p.s. I'm not affiliated in any way with commercial producers or sellers of negative ions
just an interested observer who is in the process of designing/building my own unit.
At the rip-off prices most manufacturers charge, building your own is definately the best option.
For those who don't know - ionisers use a very simple repeating Walton Cockroft type of voltage multiplier powered direct from the mains supply. For safety a series of high value resistors (usually 10Megohm) at the output to limit the current at the ioniser needle/s plus of course a low value supply fuse and a wife/- friendly enclosure
Anyone handy with a soldering iron can contruct one in a few hours for very little expense - there are plenty of g o o g l e d ioniser circuits out there.