I have had an "Air-O-Swiss 1355 Air Washer and Humidifier" for
a couple of years now, in my case chiefly as a humidifier to protect
my pianos.  It blows air through a stack of "special humidifier
discs that turn constantly through water.  This method removes many
impurities (e.g.  dust, pollen and particle-bound odors) from the air
-- as in Nature, where the rain washes the air clean.  This process
also renders traditional replacement filters unnecessary."

Although the manufacturer says it "works on the self-regulating
evaporation principle and thus it requires no additional control
systems to ensure optimal air humidity", I run mine through a plug-in
electronic humidistat which is very effective at keeping my 500
sq. ft. music room at a consistent 45% R.H. or more.  It has a two gallon
water reservoir which I have to refill about once a day to maintain
45% R.H. when the weather is at its driest.

The amount of sludge that accumulates in the bottom sump in only
a week or so is surprising.  Washing it out regularly is important of
course, even though the 1355 comes with a silver ion stick to kill
bacteria in the water.

The 1355 is well engineered and constructed, runs virtually silently
and does what it claims (it's Swiss).  Best of all it's relatively
inexpensive at about US$250.  It's available in the US and Europe.

Patrick Handscombe
Wivenhoe, Essex, UK