A Corrected Low-Cost Indoor Particle Witness: Humidity, Coincidence and Scattering Corrections for Classification Against the WHO 24-Hour PM2.5 Guideline, Below US$60

Families breathe polluted indoor air with no trustworthy affordable way to know. Low-cost optical particle counters misread through hygroscopic growth, coincidence loss and aerosol-dependent scattering. This paper designs a correction architecture for a sub-US$60 indoor witness and computes its classification accuracy against the WHO 24-hour PM2.5 guideline of 15.0 ug/m3. SIMULATED, NOT MEASURED; nothing was built or field-tested. Across six non-fog home scenarios, mean classification accuracy rises from 81.8% uncorrected to 89.7% corrected. At 92% relative humidity the hygroscopic mass growth factor is 6.75 and corrected classification is only 49.8%, so in fog-level humidity the witness must not be trusted. The component total is US$55.50.