Coway Airmega AP-1512HH

The real "hack" lies in the IKEA STARKVIND air filters. They're from a reputable manufacturer, are HEPA filters, and best of all, they only cost $14.99 CAD (price may vary in your country). This makes them great as *replacement filters for other air purifiers*. This is one of the things I love the most about IKEA - hackable IKEA products are available worldwide and can be easily sourced and adapted as "materials" instead of "products". The Starkvind air purifier appears to be airflow-constrained on the fan, and not the filters they used. I put its filter (with some cardboard glued to the edges to seal the difference in size) in a Coway Mighty AP-1512HH (often recommended) and have not noticed a huge decrease in airflow at all, even though technically there is less filter area. Based on rudimentary measurements with an anemometer, the airflow is acceptable, and after about a year or more of continuous use, it's lost about 10% of the airflow after I recently changed it. Even better, if you are so inclined, you can buy several STARKVIND filters and cut them to size/merge multiple if your purifier takes different size filters. And if you have a smaller purifier, IKEA also has the FORNUFTIG (thinner) filter for $7 that you can similarly hack.

My favorite DIY air purifier setup is buying this Conway, myself: https://www.amazon.com/dp/B01728NLRG Quiet, efficient, works super well

Sure, I use a Coway Airmega AP-1512HH set to low permanently. It doesn't move a ton of air on the low setting, but it is very quiet and the size fits pretty well next to the robot. I think getting something quiet is key as to not spook your cats, as one of mine hates the sound of loud white-noise-ish sounds. I will say I think this one works well, but you could likely find something cheaper because you don't need to much air circulation for a litter box. I just happened to already have this one.

Air purifiers and air monitors. It is shocking and disgusting how much dust and particles these air purifiers suck out of the air. The air monitors detect when to open windows for CO2, among other things. My allergies no longer exist. Edit: A few people have asked for recommendations. I recommend the Coway Airmega AP-1512HH for larger spaces and Blueair Blue Pure 411 for smaller spaces. For the air monitor I have the Qingping.