Occupational exposure limits

• Pages: 58-63

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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