Why Multi-Stage Air Filtration Is Necessary
Bangkok air contains multiple simultaneous pollutants: PM2.5 (fine particles), PM10 (coarse dust), VOC/TVOC (volatile organic compounds), formaldehyde (HCHO), CO2, NO2, and microorganisms (bacteria, viruses). No single filter addresses all of these — a staged stack must be designed.
Filter Stack Sequence
Stage 1 — Pre-filter G3/G4 (Coarse Filter): captures coarse PM10 particles >10μm, airborne fibres, hair, and lint. Lifespan 1–3 months, low cost (100–300 THB). Protects downstream filters and extends HEPA life by reducing its particle load.
Stage 2 — Activated Carbon Filter (VOC/Odor): adsorbs VOCs, formaldehyde, odours, and NO2 onto carbon pores. The carbon bed should be at least 25–50mm thick (not a thin carbon-impregnated sheet, which has poor efficiency). Lifespan 3–6 months; a VOC sensor (SGP41) can signal when breakthrough occurs.
Stage 3 — HEPA H13 (Fine Particles): filters PM2.5, PM1.0, mould spores, and pollen at ≥99.95% efficiency at 0.3μm (MPPS). The filter must be properly sealed in its frame — any bypass gap makes it ineffective. Replacement cost 800–2,000 THB, lifespan 6–12 months.
Stage 4 — UV-C Germicidal Lamp (Microorganisms): UV-C light at 253.7nm destroys bacterial and viral DNA. Requires a dose of ≥40 mJ/cm² for 99.9% inactivation (E. coli, Influenza). If the flow rate is 200 m³/h and the UV-C chamber volume is 0.02 m³, residence time is 0.36 seconds — a lamp of ≥15W is needed to deliver sufficient dose.
UV-C safety note: never expose skin or eyes directly. Use ozone-free UV-C lamps only. Lamp lifespan 8,000–12,000 hours.
Optional — Electrostatic Precipitator (ESP): uses static electricity to capture sub-micron particles (<1μm) that HEPA may miss. Efficiency >95% for PM0.1. Requires cleaning the collector plates every 2–4 weeks and produces trace ozone (<0.05 ppm per UL 867 standard).
Photocatalytic Oxidation (PCO)
Uses TiO2 + UV-A to oxidise VOCs and microorganisms — no filter to replace. However, research shows that poorly designed PCO systems can produce harmful byproducts such as formaldehyde and acetaldehyde. Always pair with an activated carbon filter.
IoT-Based Filter Monitoring
A differential pressure sensor (BMP280 or SDP31) measures pressure drop across each filter. When ΔP rises >30–50% from baseline, the filter is full and must be replaced. Home Assistant sends a LINE alert specifying which filter needs changing. A full filter reduces CADR by 30–50%.
CADR Calculation for Room Sizing
Formula: CADR = room volume (m³) × ACH ÷ 60. For a 20 sqm bedroom at 2.7m height = 54 m³, ACH = 5: CADR = 54 × 5 ÷ 60 = 4.5 m³/min = 270 m³/h. Select a purifier with CADR ≥270 m³/h.