twa calcuation

8/12/2019 TWA Calcuation

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Industrial Hygiene ConceptsPatrick N. Breysse, PhD, CIH

Peter S.J . Lees, PhD, CIH

J ohns Hopkins UniversityCopyright 2005, Patrick N. Breysse, Peter S. J. Lees, and The Johns Hopkins University. All rightsreserved. Use of these materials permitted only in accordance with license rights granted.

Materials provided AS IS; no representations or warranties provided. User assumes allresponsibility for use, and all liability related thereto, and must independently review all materialsfor accuracy and efficacy. May contain materials owned by others. User is responsible forobtaining permissions for use from third parties as needed.


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Section ATime-Weighted Averages


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Exposure Profile

8:00 9:00 10:00 11:00 12:00 1:00 2:00 3:00 4:00 5:000

2

4

6

8

10

12

14

TIME

CONCE

NTRATION

Time weighted average(TWA)

Peak

Source: Patrick Breysse and Peter S. J. Lees 4


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Time-Weighted Average Apportions the measured exposure

based on the interval of time duringwhich the exposure occurred

Can apply to any time period but istypically used for 8-hour periods of time

Also applies to short term samples

(i.e., a STEL is a 15-minute TWA)

5


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Calculation ofTime-Weighted Averages

General formula:

intervalitheofdurationt

intervalitheduringionconcentratc:where

t

tc

TWA

thi

thi

n

1i

i

n

1i

11

=

=

=

=

=

Continued 6


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Calculation ofTime-Weighted Averages

Common formula:

Compliance formula:

n21

nn2211

t...tt

ct...ctctTWA

+++

+++=

8hr

ct...ctctTWA8hr

nn2211 +++=

7


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Example ofTWA Calculation

Partial period samples (PEL=12ppm):

4 hours @ 11ppm

2 hours @ 14ppm

2 hours@ 20ppm

hr2hr2hr4

ppm20hr2ppm14hr2ppm114hrTWA

++

++=

Continued 8


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hr8ppmhr40ppmhr28ppmhr44TWA

++=

14ppm

8hr

112ppmhrTWA ==

Continued 9


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Partial period samples:

1 hours @ 11ppm 2 hours @ 14ppm

hr2hr1

ppm14hr2ppm111hrTWA

+

+=

Continued 10


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hr3ppmhr28ppmhr11TWA

+=

13ppm

3hr

39ppmhrTWAHour-3 ==

11


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Compliance andthe 8-Hour TWA

Partial period samples (PEL=12ppm):

4 hours @ 11ppm 2 hours @ 14ppm

2 hours@ not sampled

hr2hr2hr4

ppm0hr2ppm14hr2ppm114hr

TWA

++

++

=

Continued 12


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Compliance andthe 8-Hour TWA

hr8ppmhr0ppmhr28ppmhr44TWA

++=

9ppm

8hr

72ppmhrTWAHour-8 ==

13


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Section BExposure Limits for Mixtures


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Exposure Limits ELs) for Mixtures Exposure limits set for single

substances, but multiple simultaneousexposures is the industrial norm

Combined exposure limit can becalculated if:

Components have similar

toxicological effects

Combined effect is assumed to be

additive15


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Calculating EL for Mixtures Mixture in compliance if:

substanceforlimitexposureELionconcentratTWAhour-8measuredC

:where

1.0EL

C...

EL

C

EL

C

n

n

2

2

1

1

==


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Example of EL for Mixtures

complianceinNOTismixturethisso

1.0,thanlessnotiswhich25.1

75.05.0400ppm300ppm

200ppm100ppm

=

+=+

18


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Section CExposure Limits

for Extended Work Shifts


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EL 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 highexposures as allowable where

exposure periods are short

Continued 20


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EL for > 8-Hour Work Shifts Adjustments dont have the benefit of

historical use and long term observation Medical supervision during early

adjustment use advisable

Continued 21


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EL for > 8-Hour Work Shifts In simplest form, dose (concentration x

time) is held constant and newallowable concentration is calculated:

Other more complicated adjustment

calculations can account for

pharmacokinetic behavior

Xhrallowed8hrallowed TCTC =

22


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Example EL for>8-Hour Work Shifts

What is the EL for benzene over a

12-hour shift given an 8-hour EL of1.0ppm?

0.67ppmX

12Xppmhr8.0ppmhr

12hrXppm8hr1.0ppmTCTC Xhrallowed8hrallowed

=

=

==

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twa calcuation

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