Air

Old houses breathed on their own. Draughty frames, a chimney and gaps between the floorboards let fresh air in all day, and the warm air straight back out.

So we sealed them up. New windows, insulation, no draughts, for the sake of a smaller heating bill. But do not forget that the air stays in too. The carbon dioxide you breathe out has nowhere to go, so it climbs all day. Also, since I presume your home is not 100% natural, all the paint, materials and their coatings add their own fumes. By 4 in the afternoon, the mixture has built up and the air feels heavy.

A big lunch does the same thing, of course. You can test the difference in five minutes. Open two windows, leave the room, come back and see.

Six days in a Harvard building

Harvard researchers took two dozen office workers into a test building for six working days. Some days the rooms had ordinary office air. Other days they doubled the fresh air coming in and took away the things that give off excessive fumes. Nobody was told which day was which. My “hound nose” would have spoiled the experiment. Each afternoon everyone sat the same test: strategy, handling information, responding to a crisis.

On the clean-air days people scored roughly twice as high as they did on ordinary office days. The same people sat at the same desks and did the same jobs.

Two things moved the scores of the tests: the amount of fresh air coming in, and the amount of fumes still in the room. Scores rose when more fresh air came in and fewer fumes remained.

The machine in the corner

Some people opt for a purifier before they open a window... It might be useful to reconsider this order of thinking.

An air filter takes out particles. A carbon filter takes out some gases as well. Neither lowers carbon dioxide, and neither brings in fresh air. A purifier cleans the same air over and over, and each pass takes out more of the smoke or the pollen in it. It cannot help a stuffy afternoon, because that is a shortage of fresh air.

Outdoor air is usually cleaner than indoor air. All indoor air is contained in a small sealed space. It does not regenerate by itself. Outside, the wind moves the air and carries fumes away. Unless you live beside heavy traffic, or something is burning nearby, opening the house improves the air.

In winter, five minutes of a wide-open window will not hurt your purse. The cold air is in and out again before the walls, the floor and the furniture have time to cool, and they warm the room straight back up. Germans do this twice a day and have a word for it, lüften.

What better air is worth

The Harvard researchers added up the cost as well. Running the fans harder came to $14 to $40 a person a year. The extra work people got done was worth up to $6,500 each.

The best hotels and offices already spend their money here: more fresh air, plain materials, no fragrance in the corridors.

At home, a window does an exceptional job.

How to ventilate a house properly

In the order that gets you the most clean air for the least effort.

• Cross-ventilate, twice a day. Open windows on two opposite sides of the home at the same time, for five to ten minutes, morning and evening. Air has to have somewhere to go. Five minutes of a real cross-draught moves more air than an hour of one window on the latch. In winter it costs almost nothing, because the walls and the floor never have time to cool.
• Air the bedroom before you sleep in it, and again when you get up. You spend a third of your life in that one room, and it holds the whole night's breathing plus whatever the mattress and the paint give off.
• Use the extractor every time you cook. Cooking puts more into the air than anything else you do at home, and a gas hob adds its own fumes on top. Switch the fan on before the pan gets hot, keep it on for ten minutes afterwards, and open a window nearby so the fan has air to pull.
• Ventilate the bathroom until the mirror is dry. Damp air is the beginning of mould, and mould is the one indoor air problem that gets worse on its own. Keep the window open or the fan running until the glass clears.
• Let the house smell of nothing. A room with nothing added to it smells clean, and it costs nothing. That is the sixth principle of the Hearth.
• Air out anything new. New mattresses, furniture, rugs and fresh paint give off far more in their first weeks than they ever will again. Unwrap them somewhere ventilated, and keep the window open in that room for a fortnight.
• Only then consider a filter. Buy a purifier for the problems a window cannot solve: a busy road under your windows, wildfire smoke, high pollen, or a room with only one opening. Run it as well as the airing, not instead. The units I would actually buy are in the air group of the Hearth directory.

To tell whether a room needs airing, go outside for two minutes, then walk back in.

A week of open windows

Try it for a week and notice how your room feels in the afternoon.

It is one of the most reliable things I recommend. Air is one of six principles I hold the whole house to, and the rest are in The Hearth. The companion pieces are on how dark a bedroom actually needs to be and what temperature it should be.

Open two windows tomorrow morning, dear friend.