Data

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This page describes lab studies underway at UCI as well as preliminary results from our measurements. Our lab activities are closely aligned with our goal of providing scalable, croudsourced designs for face masks with carefully characterized filtration properties. We do not wish to repeat prior high-quality studies of filtration efficiencies of common household items. Some good examples of the latter include:

Testing the Efficacy of Homemade Masks: Would They Protect in an Influenza Pandemic? (August 2013), Anna Davis, et al., Peer-reviewed article in Disaster Medicine and Public Health Preparedness.

Simple Respiratory Protection—Evaluation of the Filtration Performance of Cloth Masks and Common Fabric Materials Against 20–1000 nm Size Particles (June 2010), Samy Rengasamy, et al., Peer-reviewed article from The Annals of Occupational Hygiene.

Another excellent resource is the Prof. Yang Wang’s spreadsheet on the filtration properties of common household items. Prof. Wang’s research group is actively testing materials and updates their spreadsheet daily.

This web page is organized accordingly:

Laboratory Setup and Test Procedure

The above figure shows the setup for mask and filter testing. Particles made of table salt are generated over a range of sizes from ~50 nm to 800 nm in diameter using an atomizer, which is a device that’s very similar to a medical nebulizer that some people use to treat respiratory ailments. These uncharged particles are introduced into our chamber along with a high flow of dry, clean air to achieve a relative humidity inside the chamber of ~20%. This assures that the test particles are dry. A fan inside the chamber stirs the air so insure that particles are well-mixed. For testing filter materials, we place the material in a filter holder and draw air through the filter at a flow rate that corresponds to a 10 cm/s flow velocity (also known as “face velocity”) incident upon the filter. This approximates the velocity with which air passes into a filter mask during normal breathing. For testing masks, we place a mannequin head inside the chamber. The mannequin is modified by drilling “breathing holes” in the nostrils, to which tubing is sealed. Note that a higher flow of air passes through the mannequin in order to maintain the 10 cm/s face velocity of sample particles. A pressure sensor measures the pressure drop across the filter or mask, which gives us a measure of how easy it is to breathe through the mask or filter material. Finally, we locate a tube inside the chamber to sample the chamber air.

Outside of the chamber, the two tubes (one with filtered aerosol and one with the chamber aerosol) enter a valve that directs either into an instrument called a scanning mobility particle sizer (SMPS). The SMPS measures particle concentration at 20 different particle sizes ranging from 50 to 800 nm in diameter. A complete measurement consists of the following steps:

• switch the valve to filtered aerosol and wait for the system to settle down
• measure filtered aerosol concentration as a function of size with SMPS
• switch the valve to chamber aerosol and wait for the system to settle down
• measure chamber aerosol concentration as a function of size with SMPS
• Take the ratio of the measured concentration of filtered aerosol to that of the chamber aerosol at each size. This gives us the transmission efficiency for aerosol. Typically we plot the data in terms of “filtration efficiency,” which is 1 – (transmission efficiency). A filtration efficiency of 1 means a perfect filter, and zero means no filtering at all.

Filter Media Data

Woven materials

In woven materials, individual fibers are combined to form threads and these threads are interlaced to form a fabric. Examples of common woven materials include bed linens, bandannas, and cloth napkins. Our interest in woven materials is not so much in their filtration properties, but rather their ability to support and seal actual filtration materials to the face. We provide data on the following materials (for a more comprehensive set of data please check out Yang Wang’s spreadsheet mentioned above). Note that the magnified views show a ruler with 1 mm increments.

Woven Material Filtration Efficiencies

The plot below shows the filtration efficiency for two layers of each of the above woven materials. The graph shows filtration efficiency vs. particle diameter. Filtration efficiency is defined above such that a value of “1” means that all particles are filtered away from the sample air. The filtration performance shown below is typical of other woven fabrics such as those found in bed linens and handkerchiefs.

Nonwoven materials

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Nonwoven materials are formed by combining long, individual fibers using chemical, mechanical, heat or solvent treatment. These materials are potentially ideal as filter media, since a multitude of small fibers, tangled together, create small pores to trap large particles by impaction while creating a high surface area to trap small particles by diffusion. We are primarily interested in nonwoven materials that feature both low resistance to flow and high filtration efficiencies.

Nonwoven material filtration efficiencies

As shown in the plot below, the filtration efficiencies of nonwoven materials vary greatly, presumably according to fiber thickness and overall density of the tangled fibers. It is important to consider the importance of the pressure drop across the material (dP). While the HEPA membrane from a vacuum bag performs well, the dP measured is the largest of all materials tested and therefore would not make a suitable face mask material.

Mask Data

Origami mask

The Origami mask was developed by the UCI Mask Team and is a 100% no-sew design made from easily obtained nonwoven materials. The version we tested uses WipAll model X80 disposable shop towels. As seen in the plot below, the mask is 70-80% efficient at removing 50 – 700 nm diameter particles. Importantly, this performance is achieved without any special effort made to seal the mask to the face.