Page 231 - IGC BOOK

7.3 Occupational Exposure Limit

Introduction

In general, the occupational exposure limit (OEL) represents the maximum airborne concentration
of a toxic substance to which a worker can be exposed over a period of time without suffering any
harmful consequences.

Purpose of Occupational Exposure Limits

Occupational exposure limits (OELs) provide the basis for controlling airborne contamination
of the working environment. They define standards for air quality in terms of the amount of
a particular substance which is acceptable in the atmosphere. The air quality standard is
usually a time weighted average value rather than a single measurement. It may be an
obvious point but the standards also relate to personal exposure, or that to which a worker
is actually exposed.

Long Term and Short-Term Limits

The effects due to exposure to hazardous substances depend on the nature of the substance
and the length of the exposure; some effects require prolonged or accumulated exposure
whilst other effects become apparent very quickly. WELs are therefore usually stated as:

• Long Term Exposure Limit – the level of airborne contaminant allowable over an eight-
hour period, used for substances producing chronic effects.
• Short Term Exposure Limit (15 minutes) – the level of airborne contaminant allowable over
a 15-minute period, used for substances producing acute effects.

The concentration levels are expressed as time-weighted averages (TWA). This means that
measurements are taken over the period in question and the airborne concentrations are
then averaged out.

The concept of a TWA allows concentration levels to exceed the limit, provided that there are
equivalent exposures below it to compensate. There are often no stated levels of the extent
to which the limits may be exceeded, but the general rule is that in exposures of one-and-a-
half times above the limit requires urgent improvement in control strategies.

Measuring Exposure in Units

The two main units used for measuring airborne concentrations are:

• Parts per million (ppm).
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• Milligrams per cubic metre of air (mg/m , or mg m ).

ENSIGN| Unit IG2 – Element 7 – Chemical and Biological 26
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