DIRECTION OF AIR
FLOW
Locate the hood so the contaminant is removed away from the
breathing zone of the operator.
FLOW RATE AS DISTANCE FROM HOOD
Place hood as close to the source of contaminant as possible.
The required volume varies with the square of the distance
from the source.
BLOWING VERSUS EXHAUSTING
Air blown from a small opening retains its directional effect
for a considerable distance beyond the plane of the opening.
However, if the flow of air through the same opening were
reversed so that it operated as an exhaust opening handling
the same volumetric flow rate, the flow would become almost
non-directional and its range of influence would be greatly
reduced. For this reason, local exhaust must not be contemplated
for any process which cannot be conducted in the immediate
vicinity of the hood. Also, because of this effect, every
effort should be made to enclose the operation as much as
possible.
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HELPFUL
TIPS 
