8 Hour Time Weighted Average

A Permissible Exposure Limit (PEL) is the legal limit of a chemical substance or physical amanuensis that a worker can exist exposed to during a typical viii-hr day and a standard 40-hour work week. OSHA created PELs to protect workers who operate in potentially chancy environments.

A listing of PELS can exist institute in OSHA's Standard 29CFR 1910.1000. The Standard includes three tables:

Z-one describes limits for air contaminants
Z-2 describes limits for substances that have ceiling levels
Z-3 describes limits to the exposure of mineral grit

The PELs listed in Table Z-ane are 8-hour Time Weighted Average (TWA) or Ceiling Limits (C). TWA is the boilerplate exposure of a contaminant during a specified menstruum of fourth dimension, usually 8 hours. C is the maximum commanded continuous 15-minute exposure period. Because the TWA is an average in that location could be times during an eight-hr shift when an employee is exposed to concentrations higher than the established PELs.

Below is the formula used past OSHA to decide the cumulative exposure for an eight-hour shift:

Due east = (C_a T_a+C_b T_b+. . .C_n T_n)÷8

E is the equivalent exposure for the working shift.

C is the concentration during any catamenia of time (T) where the concentration remains abiding.

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

The value of E shall not exceed the eight-hour time weighted average specified in Subpart Z or 29 CFR Function 1910 for the substance involved.

An example with values inserted would be if an employee was exposed to Substance A which has an 8-hour TWA of 100 ppm. The exposure is as follows:

Two hours exposure at 150 ppm, ii hours at 75 ppm and two hours at 50ppm (2×150 + two×75 + 4×50)÷eight = 81.25 ppm . Because 81.25 ppm is less than the 100 ppm TLV limit, the exposure is acceptable.

Determining the exposure in the instance of a mixture of air contaminants the equivalent exposure is calculated as follows:

E_m=(C₁÷Fifty₁+C₂÷L₂)+. . .(C_n÷Fifty_n)

Due east_chiliad is the equivalent exposure for the mixture.

C is the concentration of a particular contaminant.

50 is the exposure limit for that substance specified in Subpart Z of 29 CFR Office 1910.

The value of E_k shall not exceed unity (1).

To illustrate the formula, values take been inserted to the contaminants below.

Substance	Actual concentration of viii-hour exposure (ppm)	Eight-hour TWA PEL (ppm)
B	500	1,000
C	45	200
D	xl	200

Substituting in the formula, we have:

Due east_yard=500÷one,000+45÷200+40÷200
E_m=0.500+0.225+0.200
E_m=0.925

Considering Em is less than unity (1), the exposure combination is within acceptable limits.

Co-ordinate to OSHA, when dealing with contaminant exposures to reach compliance in the workplace, administrative and/or engineering controls must first be determined and implemented whenever feasible. If control measures are not feasible the next step is to determine what personal protective equipment is required to assist keep employee exposure to the contaminants to the established limits. Use of this equipment must exist determined by a competent industrial hygienist or other technically qualified person.

The information independent in this article is intended for general information purposes only and is based on information bachelor as of the initial date of publication. No representation is fabricated that the data or references are complete or remain electric current. This article is not a substitute for review of electric current applicable authorities regulations, industry standards, or other standards specific to your business organization and/or activities and should not exist construed every bit legal advice or opinion. Readers with specific questions should refer to the applicable standards or consult with an attorney.