I have now purchased 5 Blueair Pure 211 and 211 Plus air purifiers -- 1 for my home, 1 for my daughter's home, 1 for my son's home, and 2 for the classrooms for the teachers in my family.  These units are quiet and are highly rated on Consumer Reports and other sites.  I want to be as safe as possible from viruses & wild fire smoke, and hopefully have a reduction in seasonal allergy symptoms.  I very much like the sensor / auto mode on the 211 Plus model.