interior areas, provide adequate air distribution, and prevent the creation of negative air
pressures in the building. In large, open industrial buildings, general ventilation can be
achieved by roof fans used with or without gravity ventilators. The best method of
providing general ventilation in a closed building is to supply air through ductwork and
distribute it into the work areas in a manner that will provide both humidity and
Figure 4-1. Simplified schematic of a general ventilation system.
c. Dilution Selection Criteria.
(1) When considering whether dilution (general) ventilation would be
preferable to a local exhaustion system, there are limiting factors, which must be kept in
(a) The quantity of contaminant generated must not be excessive; if it
is, the volume of air required for dilution will be impracticality high.
(b) The toxicity of the contaminant must be low.
(c) Workers must be located far enough away from the point of
contaminant evolution, or the contaminant must be in low enough concentrations, so
that workers will not have an exposure that is above acceptable limits.
(d) The evolution or generation of contaminants must be reasonably
(2) Considering these factors, dilution ventilation would usually not be
recommended for control of fumes and dust, because the high toxicities often
encountered require excessively large quantities of air and because the velocity and
rate of contaminant evolution are usually quite high, resulting in unacceptably high local