The building you are in
Most of what you breathe, you breathe indoors
The EPA puts the average American indoors for roughly 90% of their time, and finds indoor concentrations of some pollutants running two to five times higher than outdoor levels. Outdoor air quality is measured continuously by a federal monitoring network and published daily. Indoor air is measured by whoever owns the building, which usually means nobody.
That asymmetry is the reason this section exists. The county pages on this site tell you what the air and the ground around you are doing. This section is about the part you can actually change.
The five
What is worth your attention, and in what order
Radon
A radioactive gas seeping from the ground through the foundation. The second leading cause of lung cancer in the US and the leading cause among people who have never smoked.
Test the house. Fix at or above 4 pCi/L.
Test from ~$15 · system $800-$2,500
Fine particles
PM2.5 from cooking, wood burning, candles, vacuuming and whatever the outdoor air brings in. Small enough to reach the deepest part of the lung.
Source control first, then ventilation, then filtration.
Filters $20-$80 · purifier $150-$700
Asbestos
In insulation, floor tiles, pipe lagging, textured coatings and roofing in millions of pre-1990 buildings. Harmless while intact and undisturbed; dangerous once it is friable.
Leave it alone unless damaged. Never sand, drill or sweep it.
Inspection $250-$800 · abatement varies widely
Mould and damp
A symptom of a water problem rather than a problem in itself. Associated with wheeze, cough and worsening asthma.
Find and stop the water, then remove the growth.
DIY under 10 sq ft · remediation $500-$6,000
Combustion
Gas cooking, unvented heaters and attached garages produce nitrogen dioxide, carbon monoxide and ultrafine particles indoors.
Vent to outside, and fit a CO alarm on every level.
CO alarm $20-$50 · range hood varies
The order matters
Source, ventilation, filtration - in that order
Indoor air quality has a hierarchy, and it is routinely done backwards by people who buy a purifier first. The EPA’s own ordering is:
Source control
Stop it entering. Seal the slab and vent the soil for radon, vent the range outside, stop the leak, do not burn things indoors. Almost always the cheapest and by far the most effective step.
Ventilation
Dilute what is left with outside air. Effective and free when the outside air is clean, and counterproductive on a wildfire smoke day, which is where the county pages come in.
Filtration
Remove what remains. Works well on particles, does nothing at all for radon or carbon monoxide, both of which are gases that pass straight through a HEPA filter.
Time-indoors and indoor-versus-outdoor concentration figures from EPA Indoor Air Quality guidance. The source-ventilation-filtration hierarchy is the EPA’s own ordering for improving indoor air.
Equipment
Knowing what is in the room
Indoor air quality monitors
Affiliate linksRead PM2.5, and often CO2 and humidity, so you can see what cooking, cleaning or a smoke day is actually doing indoors.
GoveeLife
GoveeLife Smart Air Quality Monitor with PM2.5 H5106, LED Display Wired
Temtop
Temtop S1 PM2.5 Air Quality Monitor & Indoor Thermometer
YNAK
16 in 1 Air Quality Monitor Indoor 7" Wildfire CO2 Monitor, 9 AQI+7 Alerts
Lwyinp
Air Quality Monitor Indoor, 10-in-1 Portable Smart Air Quality Tester
Prices retrieved from Amazon on 2026-08-21 and may since have changed; the price on Amazon at the time of purchase is the one that applies. As an Amazon Associate we earn from qualifying purchases, at no extra cost to you. No manufacturer pays for a place in this list.
Carbon monoxide alarms
Affiliate linksCombustion appliances produce carbon monoxide, which no filter removes and no person can smell. One alarm per level.
First Alert
First Alert Plug-in Carbon Monoxide Alarm
Kidde
Kidde Carbon Monoxide Detector, AA Battery Powered, Portable for Travel or Home, 85 dB Alarm, LED Status Indicators, 3rd Edition
Kidde
Kidde Carbon Monoxide + Explosive Gas, Natural Gas & Propane Alarm, Plug-in with 9V Battery Backup, Digital Display, 85 dB Alarm, LED Status Indicators, 3rd Edition
X-Sense
X-Sense Smoke Detector Carbon Monoxide Detector Combo, SC07, 1-Pack
Prices retrieved from Amazon on 2026-08-21 and may since have changed; the price on Amazon at the time of purchase is the one that applies. As an Amazon Associate we earn from qualifying purchases, at no extra cost to you. No manufacturer pays for a place in this list.
Start here
Find out what you are dealing with
Of everything on this page, radon is the one with a free national dataset behind it and a clear, published action level. Look up your county, then test the building.