TABLE 3.

Major laboratory studies on the filtration efficiency of masks and respirators^a

Study details	Design	Findings/conclusions	Strengths (+) and limitations (−)
Bae et al., 2020 (183) Source control:  Surgical mask  Cotton mask	4 human volunteers SARS-CoV-2 Coughing with and without mask Petri dish sampling (settle plate) Mask surface sampling	Both mask types ineffective Cotton appears superior to surgical mask Outside layer contamination >> inner contamination	(−) Implausible findings: superiority of cotton and uncontaminated inner layer (−) Ballistic particles, not aerosols (−) Confounding: cough intensities (−) Underpowered and poorly designed
Kim et al., 2020 (184) Source control:  Surgical mask  KF94  N95	7 human volunteers SARS-CoV-2 5 coughs with no mask, surgical mask, KF94 and N95 (in this order) Petri dish sampling Mask surface sampling	Surgical mask: 3 of 7 positive samples Outer and inner layer contamination KF94 and N95: 0 of 7 positive sample No outer layer contamination	(−) Implausible findings: uncontaminated inner layers of mask and respirators (−) Ballistic particles, not aerosols (−) Confounding: cough intensities and order of device testing (−) Underpowered and poorly designed
Leung et al., 2020 ( 185) Source control:  Face mask	246 human volunteers randomized to mask or no mask Influenza, rhinovirus, coronavirus Breathing and coughing Viral load in droplets and aerosols	Coronavirus: complete reduction in droplets and aerosols with mask Influenza: partial reduction in droplets but not in aerosols with mask Rhinovirus: no significant reduction with mask	(+) Similarity to clinical setting (i.e., many infected pts) (+) Viral loads quantified (+) Viral culture for influenza (not the other viruses) (−) No hypotheses provided for differential behavior of viruses (−) No fit factor
Ma et al., 2020 ( 186) Exposure control:  Surgical mask  N95  Homemade mask (paper and cloth)	Nebulizer-generated aerosols and bag as aerosol chamber Syringe-simulated human inhalation Avian influenza	Filtration efficiency N95: 99.98% Medical mask: 97.1% Homemade mask: 95.1%	(−) Particle sizes not measured but assumed from manufacturer guide (−) Unusual setup for aerosol study (nebulizer, bag, syringe) with unknown risk of bias (−) No fit factor
Patel et al., 2016 ( 187) Source and exposure control:  Natural fit and ultrafitted surgical masks  N95 with or without Vaseline seal	2 manikin heads in a chamber, 3 feet apart: Source (simulated coughing) and Receiver (simulated breathing) Nebulizer-generated and radiolabeled aerosols 3 airflow regimes	Coughing: mask or N95 on Source superior to mask or unsealed N95 on Receiver Breathing: mask on Source superior to mask or N95 on Receiver Fitting/leakage and airflow are important in Source control	(+) MMAD measured for each setup (+) Various ventilation settings (+) Loose vs tight fit for both devices (−) Vaseline seal does not adequately represent respirator fitting
Milton et al., 2013 ( 188) Source control:  Surgical mask	37 human volunteers Influenza Exhalation Viral load in droplets and aerosols	Fine particles exhaled contained more viral copies than coarse particles Viral shedding reduced by 2.8-fold (fine) and 25-fold (coarse) when using mask	(+) Similarity to clinical setting (i.e., many infected pts) (+) Viral loads quantified (+) Viral culture (on subset of fine particle samples) (−) No fit factor
Booth et al., 2013 ( 54) Exposure control:  8 types of surgical masks	Manikin head (receiver) attached to a breathing simulator Atomiser-generated viral aerosols Influenza Detection of virus in front of and behind mask	Infectious virus detected behind all masks Reduction of exposure by 1.1- to 55-fold (avg 6-fold), depending on mask Superior performance with integral visor	(+) Viral culture (+) Variety of mask types (−) Test aerosols different from natural ones (50% < 60 μm and 15% > 100 μm) (−) Unknown size of particles that penetrated mask (−) Fitting and leakage not detailed
Davies et al., 2013 ( 189) Source control:  Homemade pleated cloth mask  Surgical mask	Nebulizer-generated microbial aerosols 21 human volunteers coughing (no mask, cloth mask and surgical mask) Bacterial and viral surrogate	Surgical masks had best filtration efficiency for microbial aerosols and lowered the no. of emitted particles Fit factor: homemade half that of surgical mask	(+) Fit factor (+) Filtration efficiency measured for particles < and > 4.7 μm (−) Confusion between organism size and aerosol size
Noti et al., 2012 ( 53) Exposure control:  Surgical mask  N95	2 manikin heads attached to a coughing and a breathing simulator Nebulizer-generated aerosols Influenza	Loosely fitted respirator no better than loosely fitted mask in blocking aerosols (>50–60%) Tightly sealed masks and N95 efficient in blocking aerosols (>90–99%)	(+) Aerosol sizes measured (+) Fit factor (+) Viral culture (+) Sampling beside mouth and 3 other locations (−) Artificially high fit factor for surgical mask
Wen et al., 2013 ( 190) Exposure control:  Medical mask  N95  N99	A manikin head simulating inhalation Nebulizer-generated aerosols Phage SM702 (viral surrogate)	>97% filtration for all Low face fit factor for masks (<8) Respirators are superior when considering fit factor	(+) MMAD = 0.774 μm (+) Fit factor
Diaz and Smaldone 2010 ( 191) Source and exposure control:  Surgical mask  N95	2 manikin heads in a chamber, 3 feet apart: source (simulated exhalation) and receiver Nebulizer-generated and radiolabeled aerosols	Mask on source effective Mask on receiver not effective unless N95 with Vaseline seal	(+) MMAD measured for each setup (+) Various ventilation settings (+) Loose versus tight fit (−) Masks on both source and receiver decreased protection (implausible finding)
Johnson et al., 2009 ( 192) Source control:  Surgical mask  N95	9 human volunteers coughing Influenza Petri dish sampling	Both N95 and surgical equally effective (complete blockage)	(−) Ballistic particles of unknown size, not aerosols (−) No fit factor (−) Poor design with confounding

^aMMAD, median mass aerodynamic diameter (indicator of aerosol size); pt(s), patient(s).