I see that my last post to the blog was three months ago … where has the time gone?! During the past three months we have been busy making additional refinements to the Origami Mask. The new Origami Mask has several advantages over most cloth masks:
- You can make it out of any 9” x 9” piece of material (or 9″ x 18″ if you want to double the material). This has huge advantages in that people can select the material according to the application. If I am running or walking outside, I will choose a highly breathable material. If I am going to a crowded place, I will choose a material that has better filtering capabilities.
- It’s easy and cheap to make. All you need is the material, a stapler, some wire bands from coffee bags, and elastic strap or shoestring. No sewing skills are needed and there are only a few folds that need to be made.
- The design keeps the filter media off the face, which makes it easier to speak without sounding muffled.
- The design seals better than just about any other design that I have tested. This means no fogging of glasses, but more importantly it means that the filtration abilities are controlled by the filter media and not by leakage (as anyone who wears glasses can attest to, many masks leak).
Our mask research thus far has focused on masks as respiratory protection. That means that the masks we develop and study not only protect others from potential virus-containing droplets that you emit when you talk, cough, or sneeze, but also protect you from those emitted by others. It’s important to make this distinction because exhaled droplets are often larger than particles and droplets that we inhale. When we study mask performance for smaller, inhaled droplets and particles we are really looking at the worst-case scenario for mask performance. This is of course critically important for health care workers and essential workers, and that is why we do this research. However, for the rest of us who are not exposed to potential virus-containing aerosol particles on a day-to-day basis, just about any mask will effectively remove larger exhaled droplets and provide the effective controls needed to reduce the spread of viruses.
Looking back at my last post, I made some mysterious references to the three bears and face masks and I’m overdue in explaining myself. First, a bit of background is needed. A face mask has two very important properties that we can measure. The first is how good of a filter it is. Filters don’t have the same filtration efficiency for all particle sizes so we often list their performance for particles that are 300 nm in diameter because this is considered the “most penetrating particle size,” or MPPS. The second property is how easy it is to breathe through the mask. We measure this as the pressure drop, the difference between the outside and inside of the mask when air is drawn through it at a rate that is similar to a normal breath. The lower this pressure drop, the easier it is to breathe. For every mask that we test, we measure the filtration efficiency and the pressure drop using a test mannequin (see this link to learn how we do this). All masks leak a little where the mask meets the face and we can get a sense for how much a mask leaks by first measuring these properties for a mask that is fit directly on the mannequin head and then measuring again with the mask sealed to the mannequin head (we use duct tape). If the mask leaks, the sealed mask would have a higher efficiency and a larger pressure drop compared to the unsealed mask. One more point that I’ll make is that often the filtration efficiency and pressure drop of a mask are inter-related: a mask that is a good particle filter will have a higher pressure drop and is therefore difficult to breathe through. We can see this if we make a plot of the pressure drop versus the filtration efficiency at the MPPS for each mask we tested. When we plotted the data for a variety of commonly used masks and for the Origami Mask with different filter materials, what we find is that face masks fall into three categories and, inspired by the story of Goldilocks and the three bears, one of these is too much, one is too little and one is just right.
Let’s start with the red category of masks that are located in the upper right corner of the plot, meaning that they exhibit high filtration efficiencies and large pressure drops. Included in this category are N95 and KN95 respirators as well as surgical masks. Medical professionals know how to properly fit these masks to their faces so theirs will perform at the upper right corner of this region in the graph. Unfortunately for the rest of us, too often these masks do not fit properly. For instance, many KN95 masks use ear loops that are incapable of providing the tight seal that is required to eliminate leaks. This is why the actual size of the red region in the plot is so huge: while a medical professional can have >95% efficiency with a 100 Pa pressure drop that makes it difficult to breathe, many of us who don these masks experience efficiencies as low as 25% because of leaks that bring the pressure drop to about 40 Pa (well at least it’s easier to breathe!). Masks in the red category are saving the lives of health care workers and first responders around the world, but for the rest of us it’s just TOO MUCH … too high of pressure drop that makes leaks too likely and leads to lower overall performance to the non-professional.
Now let’s talk about the blue category of masks that are in the lower left corner of the plot, meaning they have the lowest filtration efficiencies and lowest pressure drops. This category includes the fabric masks that most people have been wearing for the past 3-4 months. Masks such as these are excellent at removing large exhaled droplets that are associated with talking, coughing, and sneezing, but as respiratory protection these masks are TOO LITTLE … filtration efficiencies that are too small, albeit with pressure drops that makes these masks comfortable to wear.
Like the story of Goldilocks and the three bears, there is a third option that’s JUST RIGHT. This is a category of masks shown in green in the plot and characterized by high filtration efficiencies but low pressure drops. The highest efficiencies may not be as high as N95 respirators, but in many cases can exceed 90%. The low pressure drop that is characteristic of these masks means that they are less likely to leak compared to the red category. It also means that these masks are easier to breathe through, so the likelihood that they will be worn properly and for long periods (if necessary) goes up. Our current mask development efforts are centered on finding low-cost, easily sourced materials for creating masks in this category. At this point, the best-performing mask in this category is a hybrid design that is constructed by sandwiching a high-efficiency air filter between two thin sheets of non-woven polypropylene fabric. This results in a mask that with filtration efficiencies that rival those of N95 masks but with a pressure drop of about 15 Pa – lower than even a cloth mask.
We are currently preparing a manuscript describing the Origami Mask for publication in a scientific journal. In the meantime, I will update the content of this website to reflect our most recent findings.
–Jim
