Here’s a question I often get asked about air purifiers: “I don’t want to hear the fan while I sleep, so can I run my air purifier for an hour and then turn it off before I sleep at night? Or should I sleep with my air purifier on at night?”

The logic is that, since the doors and windows are closed, once we clean all the air in the room, it should stay clean. This makes perfect sense! However, the key to this assumption is whether air stays clean in a closed room.

Test: How Long to Run Air Purifier in Bedroom?

To see whether you should sleep with your air purifier on, I tested how long it would take to clean the air in a sealed room. We set a Smart Air Cannon on a timer to turn on for two hours every day in a 13.5m² Beijing bedroom. The doors and windows were closed, and nobody entered the room for 6 days.

(This test is already more conservative because, in real life, people would be opening and closing the doors to enter or leave the room.)

Here’s what happened the first day after the Cannon turned on.

What happened after the Cannon went off? Did the air stay clean? Did the particulate go up slowly?

Even though the room was closed, and no one was home, after turning off the air purifier the pollution particulates went up in a hurry.

Was that first day just a fluke? Here’s what happened to indoor pollution over all six days.

Each time the purifier turned off, the air got polluted in a hurry. But just how quickly does that happen? I averaged out the six tests into this one graph:

At the 15-minute mark, the air is 50% as dirty as it was before turning on the purifier.

After 80 minutes, the air is just as dirty as it was before. This means that if you turn off your purifier before sleeping, for most of the night your air will be just as bad as not having a purifier.

Why Do The Pollution Levels Go Back Up?

This data reveals that outdoor air is constantly getting into our homes, even if the doors and windows are closed. Because our rooms aren’t perfectly sealed, dirty air is constantly entering our homes through gaps between windows, cracks in walls, or under doors, even if we can’t see or feel it.

How to Keep Air Pollution Out

You can fight this. One way to keep air pollution out is by using insulation tape to plug up gaps around windows and doors.

The Downsides of Sealing Rooms

However, there is a cost. The better sealed our homes are, the more the CO2 we breathe out will build up. That’s not a big deal if there aren’t many people in the room, but if there are several people in the room, CO2 can reach levels that will make people drowsy.

Some other pollutants coming from indoors will build up too. One example is formaldehyde coming from new furniture and flooring. Smart Air’s real-world tests show that activated carbon filters will reduce formaldehyde and other VOCs.