Michigan Administrative Code › Department - Labor and Economic Opportunity › MIOSHA › General Industry and Construction Safety and Occupational Health Standards › Occupational Health Standards › Part 301 - AIR CONTAMINANTS FOR GENERAL INDUSTRY › Section R. 325.51104 - Computation formulae

Current through Vol. 24-12, July 15, 2024
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:
(a) An employer shall compute the cumulative
					 exposure for multiple exposures to a single substance for an 8-hour workshift
					 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.
(b) An employer
					 shall compute the equivalent exposure for a mixture of air contaminants for an
					 8-hour workshift as follows:
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.