Not all air purifiers employ a negative ion generator to clean the air. Most air purifiers rely on a physical filter to trap particles and a fan to draw room air through the unit. Ionizers are often a secondary air purification system, combined with a filter for increased effectiveness.
An air ionizer works by creating charged particles that attract dust and pollen into clumps that fall out of the air. An ionizer alone, however, can only clean the air in its immediate vicinity, and is ineffective beyond a few yards. In addition, these charged clumps may stick to surfaces temporarily, but they can easily be dislodged and returned to the air column in the room.