OSHA requirements are set by statute, standards and regulations. Our interpretation letters explain these requirements and how they apply to particular circumstances, but they cannot create additional employer obligations. This letter constitutes OSHA's interpretation of the requirements discussed. Note that our enforcement guidance may be affected by changes to OSHA rules. Also, from time to time we update our guidance in response to new information. To keep apprised of such developments, you can consult OSHA's website at https://www.osha.gov.


January 23, 1997

Edwin G. Foulke, Jr., Esq.
Jackson, Lewis, Schnitzler & Krupman
2100 Daniel Building
301 N. Main Street
Greenville, South Carolina 29601-2122

Dear Mr. Foulke:

This letter is in response to your request for an interpretation on the calculation methods used under the air contaminants standard for extended work shifts. I apologize for you receiving a multitude of answers when you inquired at several field offices. In my response to your inquiry I have assumed that you are speaking to the permissible exposure limit (PEL) for a single air contaminant over an extended workshift rather than calculating a PEL for multiple chemical exposures with synergistic or additive effects.

OSHA has only two standards that specifically allow an adjustment to the PEL based upon the number of hours worked. Those two standards include 29 CFR 1910.1025 and 1926.62 - both for occupational lead exposure. No other OSHA PEL has taken this approach. All of the other OSHA PELs are based upon an 8-hour time weighted average (TWA) exposure limit, a short term exposure limit (STEL) or a ceiling exposure limit. Some contaminants may have multiple exposure limits depending upon the nature of the hazardous chemical.

For workers that work an 8-hour work shift, to evaluate their 8-hour TWA exposure level we typically attempt to first screen the exposure. If the exposure level appears to be in excess of 50% of the PEL we would then conduct personal air monitoring. In sampling a worker our industrial hygienists will, as conditions permit, sample for at least seven hours of the eight-hour work shift. The exposure level for comparison with the PEL is calculated based upon an 8-hour exposure. The balance of the exposure period (the last hour) is then calculated as a zero exposure level. For example, if sampling was conducted for seven hours for air contaminant and the concentration for that seven hours for air contaminant and the concentration for that seven hours was 500 parts per million (ppm) the exposure level for comparison with the PEL would be calculated as follows:


       TWA = 500 ppm x (420 minutes/480 minutes)
         = 437.5 ppm
420 minutes represents the sample time of seven hours
480 minutes represents the 8-hour work shift.



If OSHA's PEL for the contaminant was 100 ppm, under OSHA there would have been an overexposure.

For employees that work extended workshifts beyond 8-hours, OSHA procedures with respect to the extended workshift can take two approaches. The approach taken will often depend upon the nature of the hazardous chemical.

The first approach entails sampling what the OSHA industrial hygienist believed to be the worst continuous 8-hour work period of the entire work shift.

The second approach requires multiple samples collected over the entire work shift including the extended time period. The sampling would be conducted such that multiple personal samples were collected during the first 8-hour period and then additional samples collected for the work period beyond the initial 8-hour time period.

Unless one is dealing with lead, the PEL in this approach is calculated based upon the worst 8 hours of exposure during the workers entire workshift. The industrial hygienists would first consult with the Regional Office and possibly the National Office before issuing citations.

While OSHA only calculates a new PEL for extended workshifts for occupational exposure to lead, there are a number of procedures and practices in the literature for doing so. In the interest of providing your clients with information so that they may better protect their employees from occupational exposure to hazardous chemicals, I have enclosed selected references for your use and information.

We hope that this letter adequately addresses your concerns. We again apologize for your receiving different answers to your inquiries. We will be reemphasizing our procedures and practices with our field offices. If you have any further questions or if we may be of any further assistance please contact [the Office of General Industry Enforcement at (202) 693-1850].


Greg Watchman
Acting Assistant Secretary


[Corrected 6/8/2007]

September 6, 1996

The Honorable Joseph A. Dear
Assistant Secretary for OSHA
United States Department of Labor
200 Constitutional Avenue, N.W.
Room S2315
Washington, D.C. 20210

Re: Interpretation of Calculation Method Under Air Contaminant Standard 29 CFR 1910.1000

Dear Mr. Dear:

I would like to request a letter of interpretation regarding the calculation method for cumulative exposure under the Air Contaminants Standard. Specifically, I am inquiring about the correct method or formula used in calculating the cumulative exposure where the exposed employees normally work a shift greater than eight (8) hours. I have numerous clients whose employees either work ten- (10-) or twelve- (12-) hour shifts.

In investigating this question, I have received different opinions from different area offices. I have been advised by some that they only conduct eight- (8-) hour monitoring no matter how long the shift, and use the calculation formula set out in 1910.1000(d). Another suggestion was to use the same formula but change the eight (8) to the number of hours monitored over eight (8). Others suggested using the formula found in the lead standard at 1910.1025(c)(2). Finally, others suggested using the formula set forth in the ACG1H Threshold Limit Values (TLV) book. Based on these suggestions, I believe there is not a uniform process being utilized.

I am also interested in OSHA's current position on citing employers under the general duty clause when employees are exposed to levels of chemical substances where there are not set PEL's, but the exposure is greater than an existing TLV. I know in the past that OSHA has indicated that it would cite employers utilizing the TLV level where no PEL existed as the basis for the violation. Does OSHA still maintain that position or have all the chemical substances been assigned a PEL?

Thank you for your assistance in this matter.

Very truly yours,

Edwin G. Foulke, Jr.