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In 1974, the US Environmental Protection Agency recommended a 24-hour exposure limit of 70 dBA, taking into account the lack of a "rest period" for the ears when exposures are averaged over 24 hours and can occur every day of the year (workplace exposure limits assume 16 hours of quiet between shifts and two days a week off). [13]
The dose is represented by a percentage. A noise dose of 100% means that a person has exceeded the permissible amount of noise. Any noise exposure after the 100% noise dose may damage hearing. The exchange rate is the rate at which exposure accumulates. An addition of the exchange rate results in a halving of exposure time.
OSHA's PEL for noise exposure is 90 decibels (dBA) for an 8-hour TWA. Levels of 90-140 dBA are included in the noise dose. [4] PEL can also be expressed as 100 percent “dose” for noise exposure. When the noise exposure increases by 5 dB, the exposure time is cut in half. [5] According to OSHA, a 95dBA TWA would be a 200 percent dose. [6]
Environmental noise regulations usually specify a maximum outdoor noise level of 60 to 65 dB(A), while occupational safety organizations recommend that the maximum exposure to noise is 40 hours per week at 85 to 90 dB(A). For every additional 3 dB(A), the maximum exposure time is reduced by a factor 2, e.g. 20 hours per week at 88 dB(A).
NIOSH recommends that OSHA use an 85 dBA time-weighted average during an 8-hour period as their exposure limit. [1] An 85 dBA time-weighted average means that HPD use is recommended if an employee is exposed to an average noise level of 85 dBA or more during an 8-hour work day. NIOSH also uses a 3 dB exchange rate for time-weighted averages. [1]
Noise damage is cumulative; all sources of damage must be considered to assess risk. In the US, 12.5% of children aged 6–19 years have permanent hearing damage from excessive noise exposure. [55] The World Health Organization estimates that half of those between 12 and 35 are at risk from using personal audio devices that are too loud. [11]
The day-night average sound level (Ldn or DNL) is the average noise level over a 24-hour period. The noise level measurements between the hours of 22:00 and 07:00 are artificially increased by 10 dB before averaging. This noise is weighted to take into account the decrease in community background noise of 10 dB during this period.
Noise Monitoring: 29 CFR 1910.95(d) requires that monitoring be conducted when "any employee's exposure may equal or exceed an 8-hour time-weighted average of 85 decibels. Audiometric Testing: 29 CFR 1910.95(g) requires an "audiometric testing program" for "all employees whose exposures equal or exceed an 8-hour time-weighted average of 85 ...