Particle Reentrainment from Fibrous Filters

Abstract

When a respirator wearer breathes normally, airborne bacteria and particles may be collected by the filter medium of the respirator. If these particles are reentrained again by sneezing or by coughing during the exhalation cycle, they may reach other targets. To study this hypothesis, particle reentrainment from polymer and glass fiber filters was investigated by measuring the number of reentrained particles when loaded filters were subjected to air velocities higher than typical filtration velocities in the direction opposite to the filtration flow. The filters were loaded with mono- or polydisperse solid particles or liquid droplets. Particle loading and reentrainment were quantified by a real-time aerosol size spectrometer. The maximum reentrainment air velocity used in the tests was 500 cm/s, almost one hundred times the 6.6 cm/s filtration velocity during particle loading. The latter is typical for inhalation through a half-mask respirator at medium work load. For the test conditions, the reentrainment of 0.6–5.1 μm particles increases approximately with the square of particle size and the reentrainment velocity, and decreases with increasing relative humidity. The rise time in reaching the reentrainment air velocity has negligible influence on the degree of reentrainment. Particle and filter type were found to significantly affect particle reentrainment. The minimum reentrainment velocity decreases with increasing particle size. Electrical charges on the filter fibers significantly increase the collection of submicrometer particles, but their reentrainment is only slightly impeded by the embedded charges. The number of reaerosolized particles decreased slightly with filter thickness, which indicates that most of the reaerosolized particles are reentrained from the front layer of the filter.