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Topic 3

OCCUPATIONAL HEALTH

STANDARDS

Threshold Limit Value (TLV)

Threshold limit values (TLVs)refer to airborne concentrations of substances and represent conditions under which it is believed that nearly all workers may be repeatedly exposed day after day without adverse health effects

Because of wide variation in individual susceptibility, however, a small percentage of workers may experience discomfort from some substances at concentrations at or below the TLV

A smaller percentage may be affected more seriously by aggravation of a pre-existing condition or by development of an occupational illness

TLV (USA)

Published by ACGIH

Established in 1946 (updated annually)

Approximately 750 chemicals covered

Guidelines not law

Documentation of TLVs and BEIs

“Balancing of health considerations and cost to industry”–ACGIH 1948

Examples of TLV

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Standards of Exposures (USECHH

2000)

"permissible exposure limit" means a ceiling limit or an eight-hour time-weighted average airborne concentration or the maximum exposure limit;

"ceiling limit" means the airborne concentration that should not be exceeded during any part of the working day;

"time-weighted average" in relation to airborne concentration, means an average airborne concentration over a specified period of time;

"maximum exposure limit" means a fifteen-minute time-weighted average airborne concentration which is three times (3x) the eight-hour time-weighted average airborne concentration of the chemicals specified in Schedule I; - same as STEL (ACGIH)

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Threshold Limit Value (ACGIH)

1. TLV-TWA Threshold Limit Value – Time Weighted Average (TLV-TWA): 8 Hour time weighted average

2. Time Weighted Average– Short Term Exposure Limit (TLV-STEL): TLV for short term exposure, Contaminant concentration averaged over a 15-minute period

3. Threshold Limit Value-Ceiling (TLV-C): maximum concentration that cannot be exceeded at any time / location at work. This is an instantaneous concentration; or concentration averaged over a 15 minute period if technology does not exist to measure instantaneous concentrations

General Formula for TWA

n

i

i

n

i

ii

t

tc

TWA

1

1

ci is concentration during ith interval

ti is duration of the ith interval

Evaluating Exposure to Volatile Toxicant

by Monitoring

Worker exposures using online continuous monitoring of

air concentrations of toxicants (C), the measured time–

weighted average concentration is,

8

wt

dttCTWA08 )(

8

1

TWA is time weighted average (concentration)

C(t) is concentration in ppm or mg/m3 of chemical in air

tw is the worker shift time in hours

For workers exposed for more than 8

hour

Since the computation is normalized to 8

hours, if workers are exposed to TLV~TWA

level for 12 continuous hours, then

TWA=(8/12)(TWA8)

8-hour Time Weighted Average

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For a worker who works at various locations in

the plant, his/her exposure is measured using

intermittent samples at fixed points,

8

)(...)()(1 2

0 00 21

8

t t

n

t n

dttCdttCdttCTWA

82211

8nnctctct

TWA

...

Example

PEL for the chemical involved is 12 ppm

Partial period samples

– 4 hours @ 11 ppm, 2 hours @ 14 ppm, 2 hours @ 20 ppm

TWA calculation

14ppmTWA

hr 2)2(4

m)(2hr)(20ppm)(2hr)(14ppm)(4hr)(11pp

TWA

Health risk? Risky…why?

TLV for Mixture

Evaluation of Exposure to Volatile

Toxicant by Monitoring

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For exposure to mixture of toxicants (assuming the

effects are additive), the TWA for mixture is,

n

i i

i

n

ii

mix

TWAC

CTWA

1

1

)(

Compliance of mixture

(Health risk)

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For exposure to mixture of toxicants (assuming the

effects are additive)

012

2

1

1 .... n

n

EL

c

EL

c

EL

c

C is the measured 8-hour TWA concentration

EL is the exposure limit for substance (8h)

acceptable risk

Workers of extended hours (> 8 hour

shift)

Many worker work longer than 8 hour per day or 40 hours per week

Simplest form, dose (concentration x time) is held

constant and new allowable concentration is calculated

Other more complicated adjustment calculations can

account for pharmacokinetic behaviour

XhrallowednewhrELallowed TCTC )()( _8)(

Example

What is the PEL for benzene over a 12-hour shift given an 8-hour PEL of 1.0ppm?

0.67ppmx

hr) (12

hr) (1.0ppm)(8

)(

)( x

)()(

8

_8)(

allowed

hrallowed

XhrallowednewhrELallowed

C

TCor

TCTC

How to check if the obtained x value is correct? Common sense…

Another example

Partial period samples

– 4 hours @ 11 ppm, 2 hours @ 14 ppm

TWA calculation

So what is the 8 hours TWA? (answer: 9pm)

12ppmTWA- hour 6

hr 2)(4

m)(2hr)(14ppm)(4hr)(11pp

TWA

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CARCINOGEN

Category:

A1: Confirmed human carcinogen

A2: Suspected human carcinogen

A3: Confirmed animal carcinogen with unknown relevance to humans

A4: Not classifiable as a human carcinogen

A5: Not suspected as a human carcinogen

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BIOLOGICAL STANDARD

Analysis of substances that do not change in body tissues

– Analysis for metabolite (from metabolism process)

– Analysis of variation of enzyme or biochemical levels

Biological Exposure Indices (BEI)

– Measurement of chemical determinant in a biological media

– Examples • Acetone in urine

• S-Phenylmercapturic acid (metabolite of benzene) in urine

• n-Hexane in end-exhaled air

• Lead in blood

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Occupational Standard of

Exposure in Malaysia

AKTA KILANG & JENTERA 1967 – Peraturan-Peraturan Kilang dan Jentera (Timah Hitam)

1984 – Peraturan-Peraturan Kilang dan Jentera (Proses Asbestos)

1986 – Peraturan-Peraturan Kilang dan Jentera (Habuk Galian)

1989 – Peraturan-Peraturan Kilang dan Jentera (Pendedahan

Bising) 1989

AKTA KESELAMATAN & KESIHATAN PEKERJAAN 1994 – Peraturan-Peraturan Keselamatan dan Kesihatan

Pekerjaan (Penggunaan & Standard Pendedahan Kepada Bahan Kimia Berbahaya Kepada Kesihatan) 2000

Adjustments of PEL

Adjustments of PEL

Mathematically, correction can be made, thus requiring lower PEL (stricter)

Medical supervision during early adjustment use advisable

Should not be used to justify very high exposures as “allowable” even though exposure periods are short

Adjustments do not have the benefit of historical use and long term observation (no previous exposure data)

Apply adjustments to Exposure Limits with caution because the dose/effect relationship is not linear