Three Air Quality Sources, Cross-Referenced

What I watch

Airthings indoors. IQAir and AirNow outdoors, cross-referenced against each other. Windows stay open until the AQI crosses into moderate.

Why three sources and not one

Because outdoor air data is sparser than it looks. AirNow is the regulatory network — accurate, well-calibrated, and often reporting from a monitor some distance away. IQAir pulls in a much denser mesh of low-cost sensors, which gets closer to my actual block but is noisier per station. When the two agree I trust the number. When they disagree the disagreement is itself information, usually meaning a local source or a front moving through.

Airthings is the one that matters most, because it measures the air I am personally in rather than the air over the city.

Why PM2.5 gets a different threshold than PM10

Because the two numbers roll into one AQI figure and are not the same hazard.

PM10 is coarse — dust, pollen, road grit. Most of it is caught in the upper airway. PM2.5 is fine enough to reach deep into the lungs and cross into the bloodstream, and it is what wildfire smoke is made of. An AQI of 80 driven by agricultural dust and an AQI of 80 driven by smoke are the same number describing two different exposures, so I look at what is driving it before deciding whether the windows stay open.

Why monitoring rather than just filtering

This is the distinction that changes behavior: filtration cleans the air that is already inside. Only actual air exchange removes CO2 and VOCs.

A filter moves particulate out of the room. It does nothing about the CO2 four people breathing have been putting into it all evening, and nothing about what the furniture is off-gassing — those are dilution problems, and dilution requires outside air. Which means the decision of when to open the windows cannot be made by feel; it needs the number on both sides of the wall.