Air

Outdoor and indoor air are part of the same system

The easiest way to understand air quality is to stop treating outdoor and indoor air as two separate things. Whatever is in the air outside eventually finds its way in, through windows, doors, vents, and gaps around pipes. On a wildfire day, that's smoke. A high-pollen morning brings pollen indoors with it. The ozone that built up over hours of sunlight and traffic is the same ozone your HVAC pulls through the filter.

Outdoor air sets what you're starting with inside every building. What happens after that depends on the building: how well it breathes, what it puts into the air on its own, and whether anyone is paying attention. Two houses on the same street, breathing the same outdoor air, can have very different air inside.

The index tracks three things, and pollen is not one of them

The air quality index on a weather app follows a handful of pollutants: fine particles, ozone, and a few combustion gases. Each behaves differently. Ozone has no single source. It builds through a reaction between sunlight and other pollutants, which is why the worst ozone days arrive clear and warm rather than visibly smoggy.

Fine particles work the other way. You can smell wildfire smoke, and plumes travel thousands of miles. In June 2023, smoke from Canadian fires pushed particle levels across the Northeast and Midwest to the worst readings those cities had recorded in decades, hundreds of miles from any flame.

Pollen falls outside this system entirely. No national forecast network covers it. That's why a day rated Good on your air quality app can still be brutal if you have allergies. The app and the experience disagree because they are measuring different things.

Your home is a source, not a shelter

Indoor air carries whatever came in from outside, plus everything the household generates on its own: cooking fumes, gas burners, moisture, mold, cleaning products, and candles. In a poorly ventilated space, these build up. The EPA puts indoor levels of some pollutants at 2 to 5 times outdoor levels, and higher still during an activity like stripping paint.

A gas burner running on a winter evening with the windows shut and the range hood off is the ordinary version of this. Nothing has gone wrong, and the air still gets worse. That matters because most people spend the majority of their lives indoors, which means the household has more influence over what they breathe than the weather does.

Ventilation is the strongest tool you have indoors

On good air days, opening windows cycles out what the house has accumulated. When smoke, high ozone, or heavy pollen arrive, the opposite applies. Keep the windows closed and run an air purifier with a HEPA filter, so outdoor pollutants aren't added to whatever the house is already producing.

The harder part is knowing which day you're in. Checking outdoor air quality before you open up in the morning takes about ten seconds. It makes a real difference for anyone with asthma, allergies, or a respiratory condition, because smoke and ozone don't always look or smell like a problem until you're already feeling one.

A carbon monoxide detector covers the one indoor risk no outdoor monitor can see. Combustion without ventilation gives no warning on its own.