Indoor vs Outdoor Air Quality: Which Is Worse?

Most people assume outdoor air is the problem and indoor air is the refuge. Sometimes that is true — during wildfire smoke or rush-hour smog, staying inside with windows closed is the right call. But indoor vs outdoor air quality is not a fixed winner. Your kitchen, candles, vacuum, and even furniture can push indoor PM2.5 above what is on the street.

The useful question is not "which is always worse?" but "which is worse right now, for this room, at this hour?" A free AQI checker for your city plus a few habits covers most homes worldwide.

When outdoor air is worse

Traffic corridors, industrial zones, wildfire plumes, and winter inversions routinely push outdoor AQI into orange or red. On those days, importing outside air through open windows raises indoor PM2.5 within minutes. Ground-floor apartments beside busy roads often see higher indoor readings at night than upstairs neighbours.

If your local AQI is unhealthy for sensitive groups or worse, treat windows like a valve: close them, run a HEPA purifier if you have one, and recirculate car air on commutes.

When indoor air is worse

Cooking — especially frying and grilling without extraction — can spike indoor particles far above outdoor levels for an hour. Burning candles, incense, or wood indoors adds fine PM2.5 in a sealed room. New carpets, cleaning sprays, and unvented gas stoves release chemicals and particles that outdoor breezes would otherwise dilute.

On a green-AQI afternoon in London, Los Angeles, or Singapore, a ten-minute cross-breeze often beats an afternoon of trapped cooking fumes.

• Use extractor fans when cooking; open a window briefly after if outdoor AQI is good.
• Avoid burning scented candles or incense in small bedrooms.
• Vacuum with a HEPA filter; dry dusting stirs particles back into the air.
• Fix mould and damp — spores are an indoor pollutant AQI does not always show.

A simple decision rule

1. Check outdoor AQI on Aether for your location.
2. If outdoor is green to moderate and indoor feels stale or smells of cooking, ventilate briefly.
3. If outdoor is orange or worse, seal up and filter instead of opening windows.
4. Repeat in the evening — night-time inversions can flip the answer after sunset.