Industrial Hygiene ConceptsSection AExposure ProfileTime-Weighted AverageCalculation of Time-Weighted AveragesCalculation of Time-Weighted AveragesExample of TWA CalculationExample of TWA CalculationExample of TWA CalculationExample of TWA CalculationCompliance and the 8-Hour TWACompliance and the 8-Hour TWASection BExposure Limits (ELs) for MixturesCalculating EL for MixturesExample of EL for MixturesExample of EL for MixturesSection CEL for > 8-Hour Work ShiftsEL for > 8-Hour Work ShiftsEL for > 8-Hour Work ShiftsExample EL for >8-Hour Work ShiftsThis work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike License. Your use of this material constitutes acceptance of that license and the conditions of use of materials on this site.Copyright 2006, The Johns Hopkins University, Patrick Breysse, and Peter S. J. Lees. All rights reserved. Use of these materials permitted only in accordance with license rights granted. 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User is responsible for obtaining permissions for use from third parties as needed.Section ATime-Weighted AveragesExposure Profile8:00 9:00 10:00 11:00 12:00 1:00 2:00 3:00 4:00 5:0002468101214TIMECONCENTRATIONTimeweighted average(TWA)PeakSource: Patrick Breysse and Peter S. J. Lees4Time-Weighted Average Apportions the measured exposure based on the interval of time during which the exposure occurred Can apply to any time period but is typically used for 8-hour periods of time Also applies to short term samples(i.e., a STEL is a 15-minute TWA)5Calculation of Time-Weighted Averages General formula:intervalitheofduration tinterval i the during ionconcentrat c :wherettcTWAthithin1iin1i11===∑∑==Continued6Calculation of Time-Weighted Averages Common formula: Compliance formula:n21nn2211t...ttct...ctctTWA++++++=8hrct...ctctTWA 8hrnn2211+++=7Example of TWA Calculation Partial period samples (PEL=12ppm): – 4 hours @ 11ppm– 2 hours @ 14ppm– 2 hours@ 20ppmhr2hr2hr4ppm20hr2ppm14hr2ppm114hrTWA++∗+∗+∗=Continued8Example of TWA Calculationhr8ppmhr40ppmhr28ppmhr44TWA++=14ppm8hr112ppmhrTWA ==Continued9Example of TWA Calculation Partial period samples: – 1 hours @ 11ppm– 2 hours @ 14ppmhr2hr1ppm14hr2ppm111hrTWA+∗+∗=Continued10Example of TWA Calculationhr3ppmhr28ppmhr11TWA+=13ppm3hr39ppmhrTWA Hour-3 ==11Compliance and the 8-Hour TWA Partial period samples (PEL=12ppm): – 4 hours @ 11ppm– 2 hours @ 14ppm– 2 hours@ not sampledhr2hr2hr4ppm0hr2ppm14hr2ppm114hrTWA++∗+∗+∗=Continued12Compliance and the 8-Hour TWAhr8ppmhr0ppmhr28ppmhr44TWA++=9ppm8hr72ppmhrTWA Hour-8 ==13Section BExposure Limits for MixturesExposure Limits (ELs) for Mixtures Exposure limits set for single substances, but multiple simultaneous exposures is the industrial norm Combined exposure limit can be calculated if:– Components have similar toxicological effects– Combined effect is assumed to be additive15Calculating EL for Mixtures Mixture in compliance if:substance for limit exposureELionconcentratTWA hour-8 measuredC :where1.0ELC...ELCELCnn2211==<+++16Example of EL for Mixtures Mixture:– Methyl isopropyl ketone @ 100 ppm (TLV=200ppm)– Methylcyclohexane @ 300ppm (TLV=400ppm)– Both TLVs set for protection against anesthetic (CNS) effectsContinued17Example of EL for Mixturescompliance in NOT is mixture this so 1.0, than less not is which25.175.05.0400ppm300ppm200ppm100ppm=+=+18Section CExposure Limitsfor Extended Work ShiftsEL for > 8-Hour Work Shifts Many workers work longer than eight hours per day and 40 hours per week Apply adjustments to ELs with caution Should not be used to justify very high exposures as “allowable” where exposure periods are shortContinued20EL for > 8-Hour Work Shifts Adjustments don’t have the benefit of historical use and long term observation – Medical supervision during early adjustment use advisableContinued21EL for > 8-Hour Work Shifts In simplest form, dose (concentration x time) is held constant and new allowable concentration is calculated: Other more complicated adjustment calculations can account for pharmacokinetic behaviorXhrallowed8hrallowed TCTC=22Example EL for >8-Hour Work Shifts What is the EL for benzene over a12-hour shift given an 8-hour EL of