A quantitative respirator fit test apparatus was developed based on using a light-scattering type smoke detector for the sensing element and a clinical nebulizer for the aerosol source. The performance of three smoke detectors and nine clinical nebulizers considered for use in the final system are reported. Key design features of the apparatus include the generation of a corn oil aerosol concentration of 500 mg/cubic meter at a flow rate of 50 l/min and LED display for protection factors of 25, 50, 125 and 450. The total cost of the component parts for the apparatus is less than $300. This apparatus is designed to meet the need for a low cost, easy to use instrument for quantitatively monitoring a respirator's fit to a worker's face.