Occupational health hazard? Don’t forget the mixture calculations!

When evaluating an occupational health hazard, don’t forget mixture calculations.  Here at Affygility Solutions we perform a lot of environmental, health and safety (EH&S) audits, and during the industrial hygiene portion of the EH&S audit, one of the frequent findings is that the mixture calculations for airborne contaminants is overlooked.  In addition to being a possible Occupational Safety and Health Administration (OSHA) regulatory compliance issue (29 CFR 1910.1000(d)(2)(i), [pullquote]if those airborne contaminants pose an occupational health hazard on the same body organs, this can result in an overexposure situation, even though the results of each individual airborne contaminant is below it’s respective permissible exposure limit (PEL) or occupational exposure limit (OEL).[/pullquote]

For example:

Methanol (Methyl alcohol) has an OSHA PEL of 200 ppm; and

Ethyl Acetate has an OSHA PEL of 400 ppm.

If the 8-hour time-weighted average (TWA) exposure to methanol is 150 ppm, and the 8-hour TWA for ethyl acetate is 300 ppm, then individual each chemical substance would be under its respective permissible exposure limit.  However, let’s perform the mixture calculation to determine if there is a potential occupational health hazard.

Here’s the calculation:

(8-TWA for sub. 1/PEL for sub. 1) + (8-TWA for sub. 2/PEL for sub. 2)

Which would equal:

(150 ppm/200 ppm) + (300 ppm/400 ppm) = 0.75 + 0.75 = 1.5

occupational health hazardSince 1.5 is greater than unity, a non-compliance and overexposure situation exist.  In order to reduce the potential for an occupational health hazard, effective engineering controls must be implemented to reduce the concentration of airborne contaminants.

If you have any questions, or need additional information on potent compound safety, occupational toxicology, industrial hygiene or containment validation, please feel free to contact us at 303-884-3028 or complete the online contact form.


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About Dean Calhoun

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Dean is the President and CEO of Affygility Solutions. Affygility Solutions provides environmental, health and safety software, potent compound safety, industrial hygiene, containment validation services to the pharmaceutical, biotechnology, and medical device industry. "Dean's Google+ Profile"

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