In the beginning there was air, and it was the same everywhere.

Then someone invented shelter, which divided air into two kinds, indoor and outdoor. Walls and roofs kept storms and wildlife out, but kept in smoke from cooking. The first indoor air must have been pretty bad.

There’s a term for bad outdoor air: Air pollution.

Bad indoor air is different. It’s a more recent discovery, so there’s no succinct name for it yet. Its list of bad ingredients has little in common with the contents of (outdoor) air pollution.

Air pollutants have familiar fought-over names. Automobile exhaust. Coal-burning power plant smoke. Ozone. Wood smoke. Pollen.

Indoors, on the other hand, the hazards include –

• Cigarette smoke.
• Mold spores.
• Animal dander.
• Dust mite droppings.
• Radon gas.
• Volatile organic compounds (VOCs) released by paints, glues, foams, caulks, and other building materials.

Most people spend 90 percent of their time in buildings. So bad indoor air can sicken more people than outdoor air pollution.

Indoor air quality wasn’t too bad for most of human existence. Cooking smoke could be vented out through holes in the ceiling. Drafts blowing through cracks and chinks kept air moving. If a dwelling kept out rain, snow, and large predators, it was enough.

The oil embargo of 1974 marked a change in indoor air. Instantly, heating oil became expensive. People saw the innumerable fissures of ordinary houses leaking dollar bills. Campaigns launched to seal buildings in airtight membranes to save money.

Air leaks did waste energy, yes.

But they also had unrealized health benefits. Cracks allowed airborne contaminants to escape, and let outdoor air inside to dilute the remaining crud. When those cracks were sealed, indoor air pollutants stayed inside and built up.

What could go wrong?

Next: What does breathing all that stuff in indoor air do to people?