00 h          6.35 h         10.56 h
91 dBA          2.00 h          3.17 h          6.96 h
94 dBA          1.00 h          1.59 h          4.59 h
97 dBA        30.0 min        47.6 min          3.03 h
100 dBA        15.0 min        23.8 min          2.00 h
103 dBA         7.5 min        11.9 min          1.32 h
106 dBA         3.8 min         6.0 min        52.2 min
109 dBA         1.9 min         3.0 min        34.5 min
112 dBA         0.9 min         1.5 min        22.7 min
115 dBA         0.5 min         0.7 min        15.0 min

The table shows how quickly the rules diverge. At 85 dBA the differences hardly matter. At 103 dBA, a common level for grinding or cutting, a Canadian provincial worker reaches the limit in under 8 minutes, while the OSHA limit allows over an hour.

The law in Canada

Occupational health and safety in Canada is mainly provincial and territorial. About 90 per cent of workers fall under the law of the province or territory where they work. Federally regulated workplaces, such as banks, airlines, interprovincial trucking and rail, telecommunications, ports and federal government departments, fall under the Canada Labour Code, Part II, and the Canada Occupational Health and Safety Regulations. Each jurisdiction sets its own noise rules, although they are now broadly similar.

A closer look at Ontario

Ontario's noise regulation, O. Reg. 381/15, came into force on July 1, 2016 and extended noise protection to all workplaces covered by the Occupational Health and Safety Act, including construction, health care, schools, farms and mines, which had previously been covered unevenly. Its main requirements are short but strict.

Ontario does not mandate audiometric testing or a formal hearing conservation program in the regulation, but the general duty in section 25(2)(h) of the Occupational Health and Safety Act, to take every precaution reasonable in the circumstances, and good practice under CSA Z1007, Hearing loss prevention program management, mean that most well-run workplaces with noise above the limit have both. Inspectors may ask how the employer knows that its controls and protectors are working, and audiometric results are the most direct answer.

Program requirements beyond the limit

The exposure limit is only the trigger. What really protects workers is the set of program elements that apply once noise is above certain levels. These typically include an exposure assessment, controls, signs or posting of noisy areas, provision and selection of protectors, training, audiometric testing and records.

The general duty and due diligence

Even where a regulation is silent, such as on audiometric testing in Ontario, employers in every Canadian jurisdiction have a general duty to take every precaution reasonable in the circumstances. Supervisors have their own duties, including ensuring that workers use the protective devices the employer requires and advising workers of hazards. In a prosecution, an employer's defence is due diligence: showing that it took all reasonable steps. For noise, that means an up-to-date assessment, a documented consideration of engineering controls, properly selected protectors, training, and a way of checking that the program works.

Limits are minimums

Because current limits still leave a measurable risk of hearing loss, many employers set internal targets below the legal limit. Common choices are to use 85 dBA with a 3 dB exchange rate even where the law allows 90 dBA and 5 dB, to aim for protected exposures between 70 and 80 dBA rather than just under the limit, and to start audiometric testing and training at 80 or 82 dBA for workers whose exposures fluctuate. Buy-quiet purchasing policies, covered in Lesson 6, aim to keep new equipment below 80 or 85 dBA at the operator position.