Encyclopedia of Environmental Science and Engineering, Volume I and II

(Ben Green) #1

INDUSTRIAL HYGIENE ENGINEERING 513


A successful control technique can only be designed by a
systematic approach, looking at both hardware and soft-
ware requirements. Each aspect of the problem can be seen
as a link in a chain, if one link fails the worker can become
over-exposed to the hazard.
Using the philosophy of preferring a safe place to a safe
person, it is usual to examine first the source of the hazard
then working backwards to those at risk. When examin-
ing the suitability of any strategy it is important to be clear
about the degree of protection offered and the consequence
of any failure of the control systems.
It is usually best to consider the choice of engineering
control systems in the order in which the problem evolves,
that is: first tackle the source, then the means of transmission
and lastly the exposed population.

The Source

The purpose of dealing with the source is to reduce the
potential for emission and this can be achieved in a variety
of ways depending upon the nature of the operation.

1) The process: the problem may be eliminated by
changing the process so that no hazard is created.
2) Substitution: by replacing the toxic material for
one with a lower hazard potential.
3) Enclosure: plant and layouts can be modified to
isolate the source of emission or where possible
enclosures can be built around the sources.
4) Extract ventilation: where toxic materials are
being released into the atmosphere extraction at
the source can capture any emitted substances.
5) Suppression at source: wet methods can be used
to prevent dust being emitted provided means are
available to handle the wetted dust and drying out
is prevented.

The Means of Transmission

Once the material is released the means of transmission is
usually the air which is eventually breathed by the worker.
Thus the hazard can be reduced by one of the following
means:

1) Isolation: working systems can be devised to
ensure that at times of noxious emission the area
is unoccupied or workers are located at sufficient
distance to reduce exposure to an acceptable
level.
2) Ventilation: in the case of airborne pollutants their
concentration can be reduced by dilution ventila-
tion or the workers can be bathed in a supply of
cleaned fresh uncontaminated air; in the case of
heat the air may be treated via an air conditioning
system.
3) Shielding: certain radiations can be blocked by a
shield between the source and the worker.

The Exposed Population

Finally, the worker’s exposure can be minimized by examin-
ing his position in relation to the hazard as follows:

1) Enclosure: the worker can be enclosed in a suit-
ably treated and conditioned refuge.
2) Job Rotation: the duration of exposure and hence
the absorbed dose can be reduced by suitable job
rotation involving a larger number of workers
over a shorter period.
3) Education, Training and Method Study: the work-
ers’ awareness of potential hazards can be height-
ened by training linked to a careful examination
of the work methods that are being adopted.
4) Personal Protection: respirators, resistant cloth-
ing, hearing defenders, can be employed to reduce
contact and to shield exposed and vulnerable parts
of the body.
5) Automation: use of automation to either eliminate
or reduce the exposure time of the worker is a
costly but sometimes necessary alternative.

THE HARDWARE

Control at Source

By far the best approach to minimizing emission at source is
to modify the process or the material, provided care is taken
to ensure that alternative hazards are not created. The substi-
tution of a safer material for a toxic one has commonly been
adopted, typical examples being in the case of paint solvents
where xylene and toluene were substituted for the carcino-
genic benzene and in foundry work where shot blasting was
substituted for silicosis inducing sand blasting.
More commonly it is possible to rearrange the work pro-
cedure and methods so that emissions are minimized. Where
powders and friable materials such as minerals are trans-
ported within a process, the greatest emissions of dust usually
occur at the points of transfer from one container to another
or from one mode of transport to another. Bags of raw mate-
rial are emptied into hoppers, hoppers feed on to conveyors,
conveyors tip onto other conveyors or to bins; powders are
shovelled or blended by hand into open vessels, each stage of
the process releasing dust into the air. Where volatile liquids
are used they are often poured from one vessel to another or
left standing uncovered for periods of time. Where surface
coatings are applied, volatile solvents have to evaporate or
cure before the coating is complete. This also applies to quick
drying printing inks. In many cases the coated surfaces are
allowed to dry in the open air of the workplace.
In most cases the emission of dust and vapours can be
minimized by enclosure or by redesigning the process so that
the escape of pollutants is reduced. The enclosures can be
fitted with extract ventilation but often it is just sufficient to
enclose as much as possible allowing minimal openings for
access. If it is necessary to fit extraction then the smaller the
cross-sectional area of the openings the lower the ventilation

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