Seasonal IAQ shifts

The dominant indoor-air pressure rotates with the seasons. Winter brings tight-envelope problems: low humidity in heated homes (often below 25% indoors), elevated CO₂ from sealed bedrooms, and wood-smoke infiltration in regions where wood stoves are common. Healthcare-acquired infection rates rise in step with low indoor humidity, which is part of why hospitals are tightening winter RH standards.

Spring brings the tree- and grass-pollen seasons; the pollen species article covers the diurnal calendar. Many households open windows for the first time after winter on the first warm day, which introduces a temporary VOC pulse from off-gassing winter-stored materials and (in pollen-allergic occupants) a sharp PM₁₀ rise. The EPA ragweed indicator documents that spring pollen onset has shifted weeks earlier over the past three decades.

Summer brings the hot-humid combination, ozone-peak afternoons, and the increasingly extended wildfire season. Cooling systems run more, which generally improves indoor PM (well-maintained HVAC filters reduce infiltrating dust and pollen) but can introduce mold if condensate management is poor. Wildfire smoke is now a multi-month consideration across most of North America, not a regional issue.

Fall brings ragweed (August through first frost), the heating-system startup pulse (months of accumulated dust on coils and ducts becomes airborne in the first heating cycles), and the season-transition humidity drift. By Halloween, most cold-zone homes are back to winter patterns. Terrestream compares readings with each room's recent levels, so its sense of "normal" moves with the season instead of flagging every seasonal change as unusual.