We have built and tested an optical extinction monitor for the detection of spacecraft cabin particulates. This sensor sensitive to particle sizes ranging from a few nanometer to tens of micrometers in diameter. Designed to utilize commercial off- the-shelf components, the monitor, once calibrated, requires minimal recalibration and only periodic baseline determinations, a process which can be automated as part of the operation of the instrument. It employs no consumables. This monitor employs cavity attenuation phase shift technology and involves the use a light emitting diode coupled to a low-loss optical cavity. The Phase I project involved a proof-of-principle demonstration that demonstrated a sensitivity of less than 0.1 micrograms per cubic meter. During the Phase II project, a prototype sensor will be delivered to NASA for the purpose of laboratory and field measurements. A major goal will be to miniaturize the monitor.