Page 408 - Industrial Ventilation Design Guidebook

6,2 OCCUPATIONAL EXPOSURE LIMIT VALUES 363

The mechanisms behind the different types of risks are also quite varied,
because manufacturers may apply different conditions and agents, and each
manufacturing stage may involve different job functions and therefore differ-
ent exposure conditions. Distance to emission sources and physical parame-
ters such as rate of release, air currents, and meteorological variations have a
profound influence. The variability of exposure conditions is made even
greater by work patterns, individual practices, and simultaneous exposure to
several substances acting together.
Thus, there is a clear need to establish the relationship between the health
effects of hazardous chemical agents in the environment and the level of occu-
pational exposure to the body by means of an occupational exposure limit, in
which a reference figure for the concentration of a chemical agent is set. In
fact, occupational exposure limits (OELs) have been a feature of the industri-
alized world since the early 1950s. They were introduced, primarily in the
United States, at a time when measures to prevent occupational diseases were
considered more beneficial than compensating victims, and in this sense OELs
have played an important part in the control of occupational illnesses,
The idea behind OELs is to identify the highest level of exposure concen-
tration and the corresponding reference time period for which we can be con-
fident that there will be no adverse effects on health. However, the
concentrations and lengths of exposure at which the presence of airborne
chemical agents could damage health have not been clear for many years, and
even today there are many questions to be answered.
Generally, OELs are set in reference to the airborne route such that expo-
sure, even when repeated on a regular basis during the working schedule
throughout the working life of a normal worker, will not lead to adverse ef-
fects on the health of exposed persons or their progeny for any time—short
term, Song term, or beyond the end of the working life—as far as can be pre-
dicted from the state of knowledge.
One problem may arise in defining who is a normal worker. This
means taking account of the variability in biological response together
with the uncertain environmental data. A small percentage of workers may
experience discomfort from some substances at concentrations at or below
the OEL. Moreover, some may be affected more seriously by the aggrava-
tion of preexisting conditions or the development of an occupational ill-
ness. Therefore, OELs should not apply to special-risk workers, those who
are hypersusceptible or otherwise unusually responsive to industrial chem-
icals because of genetic factors, age, personal habits, medication, or previ-
ous exposure.
Usually, OELs are stated as the eight-hour time-weighted average concen-
tration of exposure to a substance in gaseous, vaporous, or suspended form in
the air at the workplace. Later on we will give a more precise formula.
With all this in mind, OELs should be regarded as an important part of the
overall approach to ensuring the protection of health in the workplace, providing
a criteria by which decisions can be made as to whether the airborne concentra-
tion of a given substance is sufficiently low to avoid adverse health effects.
In essence, OELs may be used for a number of purposes, for which the
principal objective is to provide standards or criteria against which the results