(1) General (dilution) ventilation. The first method involves diluting the
concentration of the contaminant before it reaches the worker's breathing zone. This
method is referred to as "general ventilation" or "dilution ventilation."
(2) Local exhaust ventilation. The second method involves capturing and
removing the contaminant near its source or point of generation, thus preventing the
release of the contaminant into the work pace. This method is called "local exhaust
b. Comparison of Methods. Dilution ventilation does not actually reduce or
eliminate the total amount of hazardous material released into the workspace. On the
workspace. Normally, local exhaust ventilation is the preferred, more effective, and
more economical method for contaminant control compared to dilution ventilation.
Section II. VENTILATION SELECTION CRITERIA
GENERAL (DILUTION) VENTILATIONS
General ventilation describes a system in which a workspace, a room, or an
entire building is flushed by supplying and exhausting a large volume of air throughout
the area. General ventilation can be quite effective in removing large volumes of heated
air or in reducing the concentration of certain atmospheric contaminants. General or
dilution ventilation can be achieved by either natural or artificial means. In actual
practice, best results are often achieved through a combination of natural and
mechanical air supply coupled with a system of natural and mechanical exhaust.
a. Natural General Ventilation. Natural means by which workspaces or
buildings may be ventilated include wind and thermal convection. These effects result
from natural pressure differences and air density differences, respectively; they each
cause natural displacement and infiltration of air through windows, doors, walls, and
other openings. (The amount of air that enters a building under a natural ventilation
scheme depends upon the wind and upon thermal effects occurring within the building.
Warmer air inside a building rises and leaks out of openings, cracks, and vents in the
upper areas; colder air leaks into the building by the same process in the lower areas.)
If it were adequate, natural ventilation would be more economical then mechanical
predictable. As a result, natural ventilation is unreliable as a primary control method.
b. Mechanical General Ventilation. Modern buildings in which industrial
operations are conducted are usually of large area, low height design, or, if they are of
multi-story design, are of masonry and glass construction. In either case, natural
ventilation forces are virtually nonexistent and mechanical ventilation must be relied
upon completely. Mechanical air supply must be provided all year round to reach