Calculating PELs and maintaining compliance

What is a permissible exposure limit?

Responding is Dennis Capizzi, product line manager, air purifying respirators, and Trish Luedtke, marketing copy writer, MSA, Cranberry Township, PA.

A permissible exposure limit is a specified legal limit for worker exposure to a chemical substance or physical agent during a typical 8-hour work shift and standard 40-hour work week. PELs were established by OSHA to protect workers operating in such potentially hazardous work environments.

Calculating PELs

OSHA’s Standard 29 CFR 1910.1000 includes the three PEL tables for air contaminants: Table Z-1 (limits for air contaminants), Table Z-2 (limits for substances with ceiling values) and Table Z-3 (mineral dusts). It is important to know which contaminant is involved and how, when, why and where it is being used to best determine which table is applicable.

A PEL usually is given as a time-weighted average, although some are short-term exposure limits or ceiling limits. A TWA is the average exposure during a specified period of time, usually a nominal eight hours. This means that for limited periods, a worker may be exposed to concentrations higher than the PEL so long as the average concentration during the eight hours remains lower.

Air contaminants without ceiling values

To determine the PEL for an 8-hour work shift, use the following formula and Table Z-1 to ascertain whether an employee is exposed above the regulatory limit for a contaminant without a ceiling value.
E = (C_aT_a + C_bT_b + ... C_nT_n)/8

In this formula:

• E is the equivalent exposure for the work shift.
• C is the concentration during any period of time.
• T is where the concentration remains constant.
• T is the duration, in hours, of the exposure at the concentration C.

The value of E must not exceed the 8-hour TWA specified in the Z tables for the material involved.

Air contaminants with ceiling values

To ascertain whether an employee is exposed above the regulatory limit for a contaminant that has a ceiling value, use Table Z-2 to determine the PEL.

Please note that a short-term exposure limit addresses the average exposure during a 15- to 30-minute period of maximum exposure in a single work shift. A ceiling limit may not be exceeded for any period of time, and is applied to irritants and other materials that have immediate effects.

Mixture of air contaminants

Use this formula to determine the equivalent 8-hour TWA exposure limit for a mixture of air contaminants:
Em = (C₁/L₁ + C₂/L₂ +...C_n/L_n)

• Em 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 Em must not exceed 1.0.

To achieve compliance

Whenever possible, it is best to use administrative controls, such as establishing exposure times for any worker entering a hazardous environment, or applying engineering controls to help provide the safest environment possible. These would include exhaust and general ventilation, enclosure of the source of emissions, modifications to processes and equipment to reduce emissions, and a substitution of chemicals that are less hazardous to worker safety.

When such measures are not feasible, the next step must be the determination of proper personal protective equipment or other necessary measures that may be required to keep the exposure of employees to air contaminants within the limits prescribed. Any equipment and/or technical measures used for this purpose must be approved for each use by a competent industrial hygienist or other technically qualified person. Respirators must comply with Standard 1910.134.

OSHA reference:
Employee exposure to any substance listed in Tables Z-1, Z-2, or Z-3 is limited in accordance with the requirements of 1910.1000.

Editor's note: This article represents the independent views of the author and should not be construed as a National Safety Council endorsement.