Coverage is local, not whole-property. A unit rated for 400 square feet cleans the air in 400 square feet — the room it is in, when the door is closed, under conditions where room air is circulating through the unit. The rest of the home receives no benefit from that unit. A three-bedroom, two-bath Florida home with an open floor plan requires a fundamentally different approach than a single unit in the corner of the living room can provide.

Sizing is frequently wrong. Consumer air purifiers are sold by room size recommendations, but those recommendations are typically calculated under ideal conditions — a square room with standard ceiling height, no furniture obstructing airflow, and the unit running at its highest fan setting continuously. Real rooms have furniture, irregular shapes, open doorways, and varying ceiling heights. Units are also frequently undersized because buyers select based on price point rather than the CADR rating required for the actual room volume. An undersized unit running at low speed may be cycling room air at a rate far below what is needed to achieve meaningful particle reduction.

HEPA filtration does not address gaseous contaminants. True HEPA filters capture particles — biological matter, dust, pollen — but do not adsorb VOCs, formaldehyde, CO₂, or other gaseous pollutants. A unit without an activated carbon stage provides no benefit for chemical contaminants regardless of its particle filtration rating. Many consumer units include a thin carbon layer that provides limited adsorption capacity and saturates relatively quickly, after which it provides no ongoing VOC reduction.

The source is not addressed. A consumer air purifier running in a room with active mold growth inside the wall reduces the airborne spore concentration in that room to some extent. It does not address the colony producing those spores. As long as the source remains, the purifier is managing a symptom rather than resolving the problem. This distinction becomes particularly important when the purpose of air purification is to support health outcomes in occupants who are already affected — a purifier that partially reduces exposure while the source continues to produce contaminants is a limited intervention.

Maintenance is inconsistent. Consumer units require filter replacement on a schedule — typically every six to twelve months for HEPA filters, more frequently for carbon stages. In practice, filters are frequently replaced later than recommended or not at all. A HEPA filter that has exceeded its service life does not fail catastrophically — it continues to move air, it may continue to capture some particles, but its efficiency drops progressively and it can begin releasing captured particles back into the air stream. Without a professional maintenance schedule, the performance of consumer units degrades in ways that are not visible to the user.