An event-driven and energy-efficient humidity sensor for environment detection and healthcare monitoring is presented. A differential CMOS-MEMS humidity device and proportion-based capacitance-to-digit readout circuit are proposed to overcome PVT variations, and in the meantime to improve sensitivity, response time, and conversion energy. This chip achieves 15.6b 20-90 %RH at 1KS/s, 6.1pJ per 0.02%RH of sensitivity, and 10ms fast response time in TSMC 0.35-m CMOS MEMS process. With variations in temperature and voltage, our proposal can minimize the errors from 40%RH to 0.2%RH and 50%RH to 0.1%RH, making it very suitable for wearable respiratory monitoring.