I buy these replacement filters as needed throughout the three years I have owned the Blue Air 211+. It is very easy to switch out and replace the original filter. The filter works as expected, just like the original filter that came with the unit.

These filters emit a petrochemical smell that does not dissipate over time. Do not purchase! (See the review for the Blueair Sense unit where this issue is mentioned in multiple reviews.)

Good original prefilter for the Bluair 211+ that I have. My cats ruined my old one with there claws.

The filter comes in a rectangular box, folded. It requires the user to disassemble the 211i MAX purifier to remove the old filter; it is pretty easy but you can watch how to do it in the app. The app will tell the user when the filter life is getting low and I was promoted at 5% to replace it. Installation is easy and the entire prefilter and top mount insert go together without difficulty. If you haven't done so in a while, I would recommend washing the pre-filter before reassembling the unit. After installation, plug in the machine and wait for it to initialize. Note that you have to press the fan speed button for 15 seconds, wait for the LED's to flicker twice, to reset the filter life; the app is remiss in that it does not have a reset function. You should have 99-100% filter life after this is done. Incidentally, with 24/7 use (16 hours at AUTO and 8 hours at "Night" setting), my original filter lasted exactly one year.

Have had this little air purifier now for about two months. Using in a small 11x11 foot bedroom where I keep a cage with two sugar gliders and have my home office. I keep the air purifier on low and the room always smells fresh now. Very impressive for a small and quiet air purifier.