An air purifier with a HEPA filter is one of the cheapest, most effective interventions for indoor air — when you actually need it. Running it 24/7 in a room with already-clean air is a waste of energy and a slow death for the filter.
A continuous PM2.5 monitor turns the purifier into a thermostat: on when needed, off when not.
What HEPA does and does not do.
HEPA filters remove particulates (PM1, PM2.5, PM10) by physical filtration, typically with 99.97% efficiency at 0.3 µm. They do not remove CO₂ (which only ventilation can fix), they do not remove VOCs (you need activated carbon for that), and they do not remove gas-phase pollutants like NO₂.
When to turn the purifier on.
- During and 30 minutes after cooking, particularly frying or grilling.
- When outdoor PM2.5 is high (Saharan dust events, wildfires, urban smog) and you cannot ventilate.
- In bedrooms during high-pollen days for sensitive sleepers.
- In renovated rooms with activated carbon filters to catch off-gassing VOCs (HEPA alone does not help here).
When to turn it off.
- When PM2.5 is already below 12 µg/m³ and there are no active sources.
- Overnight in low-PM rooms — energy use and filter cost without benefit.
- When you can ventilate instead (open windows on a low-outdoor-PM day).
Sizing the purifier to the room.
Look for the Clean Air Delivery Rate (CADR) — it is measured in m³/hour. The purifier needs to deliver ~5× the room volume in clean air per hour for fast clearance. A 30 m³ bedroom needs roughly 150 m³/hour CADR for PM2.5; smaller purifiers will work but slower.
The verification test.
This is what continuous monitoring gives you that nothing else does. Trigger a known PM event (toast on the toaster), watch the rise. Turn the purifier on. The dashboard should show measurable PM2.5 drop within minutes. If it does not, the filter is clogged, the purifier is undersized for the room, or the door is letting in too much new air from elsewhere.