People move to dense neighbourhoods for everything they offer and quietly assume the air indoors is a refuge from the street. It mostly isn’t. The U.S. Environmental Protection Agency has long noted that indoor air can carry pollutant concentrations several times higher than outdoor air, because a sealed apartment traps what gets in and adds its own from cooking, cleaning, and furnishings.
The three things worth caring about
You can ignore most of the alarmist lists. In a typical city apartment, three categories do the real damage:
- Particulate matter (PM2.5): Fine particles from traffic outside and frying inside. Small enough to reach deep into the lungs. This is the one a filter actually catches.
- Volatile organic compounds (VOCs): Off-gassing from new furniture, paint, and cleaning products. The “new flat” smell is largely this. Ventilation clears it; time reduces it.
- Carbon dioxide: Not toxic at home levels, but a useful proxy. A stuffy, headachey room with the windows shut overnight is a room that needs air, and CO2 is the easiest thing to measure.
Cooking is the big indoor source
Searing, frying, and especially gas burners throw a surprising amount of particulate into a small kitchen. If you have an extractor that vents outside, use it every time — not just when something burns. If your hood just recirculates, cracking a window during and after cooking does more than the fan does.
The single best air-quality habit costs nothing: open a window on a schedule, not just when it’s stuffy.
Ventilation beats filtration
Filters are useful, but moving fresh air through the space is the foundation. Even in winter, a few minutes of cross-ventilation — two windows open on opposite sides — flushes built-up CO2 and VOCs far faster than any device. The old wisdom of airing out the bedroom each morning turns out to be sound building science.
When a filter earns its keep
If you’re near heavy traffic, have allergies, or simply can’t open windows much, a portable HEPA unit sized to the room is the next step. The key number is the clean-air delivery rate relative to your room size — a unit rated for a space twice as large as yours, run on low, is quieter and just as effective as a small one running flat out. We go deeper on sizing in our HEPA guide, but the headline is: match the machine to the room, then mostly forget about it.