A radon monitor based on a 3 mm × 3 mm alpha particle-detecting IC fabricated in a foundry CMOS process is reported. The alpha-detecting IC consists of a 16×16 array of pn junction diodes (sense diodes) that are precharged in reverse bias and then allowed to electrically float. Radon progeny are collected on the IC using an electrostatic concentrator. On-chip comparators detect the voltage change induced in a sense diode by an alpha particle emission from radon progeny passing though a sense diode. The comparator outputs are monitored by a microcontroller which processes the data and transmits it to a microcomputer using Bluetooth. The monitor has a sensitivity of 1.12 counts per hour per 100 Bq/m^3 of radon activity. The monitor appears suitable for mass production at very low cost.