Pollen and ozone: the outdoor numbers that change your indoor advice

The Terrestream sensor reads what is in the room. But the right action (open a window, or seal up) depends on what is outside the window. Two outdoor signals the dashboard cross-references heavily are ozone and pollen.

Ground-level ozone is not emitted directly; it forms when sunlight catalyzes a reaction between NO_x and VOCs in the air. That means it peaks on sunny afternoons in summer, and it can be high outdoors when the air looks clean. EPA health-effects data show measurable lung-function impacts at 70 ppb, and asthma exacerbations well below that. On a high-ozone afternoon, opening a window brings the ozone inside.

Pollen behaves seasonally and diurnally. AAAAI notes tree pollen dominates spring, grass pollen early summer, ragweed late summer to first frost. Diurnally, dry-air pollen counts peak in early morning (4 a.m. to 10 a.m.) and again in late afternoon. The sensor sees the resulting PM₁₀ spike whenever pollen drifts indoors.

Terrestream gets outdoor pollen from Google's Pollen API and outdoor ozone from Open-Meteo's modeled air-quality data, falling back to Google's Air Quality API for the current reading (see outdoor air feeds). It will not suggest opening windows while outdoor ozone is high; high pollen adds a caution instead of blocking the suggestion. When pollen is high and your indoor PM₁₀ climbs in lockstep, closing windows and running a HEPA filter is usually the better move.