Honeywell F50F Owner's Manual page 5

F50F AND F300E ELECTRONIC AIR CLEANERS
If an ultrasonic room humidifier is used often, especially if filled
with tap (undistilled) water, the cell(s) and prefilter(s) require
more frequent washing. A white residue accumulates on the
cell(s) from the minerals in the water. This residue may also
contribute to "white dust." See About lint or "white dust" section
for more information.
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