How to Use an Air Purifier: Placement, Runtime and the Mistakes to Avoid

• Put it in the room you occupy longest with the door shut. For almost everyone that is a bedroom, not the living room.
• Leave at least 30 cm clear on every side, and more on the intake face. A purifier pushed against a wall or into a corner is throttling itself.
• Raise it slightly if the design permits — many pollutants and most cooking particulate circulate above floor level, and a unit on a low stool often performs better than the same unit on the floor.
• Keep it away from the AC's direct airstream. Placed under an indoor unit, it samples already-circulating cooled air rather than the room's stagnant corners.
• Avoid corners behind curtains, inside wardrobe alcoves and under beds. All three are common and all three are throttling.
• In a long or irregular room, two mid-sized units at opposite ends beat one large unit in a corner, because circulation rather than capacity is the constraint.

Air purification is a steady state, not an event. Pollutants enter continuously through door gaps, window leakage, cooking and people, so a purifier that runs for an hour and stops allows concentrations to climb straight back. Running continuously on a low or auto setting holds the room at a low level for a fraction of the noise and roughly the same electricity as short bursts on high.

The overnight case is the strongest of all. Eight hours in a closed room is the longest uninterrupted period most people spend anywhere, and it is when breathing rate is steady and exposure is most controllable. If you are going to run a purifier for only part of the day, run it at night.

Keep the door closed. This is not a minor optimisation: a bedroom open to a passage and living room is three times the volume the purifier was sized for, and the machine will never reach its rated performance.

Windows are the harder question, because a sealed room accumulates carbon dioxide that no purifier removes. The workable compromise in a polluted city is to ventilate briefly at the least-polluted part of the day — usually early afternoon, once the overnight inversion has lifted — then close up and let the purifier bring particulate back down within the hour.

Auto mode adjusts fan speed to a particle sensor's reading. It is genuinely useful and is the setting most units should live on, with one caveat: consumer laser sensors drift as dust settles on the optics, so clean the sensor port when the manual says to. A drifted sensor will hold the fan on low in a dirty room.

Sleep mode drops the fan to its quietest setting and usually dims or extinguishes the display. Check that it dims fully — a bright AQI ring is its own kind of sleep disturbance, and it is a common reason purifiers get switched off at night, which is the one outcome that guarantees zero benefit.