Definition
The condition of the air inside a building as it affects health and comfort — particles (PM2.5), gases and VOCs, biologicals, combustion byproducts, and radon. Controlled with four levers in order: source control, ventilation, filtration, humidity.
Why is indoor air usually worse than outdoor?
Enclosed spaces accumulate what's generated inside — cooking, cleaning, off-gassing, moisture, pets — and modern tight construction slows dilution. The EPA notes some pollutants run 2–5 times higher indoors, where Americans spend about 90% of their time.
What actually improves it the most?
In leverage order: remove sources (fix moisture, vent combustion, drop the worst products), ventilate (exhaust fans, HRV/ERV), filter (MERV 13 where airflow allows, portable HEPA in key rooms), and hold humidity at 30–50%. That EPA-backed order beats any single purchase — and the EPA specifically cautions against ozone-generating devices sold as air cleaners.
In the Puget Sound
The PNW's signature IAQ problems: damp crawlspaces feeding musty air upward all year, and wildfire-smoke stretches when outdoor air becomes the pollutant and filtration takes the lead.
Related terms
Go deeper on IAQ (Indoor Air Quality)
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Trade: Heating & Air