Indoor air quality: what is actually in your flat's air

What an occupied flat puts into its own air

No home generates air. What it does is add things to the air already inside it, at a rate set by how many people are in it and what they are doing. Five groups cover nearly all of it.

Water vapour. Breathing, showering, boiling and steaming, mopping, and the heavyweight of the list, laundry dried indoors. Vapour is the only one of the five that eventually announces itself: on cold glass, in the corner behind a wardrobe, in a bathroom that never quite dries out.

Carbon dioxide from the people in the room. Colourless and odourless, so nobody ever smells it. What makes it worth knowing about is not danger but arithmetic: it climbs while a closed room is occupied and falls when outdoor air arrives, which makes it a proxy for how much renewal a room is actually getting. Meters, readings and thresholds are a subject in their own right.

Cooking and combustion products. Frying and grilling put fine particles and odours into the kitchen; a gas hob adds the products of its own combustion on top. This is why DB-HS3 — Calidad del aire interior, the indoor air quality section of Spain's technical building code — sets a specific extraction provision for cooking fumes in kitchens, separate from the general ventilation of the dwelling. Two duties, and neither one covers for the other.

Particles that came in from outside. Traffic, works in the street, dust carried on the wind. These are not produced by the flat at all. They arrive with the air, through open windows and through every gap the building has.

Emissions from materials and products. Boards and adhesives in new furniture, fresh paint and varnish, solvents, cleaning products, bleach used in a bathroom with the door shut, scented candles and plug-in fresheners. Some of this is a slow release that fades over months; some is a short, strong burst that lasts an afternoon. Fragrance is worth singling out, because it adds to the list while giving the impression of subtracting from it.

Replacing air and cleaning air are two different operations

This is the distinction that decides which measures are worth anything to you.

Ventilation replaces. Outdoor air comes in, indoor air goes out, and everything the indoor air was carrying goes out with it — vapour, CO₂, frying smells, solvent gases and particles alike. It works by substitution, which is why it does not care what the contaminant is. That is the logic DB-HS3 is built on: a dwelling is not asked to treat its air, it is asked to exchange it continuously, along a route that runs from the dry rooms, across the flat, into the bathroom and kitchen, and out. The application guide to DB-HS3 published by the Catalan architects' association treats the transfer openings along that route as part of the installation, which is exactly what they are — a gap under a door is not a detail, it is a component.

Filtration captures. Air is pushed through a medium that holds back a share of the particles passing through it. The air itself stays in the room. Water vapour, CO₂ and most gases go through an ordinary particle filter untouched. Comparing purifiers is a separate question, but the boundary is not: a filter is something you add to renewal, never something you use instead of it.

What is in the air	Removed by replacing the air	Caught by a particle filter
Water vapour	Yes, it leaves with the air	No
CO₂ from the people present	Yes	No
Cooking smells and gases	Yes, and best captured at the hob	Only with adsorbent media
Particles, indoor and outdoor	Yes, though outdoor air brings its own	Yes, this is what it is built for
Emissions from materials and cleaning	Yes	Only with adsorbent media

A domestic air conditioner sits in neither column. An ordinary split unit takes in the air already in the room, cools it and hands it back; what travels out to the condenser is refrigerant, not your air. Cooling and renewal are separate operations, and a flat can be pleasantly cool and thoroughly unrenewed at the same moment.

What people are actually noticing when they call the air stale

You will hear this as aire carregat in Catalan and aire cargado or aire viciado in Spanish; stuffy covers the same ground in English. It is a genuine sensation and it is never one substance. Five things sit behind it, and they are separable.

• Accumulation from occupancy. A small bedroom, the door shut, two people, eight hours. Nothing has gone wrong; the room simply had no renewal while the load kept arriving. This is the version that reads worst on waking and clears within minutes of opening up.
• Humidity. Warm, damp air reads as heavy and close on the skin. On this coast that is the summer signature, and it comes with corroborating evidence: towels that stay damp, a bathroom that takes hours to dry, salt caking in the shaker.
• A smell with an actual source. Drains, last night's frying, a musty earthy note that means dampness somewhere, or the neighbours' cooking arriving through a shared vertical duct — a shunt — or across the interior light well, the pati de llum or patio de luces, that an interior kitchen or bathroom opens onto. A smell that is worst near one opening and fades as you walk away is telling you where to look.
• Still, warm air. A warm room with no air movement reads as stuffier than a cooler one holding identical air. This is why a fan feels like relief: moving air feels fresher without being any fresher, since a fan renews nothing.
• Nothing new at all — you just came back. Your sense of smell adapts to a constant background within minutes. The flat did not change while you were at work; your nose did.

Those five call for different responses, which is the whole reason for separating them. The first needs renewal, the second needs moisture removed at source as well as renewal, the third needs the source found, the fourth needs air movement or a lower temperature, and the fifth needs nothing.

Outdoor air decides when and where you open, not whether

Renewal is not optional, and outdoor conditions do not make it optional. What they change is the timing and the choice of opening.

On a street with traffic, the air at the window is not the same at rush hour as it is early in the morning or late at night. Opening wide for a short spell at a quieter hour moves the same volume of air as leaving a window ajar all day, with a smaller dose of what the street is producing. Pollen works the same way: for a household with someone sensitive to it, the season shifts the hours and the choice of window, not the principle.

The interior light well looks like the obvious alternative, and sometimes it is. But it deserves scepticism: air in a narrow shaft moves little, tends to hold humidity, and receives the extracts of the flats stacked around it. Quieter and off the street does not automatically mean better air, and in a light well the honest test is what you smell at that opening.

Two Catalan situations tighten this further. In a single-aspect flat, with every opening on one façade, there is no other side to choose — so timing carries the whole load, and internal doors have to be open for the incoming air to have anywhere to go. And on a hot, humid night with the air conditioning running, the windows are shut for hours by design; renewal then has to be arranged around the cooling rather than assumed to happen alongside it.

What you can judge yourself, and what needs an instrument

The list of things a resident can genuinely read at home is short, and it is worth knowing where it ends.

You can observe:

• Water. Condensation on glass, dark patches in corners, a wall that feels cold and damp behind furniture. Moisture is the one constituent that signs its name.
• Drying times. How long the bathroom mirror stays fogged, how long a towel takes, whether laundry indoors takes a day or three. These are crude but real measures of how fast air is being exchanged.
• Smells, in the first ten seconds. Come back after twenty minutes outside and register the first impression before you put your keys down. That is the only moment your nose is not adapted, and it is a better instrument than most people credit.
• Pattern. Whether the complaint follows an activity — cooking, showering, cleaning, drying clothes — or follows occupancy, such as a closed room with people asleep in it. That single distinction points at either a source or a renewal rate.

You cannot observe:

• CO₂. There is nothing to smell. The stuffiness people attribute to it is really a blend of warmth, humidity and odour.
• Fine particles. Invisible at the sizes that matter, whatever the light in a sunbeam suggests.
• Most material emissions. Some declare themselves for a week after painting; plenty never announce themselves at all.
• Whether the flow rate is what the dwelling was designed for. That is an instrument reading taken on site. Where a flat has a fixed mechanical ventilation system, it falls within the scope of RITE, the Spanish regulation covering thermal installations in buildings, which is where design, maintenance and inspection duties for such installations are set out; in Catalonia, companies carrying out that scope of work must be registered with the competent industrial-safety register.

One observation carries weight beyond comfort. Persistent dampness and visible mould are not a cosmetic matter: World Health Organization guidance on dampness and mould supports associating them with respiratory symptoms. That is a reason to take the signal seriously and to deal with the moisture behind it — not a reason to diagnose anything from a page, and removing existing mould is a different job from correcting the air.

The air you never make is the air you never have to move

Every source you switch off is renewal you do not have to achieve. In that order of preference:

• Cook with the hood on from the start, not after the smell arrives, and leave it running a while once you have finished. Lids on pans keep the steam in the pan.
• Do not dry laundry in a closed room. If it has to happen indoors, it happens where there is extraction or an open window, not in a shut bedroom overnight.
• Run the bathroom extract during the shower and after it, with the door closed — and check there is a gap or grille under that door, because a fan can only remove air that something else lets in.
• Ventilate while you clean, not afterwards. Never mix bleach with an ammonia-based cleaner. And treat fragrance as an addition to the air rather than a treatment of it.
• Give new materials time. Furniture boards, fresh paint and varnish release most in their first weeks. More renewal while that lasts, less need for anything else afterwards.
• Leave combustion appliances their air. A gas hob or a gas water heater needs its air supply and its flue clear. A grille that serves one of them is not a draught to be sealed. Anything involving the flue, the appliance or a shared vertical duct is for a qualified professional, and a communal shunt is never a DIY matter.

The same flat has a different air problem in February and in August

The complaint changes with the season even when nothing about the dwelling has changed, and the shape of that change is itself diagnostic.

In the cold months the windows stay shut, the moisture load has nowhere to go, and the coldest surface in the flat collects the evidence. Renewal here is short and brisk rather than prolonged, and the usual mistake is sealing trickle vents and wall grilles because they feel like a draught — which removes renewal without removing a single gram of vapour.

In mild, wet spells the outdoor air is already humid, so airing dries the flat far less than expected. Extraction at the point where the steam is produced does more than any amount of open window.

In summer the tension is direct: cooling wants the windows closed, renewal wants them open, and the night air on this coast is cooler without necessarily being drier. And when a flat is left shut for weeks — the second-home pattern along the coast — nothing is being generated, but nothing is being removed either, which is why the first impression on opening the door tends to be musty.

That seasonal shape is the most useful thing you own. If the problem disappears for part of the year and returns on cue, it is being driven by outdoor conditions and by what the household does. If it is there in February and still there in August, the renewal route through the flat is the thing to look at, and no appliance bought for the symptom will change it.