Occupational exposure limits

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58
Annex
Occupational exposure limits
1. Purpose
1.1. This annex is intended as a general introduction to exposure limits for the use
of employers and other parties concerned, and indicates where more detailed
information can be found. Although some illustrative values are quoted, it is not the
purpose of this annex to list values, because these change continually as more technical
information becomes available, and it is the responsibility of the competent authority to
specify which exposure limits should be used and how.
2. General
2.1. An exposure limit (EL) is a level of exposure which is specified by a
competent authority, or some other authoritative organization such as a professional
body, as an indicator of the level to which workers can be exposed without serious
injury. It is used as a general term and covers the various expressions employed in
national lists, such as “maximum allowable concentration”, “threshold limit value”,
“permissible level”, “limit value”, “average limit value”, “permissible limit”,
“occupational exposure limit”, “industrial hygiene standards”, etc. The exact definition
and intended application of ELs vary widely from one authority to another and the
underlying definitions and assumptions and the requirements of the appropriate
competent authority must be taken into account if they are used. For example, some
authorities have promulgated ELs which are used as legally permitted “safe” levels of
exposure and are intended to protect against “injury”, not against every health effect.
Other authorities provide for limits which are intended as guidelines or
recommendations in the control of potential workplace health hazards. An important
example of the caution to be applied in using ELs is provided in the introduction to the
annual publication Threshold Limit Values (TLVs) of the American Conference of
Governmental Industrial Hygienists (ACGIH): TLVs “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 threshold limit; a smaller percentage may be affected
more seriously”. Consequently, any EL represents a risk which is felt to be acceptable
based on a particular criterion, and where such limits are promulgated there is usually
an additional requirement to keep exposure as low as practicable, rather than simply
below the EL.
2.2. It is also important to take into account the averaging period for which the
limit is intended. Some limits are ceiling values to be continuously applied; others
apply to average exposures over a period of up to several years. A short-period limit
requires stricter control than a longer-period limit at the same exposure value. For
example, a limit applying to a month might allow the exposure to range above the value
for days at a time, provided there was a compensating period of low exposure which

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