A room of volume V with a purifier moving clean air at rate CADR and a pollution source generating G follows dC/dt = G − (CADR·η/V)·C — the standard single-zone mass-balance model behind every real CADR rating. Each stage removes a different pollutant class: HEPA mechanically traps ≥0.3 µm particles (dust, PM2.5) by interception and diffusion; activated carbon adsorbs VOC gas molecules onto its huge internal pore surface area; a UV-C / photocatalytic (TiO2) stage oxidises whatever gas molecules remain into CO2 and water. Because the stages target different pollutants, their efficiencies combine per pollutant as η_total = 1 − ∏(1 − ηᵢ) — turning off HEPA barely touches VOC, and turning off carbon barely touches PM2.5. Concentration relaxes toward equilibrium with time constant τ = V / (CADR·η): faster fans and higher efficiency stages both shrink τ, which is why the readouts settle quicker as you raise fan speed.