We spend most of the day indoors. The reason to check the outdoor number at all is to decide what to do inside.
You still need to ventilate on bad days
Keep the windows shut for days on end because of the dust outside, and the air inside becomes worse than the air outside. Carbon dioxide builds up wherever people are, and the volatile organic compounds coming off furniture, wallpaper and cleaning products have nowhere to go.
Korea's environment ministry does not tell people to stop ventilating during high-pollution spells. The advice is to do it briefly and often.
Two or three times a day, five to ten minutes each time, is enough.
Open windows on opposite sides so the air can cross the room. With only one window open, the same ten minutes achieves much less.
Pick your moment using the hourly readings — levels shift a lot within a single day.
Mopping the floor straight after helps stop the dust that came in from being kicked back up.
The biggest indoor source: cooking
More often than you would expect, the problem is not the air outside but the pan. Frying and grilling send indoor PM2.5 up several-fold, sometimes by a factor of tens, because oil breaking down at high temperature creates very fine particles.
Turn the extractor on before you start cooking and leave it running for at least ten minutes afterwards. Those particles stay airborne longer than you would think.
When frying or grilling, cracking a window is worth it even if the outdoor reading is poor.
Smoky dishes such as grilled fish are worth pushing to a time when you can ventilate properly.
Vacuuming, changing the bedding, and burning candles or incense also raise indoor dust. A vacuum can blow fine particles back out through its exhaust, so a model with a HEPA filter helps — and following up with a damp mop helps more.
Choosing an air purifier
In order of what actually matters:
A HEPA filter — the grade that removes at least 99.97% of 0.3 µm particles. For fine dust, this is the baseline.
Rated room size — the unit only performs as advertised if your room is smaller than its rating, which is measured under ideal conditions. Allow about 1.5× the space you actually want to clean.
Certification — in Korea, the Korea Air Cleaning Association (CA) mark means the performance has been verified.
Filter replacement — filters are consumables. A purifier with an exhausted filter is just a box holding dust. HEPA filters typically last six months to a year.
Put it slightly away from the wall where air can circulate. In a corner or behind furniture the intake is blocked and performance drops. It clears a room fastest with the door closed.
Humidity: 40–60%
Dry air lets dust stay airborne and dries out the lining of your nose and throat. Once that lining dries, it filters inhaled particles less effectively. Go above 60% and you start growing mould and dust mites instead.
If you use a humidifier, wash the tank and change the water daily. An unmaintained humidifier sprays bacteria into the room. Hanging a damp towel or drying laundry indoors has a similar effect with none of that risk.
When you come home
Shake out your coat at the door.
Wash your hands and face; rinsing your nose with water clears out what is left.
Drinking water regularly keeps those membranes from drying out.