Intuitively, you would think that an exposure limit such as the Threshold Limit Values (TLVs) from the American Conference of Governmental Industrial Hygienists (ACGIH) that are based on an 8-hour workday may not be protective for workers on a 12-hour shift. You would be right because there are two things that happen in this case. First the workers would inhale airborne contaminants longer and be exposed to more chemicals in the day. In addition, the workers would have less time away from work. Time away from work gives the body time to clear contaminants from the body and to recover.
The TLV book that the ACGIH publishes annually recognizes this issue and notes that there are numerous mathematical models to adjust for unusual work schedules. The ACGIH directs us to a model by Brief and Scala that is easier to use than some of the more complex models based on pharmacokinetic actions. This model has two terms that produce a reduction factor to lower the TLV or other exposure limit. The first factor is for the time over eight hours and the second factor is for the reduction in recovery time. Here is the formula.
Where RF = reduction factor to apply to the TLV by.
h = the number of hours worked per day.
For 8 hours which is what you would expect.
For 12 hours, which means the recommended 12-hour shift TLV would be half of the 8-hour sift TLV.
While this approximation is more conservative than other models, the Brief and Scala paper recommends medical surveillance for a reasonable period until is it assured that the modified limits do indeed protect the workers. This additional control is more necessary if workers are in an environment that is close to the recommended TLV. Note that this model is designed for work shifts longer than eight hours not shorter times.
 Brief RS; Scala RA: Occupational health aspects of unusual work schedules: a review of Exxon’s experiences. Am Ind Hyg Assoc J. 47 (4) : 199-202 (1986)