Flow imaging microscopy-based method for rapid, high-throughput measurement of fiber count and length distributions in air

Assessing airborne fiber length and number in air samples is crucial for evaluating workplace exposure to asbestos and elongate mineral particles (EMPs). Growing concerns about noncommercial EMPs highlight the need for efficient monitoring methods. Phase Contrast Microscopy (PCM), used in the National Institute for Occupational Safety and Health (NIOSH) Method 7400, is a standard technique but is labor-intensive and time-consuming, examining only about 0.2% of the filter area and yielding 100-200 fiber counts. This study evaluates flow imaging microscopy (FIM) as a rapid, high-throughput alternative for measuring fiber number and length distribution. To validate its accuracy, monodisperse polystyrene latex standards (5-50 µm) were analyzed using 4X and 10X objective lenses. Test glass fibers were prepared as (i) suspensions in deionized water and (ii) aerosols collected on cascade mesh micro-screens to produce fibers of varying lengths. FIM demonstrated accurate sizing for spherical particles (5-50 µm), with biases under 13% for 4X and 3% for 10X. Counting accuracy biases were below 22% for 4X and 10% for 10X, with relative standard deviations (RSDs) of 4.7% and 9.0%, respectively. Fiber length distributions at 10X showed geometric mean lengths of 8.0-26 µm, closely agreeing with PCM (average bias ∼16.6%). Comparisons of fiber density showed that discrepancies between the two methods decreased as fiber counts increased, highlighting the significance of high-throughput measurement with FIM. The results indicate that FIM's high-throughput ability shows potential for analyzing workplace air samples more quickly and cost-effectively, while still providing superior counting statistics.