Averages 42.0 hours a week over the rotation; the busiest calendar week is 84.0 hours.
3
The adjusted limit
Use 0.250 ppm on this schedule
Alberta requires Brief and Scala, daily: a factor of 0.500 on the 8-hour limit of 0.500 ppm.
8-hour limit
0.500
ppm
Factor
0.500
required here
Adjusted limit
0.250
ppm
Excursions
0.750
ppm, 30 min max; never 1.25
Model
Factor
Adjusted
How
Brief and Scala, dailyrequired
0.500
0.250 ppm
(8 ÷ 12) × ((24 − 12) ÷ 16)
Brief and Scala, weekly
0.313
0.156 ppm
busiest week 84 h: (40 ÷ 84) × ((168 − 84) ÷ 128)
BC reduction factor
0.500
0.250 ppm
the band for a 12-hour shift
Quebec (IRSST)
0.952
0.476 ppm
category III, weekly; average week 42 h
How the daily factor falls with the shift
Brief and ScalaBC bandsQuebec daily (8 ÷ hours)
Alberta’s bulletin CH055 recommends also working out the weekly factor and using the stricter of the two.
A Director’s approval (2023) lets respirable crystalline silica be adjusted by the Quebec model instead: 0.024 mg/m³ for a 42-hour average week such as 14 on 14 off at 12 hours.
15-minute and ceiling limits are not adjusted.
Ceiling and 15-minute limits are never adjusted, and no model raises a limit for a short shift.
Sample over the whole shift and compare the result with the adjusted limit, not the 8-hour one.
The rule · Alberta
Mandatory for a shift over 8 hours: Brief and Scala daily, unless the substance has a “3” (limit based on irritation) in Table 2. Another method needs a Director’s approval.
Quebec categories are from the IRSST guide (3rd edition); confirm a substance’s category in the current edition.
A summary to plan with. The regulation and your employer's procedures govern.
Pick the substance, enter its 8-hour exposure limit and your shift and rotation, and this calculator adjusts the limit by every recognised model (Brief and Scala, BC's reduction factors and Quebec's IRSST method) and picks out the one your province or territory requires.
Why long shifts need a lower limit
Exposure limits are set for 8 hours a day and 40 a week, with 16 hours to recover between shifts. A 12-hour shift means more exposure and only 12 hours to clear it before the next one, so the same average concentration puts more into the body. The models lower the limit to give the same protection.
Brief and Scala allows for both the longer exposure and the shorter recovery. It is the most protective, and Alberta, New Brunswick and Yukon require it.
BC uses fixed factors by shift length: 0.7, 0.5, 0.25 and 0.1.
Quebec's method depends on how the substance harms: not adjusted for irritants and ceilings, by the day for short-term effects, by the average week for long-term ones. Ontario allows it.
Sample over the whole shift
Compare a full-shift sample with the adjusted limit, not the 8-hour one. A 12-hour shift sampled for 8 hours, or compared with the 8-hour limit, can pass when the worker is over.
Long shifts in the heat too? Plan the breaks with the heat stress calculator, and see our training for WHMIS and exposure control.
Sources: Alberta OHS Code s.18 and Schedule 1 Table 2, bulletin CH055 and the 2023 silica approval; BC OHS Regulation s.5.50 and Guideline G5.50; Quebec RSST Schedule I and the IRSST guide (3rd edition); Ontario Reg. 833 Schedule 1 s.6; New Brunswick Reg. 91-191 s.24.1; Newfoundland and Labrador s.42; Yukon s.27(4); Saskatchewan s.21-6; the NWT and Nunavut s.314; Manitoba s.36.5. The Brief and Scala weekly factor here uses the busiest calendar week, the conservative reading.
Frequently asked questions
How do you adjust an exposure limit for a 12-hour shift?
With the Brief and Scala model, multiply the 8-hour limit by (8 ÷ 12) × ((24 − 12) ÷ 16), which is 0.5: a 0.5 ppm limit becomes 0.25 ppm. Alberta requires this for shifts over 8 hours unless the limit is based on irritation. BC uses a fixed factor of 0.5 for shifts over 10 and up to 12 hours. Quebec’s method gives 8 ÷ 12 = 0.67 for substances with short-term effects.
What is the Brief and Scala model?
It lowers an 8-hour exposure limit for longer shifts to allow for both the longer exposure and the shorter recovery time before the next shift. The daily factor is (8 ÷ h) × ((24 − h) ÷ 16), where h is the hours worked in the day; the weekly factor is (40 ÷ h) × ((168 − h) ÷ 128) for the hours worked in a week. It gives 0.7 for 10 hours, 0.5 for 12 and 0.36 for 14.
Do you adjust exposure limits for a 14 on 14 off rotation?
Yes for the daily shift length: on 12-hour shifts the Brief and Scala factor is 0.5 in Alberta. Under Quebec’s method, substances with long-term effects use the average week over the rotation, which is 42 hours for 14 on 14 off at 12 hours (a factor of 0.95). Alberta has approved that method for respirable silica, giving 0.024 mg/m³.
Are ceiling and short-term limits adjusted for long shifts?
No. Every Canadian rule adjusts the 8-hour average only. Ceiling and 15-minute limits stay as they are, and no method raises a limit for a shift shorter than 8 hours.
Keep exposure assessments and sampling records with the rest of your program
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