"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 PM2.5 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.
Open without hesitation when: outdoor AQI is in the green band and indoor CO2 is above 1,100 ppm; or indoor VOC index is sustained above 200 after cooking or cleaning; or indoor humidity is below 30% in winter and outdoor dew point is moderate; or it's the 2-6 hour clean-air window right after a sustained rain. The dashboard tags these conditions and suggests proactively.
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. The dashboard's ventilation suggestion accounts for room geometry where it has it.
References
- EPA - AirNow: AQI Basics www.airnow.gov
- EPA - Wildfire smoke course www.epa.gov
- EPA - Health effects of ozone www.epa.gov
- AAAAI - Pollen allergy guide www.aaaai.org
