Thats a great question! You really dont know. You know it's cleaner than it was... The only way tobe sure is to have outside air circulating, and even that can be dirtier than you'd think. There are testing gadgets that will tell you, but they are expensive. Just keeping the filters clean is about the best you can reasonably do.

A question I have is how do
: you know when the air is really clean since you can't see the smallest
: particles?