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# 1910.1000(d) Computation Formulae

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The computation formula which shall apply to employee exposure to more than one substance for which 8-hour time weighted averages are listed in subpart Z of 29 CFR part 1910 in order to determine whether an employee is exposed over the regulatory limit is as follows:

E = (C

Where:

E is the equivalent exposure for the working shift.

C is the concentration during any period of time T where the concentration remains constant.

T is the duration in hours of the exposure at the concentration C.

The value of E shall not exceed the 8-hour time weighted average specified in subpart Z of 29 CFR part 1910 for the substance involved.

_{a}T_{a}+C_{b}T_{b}+. . .C_{n}T_{n})÷8Where:

E is the equivalent exposure for the working shift.

C is the concentration during any period of time T where the concentration remains constant.

T is the duration in hours of the exposure at the concentration C.

The value of E shall not exceed the 8-hour time weighted average specified in subpart Z of 29 CFR part 1910 for the substance involved.

To illustrate the formula prescribed in paragraph (d)(1)(i) of this section, assume that Substance A has an 8-hour time weighted average limit of 100 ppm noted in Table Z-1. Assume that an employee is subject to the following exposure:

Two hours exposure at 150 ppm

Two hours exposure at 75 ppm

Four hours exposure at 50 ppm

Substituting this information in the formula, we have

(2 × 150 + 2 × 75 + 4 × 50) ÷8 = 81.25 ppm

Since 81.25 ppm is less than 100 ppm, the 8-hour time weighted average limit, the exposure is acceptable.

Two hours exposure at 150 ppm

Two hours exposure at 75 ppm

Four hours exposure at 50 ppm

Substituting this information in the formula, we have

(2 × 150 + 2 × 75 + 4 × 50) ÷8 = 81.25 ppm

Since 81.25 ppm is less than 100 ppm, the 8-hour time weighted average limit, the exposure is acceptable.

In case of a mixture of air contaminants an employer shall compute the equivalent exposure as follows:

E

Where:

E

C is the concentration of a particular contaminant.

L is the exposure limit for that substance specified in subpart Z of 29 CFR part 1910.

The value of E

E

_{m}=(C_{1}÷L_{1}+C_{2}÷L_{2})+. . .(C_{n}÷L_{n})Where:

E

_{m}is the equivalent exposure for the mixture.C is the concentration of a particular contaminant.

L is the exposure limit for that substance specified in subpart Z of 29 CFR part 1910.

The value of E

_{m}shall not exceed unity (1).To illustrate the formula prescribed in paragraph (d)(2)(i) of this section, consider the following exposures:

Substituting in the formula, we have:

E

E

E

Since E

Substance | Actual concentration of 8-hour exposure (ppm) | 8-hour TWA PEL (ppm) |
---|---|---|

B | 500 | 1,000 |

C | 45 | 200 |

D | 40 | 200 |

Substituting in the formula, we have:

E

^{m}=500÷1,000+45÷200+40÷200E

^{m}=0.500+0.225+0.200E

^{m}=0.925Since E

^{m}is less than unity (1), the exposure combination is within acceptable limits.### Related Code Sections

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