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Dilution ventilation controls pollutants generated at a worksite by ventilating
the entire workplace. The use of general ventilation distributes pollutants, to some
degree, throughout the entire worksite and could, therefore, affect persons who are
far from the source of contamination.

Dilution ventilation can be made more effective if the exhaust fan is located
close to exposed workers and the makeup air is located behind the worker so that
contaminated air is drawn away from the worker's breathing zone.
See Figures 8.1, 8.2 for examples of good and poor dilution ventilation design.

When used to control chemical pollutants, dilution only situations where:
1) the amounts of pollutants generated are not very high,
2) their toxicity is relatively moderate, and
3) workers do not carry out their tasks in the immediate vicinity of the
source of contamination.
It is therefore unusual to recommend the use of general ventilation for the
control of chemical substances except in the case of solvents which have admissible
concentrations of more than 100 parts per million.

The limitations of dilution ventilation.
As a method for protecting workers, it is important to know that dilution
ventilation:
1) does not completely remove contaminants;
2) cannot be used for highly toxic chemicals;
3) is not effective for dust or metal fumes or large amounts of gases or
vapours;
4) requires large amounts of makeup air to be heated or cooled;
5) is not effective for handling surges of gases or vapours or irregular
emissions.
Regular "floor" or "desk" fans are also sometimes used as a method of
ventilation, but these fans typically blow the contaminant around the work area
without effectively controlling it. Opening doors or windows can be used as dilution
ventilation, but again, this method is not reliable since air movement is not
controlled.
As a general note, the air or "volumetric" flow rate of dilution ventilation
depends largely on how fast the contaminant enters the air as well as the efficiency
that fresh air mixes with workroom air.

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