Time of day is one of the most useful pieces of context for air-quality data. Several air-quality parameters have strong diurnal patterns governed by the solar cycle, and several human responses to air quality (sleep, alertness, exercise) are governed by the same.
Outdoor ozone is the canonical example: it is photochemically produced and peaks in the mid- to late afternoon on sunny days, dropping to near-zero overnight. The "open a window" suggestion respects this; opening at sunrise on a forecast-high-ozone day is meaningfully different from opening at 3 PM. Terrestream takes that timing from the hourly ozone forecast for your home's location.
Indoor patterns mirror occupancy. CO2 follows breathing density, which follows the sleep-wake cycle. Cooking pulses cluster around meal times. Cleaning happens in characteristic windows (weekend mornings for many households, evenings for others). For CO2, Terrestream learns each room's usual level by hour and day of the week from its own history, so a reading that is unusual for that time can stand out even when it would be ordinary at another hour.
Human circadian biology brings its own air-quality dependencies. Lux exposure during the day sets the circadian clock (see lux); insufficient daytime light is the most common circadian-rhythm disruptor in office workers. UV index pairs with lux for outdoor decisions (see UV index). Keeping these rhythms in mind is how air-quality decisions actually get made: in the context of "what time is it and what is the sun doing right now?"
References
- WHO - Circadian rhythms and health www.who.int
- IES - Physiological and behavioral effects of daytime interior lighting (RP-46-25) store.ies.org
- NOAA Global Monitoring - Solar position calculator gml.noaa.gov
- EPA - Ground-level ozone basics www.epa.gov
