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Noise - Occupational Exposure Limits for Extended Workshifts
Are there guidelines for noise exposure on shifts longer than 8 hours?
How do I calculate the exposure limit?
Which method do I use?
What are some other things to consider about noise exposures over extended work shifts?
Does CCOHS have other information on noise or noise regulations?
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 Noise - Occupational Exposure Limits for Extended Workshifts

Are there guidelines for noise exposure on shifts longer than 8 hours?

Most standards and guidelines concerning noise exposure are based on an 8-hour work shift and also provide levels for shorter working days. In real life conditions, longer working days are common. When calculating exposure limits for an extended work shift such as a 12-hour shift, one must take into account information on health effects related to noise exposure and those related to a 12-hour shift work. The final answer has to come from a study of actual work places that might have experimented or adapted such work practices.

A change from an 8-hour shift to a 12-hour shift must consider the following issues:

  • Eight-hour time-weighted noise exposure level in dB(A)
  • Problems related to use of hearing protectors for such a prolonged work shift
  • Combined effect of stress factors related to a 12-hour shift and noise
How do I calculate the exposure limit?

1) Equal energy rule

Many regulatory agencies recommend a time-weighted average (TWA) sound level of 85 dB(A) to 90 dB(A) as a noise exposure limit for 8-hour work day.

The ISO Standard 1999-1990 produced by the International Organization for Standardization recommends the use of the equal energy principle (3 dB exchange rate) in calculating the TWA for a work shift:

Limit for a given shift = 90 - 10 log (T/8)

where T = duration of work shift in hours. Results of such calculation for various extended work shifts are listed in Table 1.

Table 1 also shows the noise exposure limit for extended shifts when the 5 dB exchange rate is used. The formula used for calculating these exposure limits for extended shifts is:

Limit for a given shift (5 dB rule) = 90 - 16.61 log (T/8)

Table 1 - TWA Method

Work Shift Duration (Hours)

Noise Exposure Limit, dB(A)
[Criterion level = 90 dB(A)]

Using 3dB exchange rate

Using 5 dB exchange rate

8 90 90
9 89.5 89.2
10 89.0 88.4
11 88.6 87.7
12 88.2 87.1
13 87.9 86.5
14 87.6 86.0
15 87.3 85.5
16 87.0 85.0

The noise exposure limit for a 12-hour shift, based on the equal energy rule, is 88.2 dB(A). In other words, if the noise level is kept below 88 dB(A) then, according to equal energy concept, the maximum permissible limit is not exceeded.

2) An alternative method

An alternative method, called the Brief and Scala method, is sometimes used to calculate TLV (Threshold Limit Values) for chemicals but it can also be used to calculate modified noise exposure limits for extended work shifts. This method is more conservative than the TWA method described above. It takes into account the decreased hours of recovery. The exposure limits for extended shifts, based on this method, are listed in Table 2.

Table 2
Noise Level for extended shift according to Brief and Scala Method using the 3dB exchange rate and criterion level = 90 dB(A)

Duration of Work shift (h)

Noise Exposure Limit (dB(A))
8 90.0
9 89.2
10 88.4
11 87.7
12 87.0
13 86.3
14 85.5
15 84.8
16 84.0

With this method, the limit for a 12-hour work shift is 87 dB(A), which is lower than that allowed by the TWA method.

Which method do I use?

The authority responsible for noise regulation recommends the acceptable method for calculating the noise limit for an extended work shift. For example, in Ontario, noise exposure limits are provided by the Ministry of Labour. You should contact the agency responsible for health and safety regulations applicable to your workplace and inquire about the recommended procedures for calculating exposure limits for extended work shifts.

What are some other things to consider about noise exposures over extended work shifts?

  • The consequence of an extended work shift on hearing loss is not known. The 90 dB(A) limit was determined for an 8-hour shift and the effect of the same noise dose spread over a 12-hour shift remains to be evaluated. This would include studying the effects of shorter recovery times between shifts.
  • If hearing protectors are to be used, the feasibility of their proper and efficient use during an extended shift may need some thought. It is a well-known fact that there is a wide variation in the effectiveness of different protectors. Problems related to comfort, especially for extended periods, must be considered as well.
  • The stress related to a 12-hour shift has been studied by a number of researchers. There have been mixed feelings about the acceptability of the 12-hour shift in general.
Does CCOHS have other information on noise or noise regulations?

If you came to this page directly from another site, you missed seeing a list of other topics about noise. You can see a list of these other topics by going to the Physical Agents topics page.

Information on hearing protectors is available in the "Personal Protective Equipment" section of OSH Answers.

The NOISE LEVELS Database, available on CD-ROM and diskette, contains noise level measurements of actual work situations associated with different types of equipment and occupations in various industries.

If you want to find articles, research papers, government reports and other information that deal with the effects of noise exposure in certain situations, noise control, hearing protection, hearing conservation programs, etc., you can search our bibliographic databases like CISILO, NIOSHTIC, and HSELINE. Information on other CCOHS bibliographic databases and full-text products is accessible from the Health and Safety Studies, Research and Directories web page.

You can also link to descriptions of legislation-related information products by visiting the Legislative and Regulatory Information web page.

Free searches of Canadian enviroOSH Legislation are available at:


You can view the titles retrieved by your search but you must be a subscriber to display the full text of the legislation.

You can search for information related to noise that the Labour Canada Program at HRDC and several provincial and territorial OH&S agencies have put on their web sites by searching the CanOSH web site.

Document last updated on January 22, 1999

Copyright ©1997-2004 Canadian Centre for Occupational Health & Safety

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