PM2.5 means "particulate matter 2.5 micrometers in diameter or smaller": about 1/30th the width of a human hair. The size matters. Anything larger gets caught in nose hair and the upper airway; PM2.5 goes deep into the lungs, and the smallest fractions (PM1.0 and below) cross into the bloodstream.
It is the most strongly health-linked air pollutant we measure. The Lancet Global Burden of Disease attributes millions of annual deaths to ambient PM2.5 exposure: heart disease, stroke, COPD, lung cancer, and increasingly dementia and adverse birth outcomes.
Two different thresholds matter. The U.S. EPA NAAQS annual standard is 9 µg/m³ (tightened in 2024). The WHO 2021 guideline is 5 µg/m³. Indoors, sources include cooking (especially gas and frying), candles, wood smoke, vacuuming without HEPA, and outdoor air drifting in during wildfire smoke or high-traffic conditions.
What to do: cooking on the stovetop, run the range hood and crack a window. During wildfire smoke, close everything and run a HEPA box-fan filter. The device reports PM1.0, PM2.5, PM4.0, and PM10 so you can see whether a spike is fine combustion particles (smaller) or coarse dust (larger).
Terrestream Intelligence · near you
What a Terrestream would flag in Columbus, OH today
Situation
Outdoor smoke intrusion
Smoke advisory conditions · AQI 48 · PM2.5 5 µg/m³ · 65° · near Columbus, OH
Wildfire smoke is likely reaching your area. In a home like this one, a Terrestream would flag fine particles drifting upward indoors and recommend closing up and filtering — then keep watching whether indoor air is holding cleaner than outside.
- PM2.5 (indoor) 12 µg/m³ Moderate
- CO₂ (indoor) 640 ppm Moderate
Recommended action Now
Seal and filter
- Close windows
- Run an air cleaner
Keep windows and doors closed, run an air cleaner, and avoid adding indoor particles (frying, candles, vacuuming). The goal is holding indoor PM below outdoor.
For health guidance and any stay-indoors or evacuation instructions, follow AirNow and your local authorities.
Illustration: outdoor conditions are real, approximate-area data; indoor readings are modeled, not measured. No device is reporting from your home. Not medical advice.
References
- EPA - National Ambient Air Quality Standards for PM www.epa.gov
- WHO Global Air Quality Guidelines (2021) www.who.int
- EPA - Health & environmental effects of PM www.epa.gov
- Lancet - Global Burden of Disease (PM2.5) doi.org
