Birch trees and the grass patch release pollen and mould-spore
particles (yellow-green haze) that the wind carries toward the
house. Particles that reach the open window raise indoor
exposure; an air purifier near the window removes a fraction of
the particles that pass through its radius each second. Rain
wets and grounds airborne particles, cutting the concentration
quickly; dry, windy weather lets it build up.
C' = E·(1 − f) − k_settle·C − k_rain·C (C = airborne conc., grains/m³)
risk = Low < 60 ≤ Moderate < 150 ≤ High < 300 ≤ Very High
symptom' = a·max(0, C_local − threshold) − b·symptom
- Pollen & spore release — emission rate E from trees and grass before filtration.
- Wind speed — how fast the plume drifts from the park toward the house; higher wind spreads pollen further and faster.
- Purifier filtration — fraction f of particles removed per pass through the purifier's radius (HEPA-style filtration efficiency).
- Rain toggle — rain scavenges airborne particles onto the ground (k_rain), mirroring how a shower clears a high pollen count.
This mirrors real allergen monitoring: pollen counts and mould
spore counts are tracked by public health networks, HEPA air
purifiers and closed windows reduce indoor exposure, and rainy
days reliably lower same-day allergy symptom reports.