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. At OSHA's permissible exposure limit of 90 dBA, NIOSH estimates that roughly a quarter of workers exposed over a working lifetime would develop a material hearing impairment, and even at 85 dBA the excess risk is about 8 percent. The limits reflect a judgment 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 the United States

In the United States, the Occupational Safety and Health Act of 1970 gives OSHA authority to set and enforce standards for most private-sector employers, either directly or through state plans that must be at least as effective as federal OSHA. Several state plans, including California, Washington and Oregon, have their own noise rules that follow the federal model with some differences. Mining is regulated separately by the Mine Safety and Health Administration (MSHA), and some transportation sectors fall under other federal agencies. Public-sector workers are covered only in state-plan states.

A closer look at 29 CFR 1910.95

The general industry noise standard is the most detailed US rule and the model for most employer programs. Its main requirements are these.

A long-standing OSHA enforcement policy has generally allowed employers to rely on hearing protectors within an effective hearing conservation program, rather than being cited for missing engineering controls, unless exposures are very high. A 2010 proposal to change that interpretation was withdrawn in 2011. Employers should not treat the policy as permission to ignore engineering controls: the regulation still requires feasible controls above the PEL, and good practice and many internal standards apply the NIOSH limit of 85 dBA with a 3 dB exchange rate.

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 standard is silent, such as on detailed program elements in construction, the OSH Act's general duty clause requires employers to keep the workplace free from recognized hazards likely to cause death or serious physical harm. Supervisors are the employer's agents for enforcing protector use and safe practices. A well-documented program, including monitoring results, a consideration of engineering controls, protector selection, training records and audiometric follow-up, is the employer's best evidence of compliance and the best way to protect workers.

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.