Source control with masks and respirators in hospitals: exhalation aerosol shedding, jet suppression, and near-/far-field persistence

Infectious exhalation aerosol source control using masks and respirators in hospitals depends on the aerosol shedding and its dispersion.

In a simulation study with a headform, we used a leakage testing system for total outward leakage (TOL) and count median diameter (CMD) and a separate hood for horizontal exhalation-jet velocity at 1, 2, and 3 ft (0.30, 0.61, 0.91 m). Neutralised NaCl aerosols were generated to mimic human exhalation shedding. Five devices for source control - surgical mask (SM), non-valved N95 filtering facepiece respirator (N95 FFR), valved N95 (FFRV), elastomeric half-mask respirator (EHMR), and EHMR with an SM overlay (EHMRSM) - were tested under three faceseal conditions (0%, 50%, and 100%) and three flow rates (17, 28, 39 L min^-1). We derived a near-field (NF) emission index and a far-field (FF) persistence index.

Device type significantly affected TOL and CMD (P < 0.01). The N95 FFR minimised TOL (≈5-9%) and eliminated horizontal jets, yielding near-zero NF and the lowest FF. The EHMR had the highest TOL (≈82-88%) and persistent jets, producing the largest NF and FF; FFRV was intermediate. The SM and EHMRSM collapsed jets (NF≈0) but retained moderate to high TOL, sustaining elevated FF. Leaked particles were predominantly submicron (CMD ≈ 0.42-0.61 μm).

Gravitational loss (≈0.02-0.05 h^-1) is negligible compared with clinical ventilation to clean leaked aerosols, yielding orders of magnitude longer clearance times, so settling alone cannot deliver timely clearance. In practice, low-leakage devices - preferably non-valved N95s - should be paired with adequate air-change rates.