Bought a cheap small air purifier, thinking it would suck the dust in the air but over time, dust is still settling on the desk's surface, even next to the purifier. Is it because it's a cheap small one or are air purifiers not supposed to suck the dust out of the air?

This post is an expanded guide from a previous Reddit post that had some purifier-related calculations.

What is CADR

CADR (clear air delivery rate) is a value reflecting the effective filtration rate of a purifier where larger values means the purifier can filter larger rooms. Manufacturers will usually disclose their claimed CADR value, which is often given as three values representing different air contaminant types.

In addition to effective filtration rate, CADR is also used in determining sizing purifiers to rooms as it is a figure of merit that quantifies the effective volumetric flow rate of clean air. WARNING: While this is a figure of merit, it also represents a theoretical volumetric flow rate of 100% clean air which means that it can be in units of cubic feet per minute (