Methodologies that have been used in a number of passive collectors and active detectors for measuring ambient carbon fibers are summarized. The need for improved performance in terms of specificity, sensitivity, detection efficiency, accuracy, precision, speed, stability and serviceability have been identified. The design principles for a new detector based on the technique of measuring the induced charge acquired by a conducting fiber in an electric field have been discussed in detail. A prototype of such a detector has been tested to cover the measurement of fiber length distributions between 0.08 mm and 9 mm, and fiber concentrations between 1-1,000,000 fibers/cu m of air. The precision of the instrument has been demonstrated to be about 2% for fibers in the 1-9 mm range and approaching one fiber diameter (or 10 micrometers) for the submillimeter fibers.