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The approach taken will often depend upon the nature of the hazardous chemical. If OSHA's PEL for the contaminant was 100 ppm, under OSHA there would have been an overexposure.įor employees that work extended workshifts beyond 8-hours, OSHA procedures with respect to the extended workshift can take two approaches. = 437.5 ppm 420 minutes represents the sample time of seven hoursĤ80 minutes represents the 8-hour work shift. TWA = 500 ppm x (420 minutes/480 minutes) 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: The balance of the exposure period (the last hour) is then calculated as a zero exposure level. The exposure level for comparison with the PEL is calculated based upon an 8-hour exposure. In sampling a worker our industrial hygienists will, as conditions permit, sample for at least seven hours of the eight-hour work shift. If the exposure level appears to be in excess of 50% of the PEL we would then conduct personal air monitoring. Some contaminants may have multiple exposure limits depending upon the nature of the hazardous chemical.įor workers that work an 8-hour work shift, to evaluate their 8-hour TWA exposure level we typically attempt to first screen the exposure. 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. No other OSHA PEL has taken this approach.
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10.62 - both for occupational lead exposure. OSHA has only two standards that specifically allow an adjustment to the PEL based upon the number of hours worked. 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. I apologize for you receiving a multitude of answers when you inquired at several field offices. 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.