Protocols, policies and procedures may evaluate the ability of masks to protect the wearer from breathing in infectious or dangerous materials where the mask functions as personal protective equipment (PPE). In other instances masks are evaluated for their ability to protect others from the mask wearer. The latter is also known as respiratory source control which is a measure which aims to protect others from exposure to potential infectious particles that the wearer may breath out.
Key equipment and materials: Laser, box, computer algorithm, laminar HEPA-filtered air
Researchers: Martin Fischer
Martin Fischer and colleagues at Duke University tested masks for respiratory source control using a black box, laser and camera. The laser beam forms a thin sheet of light that shines through slits on the sides of the box from left to right. A person wearing various types of masks speaks through a hole in the front of the box and a cell phone on the back of the box records light scattered by the respiratory droplets that cut through the laser beam as they talk. A computer algorithm counts the droplets. The study showed that N95 masks performed the best and three-layer surgical masks and cotton masks performed well. To clear droplets from the box between experiments, laminar HEPA-filtered air was continuously fed into the box from above through a duct with 25 cm x 25 cm cross section. The supplied air was expelled through the light sheet slits and speaker hole. There was a slight positive pressure in the box which cleared droplets and prevented dust from entering. Neck fleeces or gaiter masks were least effective and resulted in a higher number of respiratory droplets as the material appeared to break down larger droplets into smaller ones.
Key equipment and materials: Mannequin, fog machine, glycerin, laser pointer
Researchers: Siddhartha Verma, Manhar Dhanak, and John Frankenfield
Researchers at the Department of Ocean and Mechanical Engineering at Florida Atlantic University examined the blocking of emulated coughs and sneezes by different mask materials and designs using qualitative visualizations. To simulate a cough a mammequin’s head was connected to a fog machine that creates vapor from water and glycerin and a pump was used to expel vapor through the mannequin’s mouth. The vapor flow was visualized using a laser sheet that was generated by passing a green laser pointer through a cylindrical rod. The simulated coughs appeared as glowing green vapor. The study found that well-fitted homemade masks with multiple layers of quilting fabric and off-the-shelf cone style masks were most effective in reducing dispersal of droplets.
Key equipment and materials: Cadaver head, joss incense sticks, high-resolution camera, digital image processing, Ambu mask, NAPR
Researchers: Tawfiq Khoury, James Evans
A research study based at Thomas Jefferson University Hospital, Philadelphia, USA tested aerosolized particle escape from masks using simulated breathing conditions and the efficacy of a negative-pressure environment around the face in preventing particle escape. The finding are relevant to protection of healthcare providers from infectious patients. A fixed cadaver head was placed in a controlled environment with a black background and small-particle aerosols of diameter 0.28 µ were created with joss incense sticks. When smoke was passed through the cadaver head, images were taken with a high-resolution camera. Digital image processing was used to calculate relative amounts of small-particle escape from various mask including a standard surgical mask, a modified Ambu mask and a negative airway pressure respirator (NAPR). While significant aerosolized particles escaped with no mask, surgical mask and NAPR without suction, the application of suction applied to the NAPR created a negative-pressure system where not particle escape was noted.
Key equipment and materials: G-II bioaerosol collecting device, RT-PCR reagents and instruments
Researchers: Benjamin J. Cowling, Donald K. Milton
Researchers from University of Hong Kong, WHO Collaborating Centre for Infectious Disease Epidemiology and Control and University of Maryland conducted a study on the amount of respiratory virus in exhaled breath of participants with medically attended acute respiratory virus illnesses. Participants were randomized to wearing or not wearing a surgical face mask (Kimberley-Clark, cat. No. 62356) during 30 minutes of exhaled breath collection. Exhaled breath particles were captured and categorized into two size fractions used a G-II bioaerosol collecting device. Viral shedding was detected in breath samples using reverse transcription PCR (RT-PCR) in respiratory droplet and aerosol droplet samples collected with or without a face mask. The study indicated that surgical masks reduced emission of influenza virus particles in respiratory droplets but not aerosols and coronavirus in both droplets and aerosols.
Key equipment and materials: Air brush, micropump nebulizer, influenza virus, motor and metal bellows, digital breathing machine, spray droplet size analyzer
Researchers: William G. Lindsley
Face shields as a protection to the wearer were assessed by researchers at NIOSH using a coughing patient simulator and a breathing worker simulator with or without a face shield. The testing used influenza-laden cough aerosol with a volume median diameter of 8.5 μm. Some experiments used an air brush to produce a cough aerosol at volume median diameter of 8.5 μm. For other experiments a micropump nebulizer was used to produce aerosol at volume median diameter of 3.4 μm. The cough airflow was produced by metal bellows driven by a computer-controlled motor. A digital breathing machine (Warwick Technology) was used to simulate a respiring health care worker. Concentration of aerosol particles inhaled during breathing were measured with a spray droplet size analyzer (Spraytec Analyzer with a 300 mm lens and an Inhalation Cell). When face shields were tested, optical counters were used to draw aerosol samples through tube inlets between the face shield and the model head. The experiments determined the amount of influenza virus inhaled by the breathing simulator and deposited on the face shield.
Key equipment and materials: Six-jet atomizer, diffusion dryer, stainless steel chamber, model head, scanning mobility particle sizer, vacuum pump, differential mobility analyzer, condensation particle counters
Researchers: Jing Wang
Researchers at University of Switzerland modified a snorkel mask with a 3D-printer adapter to test for protection against COVID-19 aerosols. Their experimental setup to test size-dependent particle penetration in the mask included a chamber containing the mask mounted on a dummy head. Sodium chloride particles were generated using a six-jet atomizer. The flow passed through a diffusion dryer (Kr-85 neutralizer) and injected into the stainless steel chamber with the model head and mask. Aerosol was mixed with filtered air at the chamber entrance. The airflow was pumped out of the mask at a setting equivalent to heavy breathing. The particle concentration was measured from a lower chamber via a sampling line that ran through the dummy head. A scanning mobility particle sizer was used to measure aerosol size distribution.
ASTM International is an international standards organization providing public access to ASTM standards used in production and testing of personal protective equipment including face masks, at no cost. The following standards and methods are available:
- ASTM F2299/F2299M-03(2017) Standard Test Method for Determining the Initial Efficiency of Materials Used in Medical Face Masks to Penetration by Particulates Using Latex Spheres
- ASTM F2101-19 Standard Test Method for Evaluating the Bacterial Filtration Efficiency (BFE) of Medical Face Mask Materials, Using a Biological Aerosol of Staphylococcus aureus
- ASTM F2100-19 Standard Specification for Performance of Materials Used in Medical Face Masks
- ASTM F1862/F1862M-17 Standard Test Method for Resistance of Medical Face Masks to Penetration by Synthetic Blood (Horizontal Projection of Fixed Volume at a Known Velocity)
- ASTM F1494-14 Standard Terminology Relating to Protective Clothing
European Standards include requirements are test methods EN14683:2019 Medical face masks and EN 149:2001 + A1:2009 Respiratory protective devices. In Europe, mask tests are performed by accredited labs and approved by the relevant National Accreditation Body.
The National Institute of Occupational Health and Safety (NIOSH) set the standard for filtration efficiency for N95 respirators.
The National Personal Protective Technology Laboratory (NPPTL) tested ten respirators from each of a variety of models using a modified version of the NIOSH Standard Test Procedure (STP) TEB-APR-STP-0059 where only particulate filter efficiency was assessed. The testing procedure involves challenging respirator filters with a sodium chloride aerosol at 25oC and 30% relative humidity after having been neutralized to the Boltzmann equilibrium state. An aerosol generator produces a constant stream of particles. A heater and neutralizer condition the aerosol by removing moisture and charges. An upstream photometer measures the particle concentration by determining the amount of light scattering and converting that to a voltage. Downstream of the filter a second photometer quantifies the fraction of particles that pass. The downstream particle concentration is divided by the upstream concentration. The flow rate and pressure drop are also measured.
Key equipment and materials:
- TSI Model 8130 or 8130A Automated Filter Tester or equivalent instrument
- The filter tester includes an aerosol generator, heater and neutralizer for conditioning the aerosol, photometers to measure particle concentration via light scattering
- Microbalance accurate to 0.0001 grams
- Type A/E glass fliters, 102 mm diameter, high efficiency filters with a 1 micron pore size
- Timer
- 2% sodium chloride solution in distilled water
- Temperature and humidity chamber
- Respirator filter holder
- Thermal printer or optional data acquisition system
- TSI, Green Line Paper, part number 813010
The China National Accreditation Service for Conformity Assessment (CNAS) has a list of accredited laboratories. Nice laboratories are certified for testing under EN 149 and/or EN 14683.
The Particle Science and Technology Laboratory at Southwest Research Institute (SwRI) in San Antonio, Texas includes mask testing as one of its services. The mask testing is done in their particle lab. SwRI's laboratory is ISO/IEC 17025-accredited by the American Association for Laboratory Accreditation to calibrate devices that measure particles as small as 10 nanometers in diameter.
