Summary: We just opened the box, took our old pre-filter off and saw that we didn't have to try to figure out where to cut it because there was a dotted/precut line that measured exactly to our pre-filter's specifications. It was easy to put back on. It has really made a difference in how our purifier in our office works.

Summary: When you've got long-haired cats in the house, a quality filter like the Honeywell 17005 can helpp, especially in rooms with electronics. The permanent filters work, but it's these carbon prer-filters that help pull the most cat hair (and dander) out of the air. They work well, can sort-of be cleaned (removing cat hair from them after a few days is like removing lint from a clothes dryer), but eventually need to be replaced.

The 38002 kit comes with enough material to make 2 pre-filters, so for the price (now that I'm using permanent filters and these pre-filters are the only \"disposable\" parts of the whole purifier), it's a very cost-effective way to keep a good-sized room HEPA-filtered clean.