Can I use an air purifier in a larger than recommended room?
I was wondering if I could use an air purifier recommended for use in rooms up to 418 square feet in a basement that is between 450 and 500 square feet.
Would there be a decrease in performance? If so, how much?

It’s not that much of a difference, maybe 10%. Don’t sweat the details, it’ll work just fine.
Not much loss in performance. Being a basement, there will not be huge changes in the air (such as an upstairs room) in the room either. This will lessen the work the unit needs to do.
The size of the cleaning area is always provided for the air purifier running on High speed setting. No doubts, in most air purifiers this is the noisiest setting, and it is doubtful that you’ll be willing to run the unit on this setting for a long time.Therefore, to make sure that the air purifier maintains the required number of air exchanges per hour while operating on a quieter Medium speed setting, select the air purifier designed for the room size at least 25% to 30% larger than the actual room size you look the air purifier for.
regards,
mchelaid
http://www.thehunterairpurifier.com/