Air purifier or fresh air? What each one does for a stuffy bedroom

An air purifier cleans the air that's already in the room: it pulls it through a filter and blows it back out. It doesn't bring any outside air in. The stale feeling in a closed bedroom by morning comes from too little outside air, and CO₂ is the easiest way to see that build-up. The only thing that clears it is outside air. So a purifier and fresh air do different jobs, and a stuffy bedroom usually needs the second one.

What a purifier does

A portable air cleaner is a fan and a filter in a box. The EPA puts it plainly: "Most filters are designed to filter either particles or gases." The common kind, often sold as HEPA, catches particles such as dust, pollen and smoke.

Some purifiers add a second filter for gases, usually activated carbon, aimed at smells and chemical vapors. The EPA says activated carbon "can be effective, provided that there is a large amount of material used in the filter." It also says research hasn't yet shown that other technologies, such as plasma, photocatalytic oxidation or UV light, remove gases effectively.

Why it can't lower CO₂

CO₂ is the gas you breathe out. A purifier takes the room's air in and sends the same air back, minus some particles, so nothing from outside replaces it. Household purifier filters aren't designed to remove CO₂, and the CO₂ stays in the room until air from outside dilutes it.

That's why you can run a purifier all night and still wake up to a close, stale room. If you have a CO₂ monitor, you can see it for yourself: the night's number barely changes.

The EPA's own advice puts the two in order. "The most effective ways to improve your indoor air are to reduce or remove the sources of pollutants and to ventilate with clean outdoor air." Filtration, it says, "can be an effective supplement to source control and ventilation," but "does not replace the need to control pollutants and ventilate."

Side by side

When a purifier earns its place

• Smoky days. On wildfire-smoke days, outside air is the problem. The EPA's advice for smoke is to keep doors and windows closed, set any fresh-air intake to recirculate, and "run a portable air cleaner as often as possible on the highest fan speed." Then air the house out "when smoke clears, even temporarily."
• Dust, pollen and pets. Particles a filter can catch.
• Alongside fresh air. A balanced ventilation system (an HRV or ERV) filters the outdoor air it brings in, and a purifier can still run in the room. They don't compete.

Skip any purifier that makes ozone on purpose. The EPA says to "avoid portable air cleaners and furnace/HVAC filters that intentionally produce ozone," and notes that no federal agency has approved them for use in occupied spaces.

What lowers CO₂ in a bedroom

Outside air, in whatever form you can keep doing every night:

• The door open a few inches, if the rest of the house is well ventilated.
• The ventilation you already have, on boost or running all night.
• A window cracked, when weather, outdoor air, noise and security allow.
• A balanced system with a fresh-air supply into the bedroom, when the free changes aren't enough.

Try one per night and compare each with your usual nights. Free fixes to try first has the details, and the goal is 800 ppm or less, on average, while you sleep.