Venta in Germany makes a very good air washer that I am thinking of
purchasing to replace my current humidifier, which is at the end of
its life.  Their web site is http://www.venta-airwasher.com/ and the
unit is available at Bed, Bath and Beyond in New York City for $199.
They may be less expensive elsewhere.

The Air-O-Swiss 1355 air washer which Paddy Handscombe mentioned can
be found at http://www.purennatural.com/prod_desc.php?prod_id=757
for $249.  Interestingly, the web site states that it cannot be
shipped outside of the United States.

Both of these products humidify and filter the air through water.
In both cases the base needs to be emptied and cleaned of sludge about
once a week.  Neither one ionizes the air or changes its chemical
composition, as far as I can tell.  Both claim to be almost silent
in their operation.  Any further information would be appreciated.

Vincent Morgan, New York City