Air quality discussion focuses on outdoor levels, which is where the measurements and the policy attention are. Most people spend the large majority of their time indoors, and indoor concentrations are what actually determine exposure.
The relationship between the two is not simple, and several common assumptions are wrong.
Indoor air isn't separate air
The first thing to understand: fine particulate matter penetrates buildings readily. In a typical home without specific filtration, indoor concentrations track outdoor levels with some lag and some reduction.
How much reduction depends on how airtight the building is. A well-sealed modern building achieves more; an older building with gaps achieves less.
Which means that on a high-pollution day, staying indoors helps somewhat and does not protect you the way people assume. Without filtration, you're breathing a substantial fraction of what's outside.
Indoor sources matter too
Several significant sources exist inside, and some are large.
Cooking. Frying, grilling and high-heat cooking generate substantial fine particulate matter. Gas cooking additionally produces nitrogen dioxide. Peak indoor concentrations during cooking frequently exceed outdoor levels considerably, even in polluted cities.
Solid fuel burning. Where used for cooking or heating, this is among the largest health exposures globally and dwarfs most other indoor sources.
Smoking. Obvious and enormous.
Incense and candles. Generate particulate matter at meaningful levels. Frequently used in enclosed spaces without ventilation.
Cleaning products and volatile compounds. A different class of pollutant from particulates, with different effects.
What works
Extraction while cooking. The highest-value intervention for most households. An extractor hood vented outside, used every time, removes the pollutant at source. A recirculating hood that filters and returns air to the room does much less.
Opening a window while cooking is a reasonable substitute where outdoor air is clean, and a poor one where it isn't.
Filtration. A HEPA filter removes fine particulates effectively. The critical variables are the unit's clean air delivery rate relative to the room size, and whether it's actually run.
Undersized units in large rooms achieve little. A unit rated for a small bedroom will not clean a living room, regardless of what the marketing implies.
Running it continuously in the room where you spend most time — typically a bedroom overnight — is more valuable than moving it around, because sustained reduction matters more than brief periods of clean air.
Sealing. Reducing infiltration through gaps around windows and doors reduces how much outdoor pollution enters. Cheap and effective, with a ventilation caveat below.
What doesn't work
Houseplants. The frequently cited research on plants removing pollutants was conducted in sealed chambers and does not scale to real rooms. The air exchange achieved by realistic numbers of plants is negligible compared with ordinary ventilation. Plants are pleasant; they are not air purification.
Ozone generators. Marketed as air cleaners, they produce ozone, which is itself a respiratory irritant at relevant concentrations. Health authorities generally advise against them for occupied spaces.
Ionisers without filtration. Cause particles to settle on surfaces rather than removing them, and some produce ozone as a byproduct.
Cloth masks against fine particulates. Ineffective for particles in the size range that matters. Properly fitted respirator-grade masks work; loose cloth coverings do not.
The ventilation tension
A genuine complication. Sealing a building reduces incoming pollution and also reduces the removal of indoor pollutants — carbon dioxide, moisture, and volatile compounds from furnishings.
An airtight home without mechanical ventilation accumulates indoor-generated pollutants, and elevated carbon dioxide has documented effects on cognitive performance.
The resolution is mechanical ventilation with filtration — bringing in outdoor air through a filter. That's standard in some building codes and rare in retrofit situations, where the practical compromise is filtering the air you have and ventilating when outdoor conditions are better.
Measuring it
Consumer air quality monitors have become affordable and they vary considerably in accuracy. Cheaper optical sensors give useful relative readings — showing when levels rise and fall — and shouldn't be treated as calibrated measurements.
Even so, a monitor is genuinely useful because it makes invisible things visible. Watching the reading spike during cooking, and drop when the extractor runs, changes behaviour in a way that reading advice doesn't.
The most common discovery people report is that their own cooking produces higher indoor levels than the outdoor air on a bad day. That's a useful thing to know, and it points at the intervention with the best return for the least money.
Filter maintenance
The part that determines whether any of this works and gets neglected once the purchase excitement fades.
HEPA filters load with particulates and their airflow drops as they do. A clogged filter in a running unit is moving very little air, which means very little cleaning, while the indicator light continues to suggest everything is fine.
Replacement intervals quoted by manufacturers assume moderate conditions. In heavily polluted environments the useful life can be considerably shorter, and running a unit continuously shortens it further.
Pre-filters, which catch larger particles before the main filter, can usually be washed and doing so monthly extends the expensive filter's life meaningfully. It takes ten minutes and almost nobody does it.