77
Limit Country Union
TLV – Threshold Limit Value
USA MAK - Maximale Arbeitsplatz-Konzentration
Germany MAC
Russia WEL
– Workplace Exposure Limit United Kingdom
IOELVs Indicative Occupational Exposure Limit Value Europe OES
– Occupational Exposure Standards Australia
WES – Workplace Exposure Standards
New Zealand
7.3.1 Quantifying Airborne Concentrations of Chemical Agents
Airborne contaminants can be quantified in several ways, and these relate to the relevant hygiene standards:
By volume - atmospheric concentration in parts per million ppm By weight - milligrams of substance per cubic metre of air mgm
3
. There is a correlation between ppm and mg.m
-3
:
Conc by weight mgm
3
= Conc by volume ppm x Molecular weight
24.06 at 20°C and 760 mm Hg 1 atmosphere pressure
Numerical - for fibres, fibres per millilitre of air fibresml
7.3.2 Categories of Exposure Limits
Long Term Exposure Limits are expressed as a Time Weighted Average TWA normally over an eight hour period. This allows for exposures to vary
through the working day so long as the average exposure does not exceed the limit.
Short Term Exposure Limit STEL normally over a 15 minute period are used when exposure for short periods of time occurs.
Ceiling Limits are sometimes used and are concentrations that should not be exceeded during any part of the working exposure.
78
7.3.3 Skin Notation
Substances that have been assigned a “Skin” notation can have a contributing
exposure effect by the cutaneous route including mucous membranes and eyes either by airborne, or more particularly, by direct contact of the
substance with the skin. The exposure limits for such substances relate to exposure via inhalation only; they take no account of absorption via skin
contact.
7.3.4 Effects of Mixed Exposures
Where mixed exposures occur the first step is to ensure adequate control of exposure for each individual substance. WELS for defined mixtures should be
used only where they are applicable and in addition to any relevant individual WELS. They should not be extended to inappropriate situations. It is then
necessary to assess whether further control is needed to counteract any increased risk from the substances acting in conjunction. Expert assessments
for some particular mixed exposures may be available and can be used as guidelines in similar cases. In other cases, close examination of the
toxicological data will be necessary to determine which of the main types of interaction if any are likely for the particular combination of substances
concerned; the various types should be considered in the following order. Synergistic substances: known cases of synergism are considerably less
common than the other types of behaviour in mixed exposures. However, they are the most serious in their effects and require the strictest control. They are
also the most difficult to assess and wherever there is reason to suspect such interaction, specialist advice should be obtained.
Additive substances: where there is reason to believe that the effects of the constituents are additive, and where the WELS are based on the same health
effects, the mixed exposure should be assessed by means of the formula; C
1
+ C
2
+ C
3
…. 1 L
1
L
2
L
3
79 where C
1
C
2
etc. are the time weighted average TWA concentrations of constituents in air and L
1
, L
2
etc. are the corresponding WELs. Where the sum of the CL fractions does not exceed 1, the exposure is
considered not to exceed the notional exposure limit. The use of this formula is only applicable where L
1
, L
2
etc relate to the same reference period in the list of approved WELs. This formula is not applicable where the lead health
effect is cancer or respiratory sensitisation. For mixtures containing these substances the overriding duty is to decrease exposure so far as is
reasonably practicable. Independent substances: where no synergistic or additive effects are known
or considered likely, the constituents can be regarded as acting independently and the measures needed to achieve adequate control assessed for each
separately. The controls needed for the mixture will be those for the component requiring the tightest control.
7.3.5 Calculation of exposure with regard to the specified reference periods The 8-hour reference period
The term 8-hour reference period relates to the procedure whereby the occupational exposures in any 24-hour period are treated as equivalent to a
single uniform exposure for 8 hours the 8-hour time-weighted average TWA exposure.
The 8-hour TWA may be represented mathematically by: C
1 x
T
1
+ C
2
x T
2
+ ……. C
n x
T
n
8 where C
1
is the occupational exposure and T
1
is the associated exposure time in hours in any 24-hour period.
80 Example 1
The operator works for 7h 20min on a process in which he is exposed to a substance hazardous to health. The average exposure during that period is
measured as 0.12 mg.m
-3
. The 8-hour TWA therefore is -
7h 20min 7.33 h at 0.12 mg.m
-3
40min 0.67h at 0 mg.m
-3
That is: 0.12 x 7.33 + 0 x 0.67
8 = 0.11 mg.m
-3
The short-term reference period
Exposure should be recorded as the average over the specified short-term reference period usually 15 minutes and should normally be determined by
sampling over that period. If the Exposure period is less than 15 minutes, the sampling result should be
averaged over 15 minutes. For example, if a 5 minute sample produces a level of 150 ppm and is immediately followed by a period of zero exposure
then the 15-minute average exposure will be 50 ppm. That is:
5 x 150 = 50 ppm
15
Exposure period is 15 minutes or longer
Measurements should be taken over a 15 minute period and the result is the 15 minute average exposure. Measurements for periods greater than 15
minutes should not be used to calculate a 15 minute average exposure, but if the average exposure over the longer period exceeds the 15 minute exposure
limit, then this limit must have been exceeded over the 15 minute period.
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7.4 Biological Monitoring Guidance Values
