PM2.5 stands for particulate matter 2.5 microns and smaller. For scale, a human hair is about 70 microns wide. These particles are small enough to get deep into the lungs, and some can pass into the bloodstream, which is why health agencies pay close attention to them.
Where it comes from indoors
- Cooking, especially frying and searing
- Candles, incense and fireplaces
- Wildfire smoke and outdoor pollution that leaks in
- Attic dust pulled in through leaky return ducts
- Vacuuming without a sealed, high-efficiency vacuum
How much is too much?
The U.S. EPA's annual outdoor standard for PM2.5 is 9 micrograms per cubic meter, and the World Health Organization recommends even lower. Indoors, short spikes from cooking are normal. What we look for is a high baseline, or spikes that take hours to clear.
What lowers it
- Source control first. A range hood that actually vents outside, used every time you cook.
- Sealed ductwork, so the system is not pulling dust from the attic.
- Properly sized filtration, ideally MERV 13 in a media cabinet that the system can breathe through.
- Running the fan when the air needs it. A monitor connected to the system can do this automatically.
Portable purifiers can help in a single room, but in most homes the bigger wins come from fixing the source and the ductwork, then filtering the whole house.
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