Imagine if the water coming into your home was only purified for a few hours each day. You probably wouldn't be very happy. Now, think about the air you breathe only being purified occasionally. That doesn't sound like the healthiest option. For this reason, most air purifiers are designed to run 24/7. However, it is wasteful to leave something running when it is not needed. For this reason, some air purifiers (generally those for which you would need to spend more than $100) have a built-in sensor that measures the air quality. If the air in the room is clean, the unit will shut off so it doesn’t run needlessly.