Open vs. closed windows

"Should I open a window right now?" is the most common decision the dashboard answers. The naive heuristic, "indoor air feels stuffy, so open one", is correct surprisingly often and wrong consequentially the rest of the time. Whether opening helps depends on six outdoor signals and three indoor ones.

Suppress the impulse to open when: outdoor PM_2.5 is above 35 µg/m³ (e.g., wildfire smoke); outdoor ozone is in the AQI orange band or higher (sunny summer afternoons in urban areas); outdoor pollen of your sensitive species is high (see pollen species); outdoor SO₂ or NO₂ is elevated near you (industry, dense traffic); outdoor dew point is higher than indoor dew point (opening would add humidity); or the outdoor temperature would push your indoor comfort outside the ASHRAE 55 envelope.

Opening usually helps when the outdoor air is clean and the room has built up something of its own: CO₂ that has stayed above about 1,000 ppm, a VOC or NO_x rise from cooking or cleaning, fine particles from an indoor source while the outdoor level is low, or a room that is too warm, too humid, or too dry when the outdoor air would help. Terrestream's window guidance in the app weighs those indoor reasons against outdoor conditions like the ones above and gives each room an answer, from "Opening windows may help" to "Keep windows closed," and it can suggest a time to air out before a forecast change.

For ambiguous cases (outdoor AQI moderate, indoor CO₂ 900 ppm), a 5-minute cross-ventilation purge often gets you most of the way without significant outdoor-pollution intake. Two windows on opposite sides of the house, both fully open for 5 minutes, exchange more air than one window cracked for an hour. If an airing clears CO₂ slowly, Terrestream suggests opening windows on opposite sides.