The guy who repaired my parents furnance suggested micro-biocide
filters. I usually buy the pleated type for them at Lowes or Depot
for about $15.00 a pop, but I don't know if they are treated or not.
He also suggested buying an electrostatic air filter unit which
attaches to the furnace for addtional filtration. From what I've read,
older homes or ducts (my parents house is 60 years old) aren't
designed to work well with these.Any suggestions? Thanks.