Mich. Admin. Code R. 325.51104
Rule 4. The computation formulas that apply to employee exposure to 1 or more substances that have an 8-hour, time-weighted average listed in table G-1-A or G-2 to determine whether an employee is exposed in excess of the exposure limit are as follows:
E = (C1T1 + C2T2 + ... CnTn) ÷ 8 hours
Where:
E is the cumulative exposure for an 8-hour workshift.
C1 is the substance concentration during the first period of time "T" where the concentration remains constant.
C2 is the substance concentration during the second period of time "T" where the concentration remains constant.
T is the period of time in hours for which the substance concentration C remains constant.
The value of E shall not exceed the 8-hour, time-weighted average limit for the substance as specified in table G-1-A or G-2.
To illustrate the formula for a cumulative exposure to a single substance, assume that Substance A has an 8-hour, time-weighted average exposure limit of 100 ppm noted in table G-1-A. Assume that an employee is subject to the following exposures over an 8 hour workshift:
Two hours' exposure at 150 ppm
Two hours' exposure at 75 ppm
Four hours' exposure at 50 ppm
Substituting this information into the formula:
E = [(150 ppm X 2 hrs) + (75 ppm X 2 hrs) + (50 ppm X 4 hrs)] ÷ 8 hrs
E = [300 ppm·hrs + 150 ppm·hrs + 200 ppm·hrs] ÷ 8 hrs
E = 650 ppm·hrs ÷ 8 hrs = 81.25 ppm
Since the cumulative exposure of 81.25 ppm is less than the exposure limit of 100 ppm, then the employee's 8-hour workshift exposure is acceptable.
Em = (C1 ÷ L1 + C2 ÷ L2) + ... (Cn ÷ Ln)
Where:
Em is the equivalent exposure to the mixture of air contaminants during an 8-hour workshift.
C1 is the average 8-hour concentration of the first substance.
C2 is the average 8-hour concentration of the second substance.
L is the 8-hour, TWA exposure limit for that particular substance. The value of Em shall not exceed a value of one (1.0).
To illustrate the formula for a mixture of air contaminants, assume the following exposures:
Substances in mixture | Average concentration of 8-hour exposure (C) | 8-hour TWA exposure limit (L) |
Substance A | 500 ppm | 1,000 ppm |
Substance B | 45 ppm | 200 ppm |
Substance D | 40 ppm | 200 ppm |
Substituting this information into the formula:
EM = (500 ppm ÷ 1,000 ppm) + (45 ppm ÷ 200 ppm) + (40 ppm ÷ 200 ppm)
Em = 0.500 + 0.225 + 0.200
Em = 0.925
Since the value of Em did not exceed one (1.0), the employee's 8-hour workshift exposure to the mixture of air contaminants is acceptable.
Mich. Admin. Code R. 325.51104