Do I really need a smart air quality monitor, or is a cheap PM2.5 meter enough?

A cheap PM2.5 meter tells you about particulate matter only. Smart monitors add CO2 (the single best proxy for ventilation in occupied rooms), VOCs, humidity, and in higher-tier units radon — and they log trends over weeks so you can correlate spikes with cooking, HVAC cycles, or outdoor smoke. Specialist subreddits like r/AirQuality consistently push buyers toward units with an NDIR CO2 sensor specifically, because the cheap MOX-based 'eCO2' estimates in budget meters are widely considered unreliable.