 How do respirators and face masks prevent harmful particles from getting into your lungs? And what makes them more or less effective at doing the job they're designed for? Face masks and their higher performance cousins, respirators, are designed to prevent users from inhaling harmful airborne particles, whether these are dust particles or pathogen carrying droplets. They can also reduce the amount of harmful particles someone expels into the air around them, especially through coughs and sneezes. But the physics behind how they work is not necessarily that obvious. It's tempting to think of the filter material that respirators and face masks are made of as a microscopically fine sieve that physically blocks small particles from entering your mouth and nose. Unfortunately, the science isn't quite this simple, though. The materials that commercial respirators and face masks are made of usually consist of tangled mats of fine fibers. These create convoluted pathways that the air you inhale has to pass through, along with any particles and droplets being carried along by it. For particles big enough to be seen by the naked eye, these fibrous mats actually do behave a little like a sieve by physically blocking them from getting through, but this is pretty much where the comparison ends. This is actually good news, as a sieve that was fine enough to physically block the particles that you can't easily see, and the ones that are often the most dangerous, would be near impossible to breathe through. For many of these particles, it's their inability to follow the convoluted pathways through the filter material that leads to them being captured. To get a handle on this, imagine a car speeding along a winding road. The car fits easily on the road, but if it's going too fast, it won't be able to navigate the bends, and you just know that at some point it's going to crash. It's the same for small particles speeding through a filter. As the air twists and turns, the particles can't keep up, and ultimately, they end up crashing out and being caught. Because of their inertia, which just like the car depends on how massive they are and how fast they're going, they can't stay the course. But how about the smallest, lightest particles, those that can easily slip along the winding path the air takes? Amazingly, these are also captured efficiently by a well-designed filter material, but the way they're captured is very different. Because really small particles have so little mass, they dance around as they're bombarded by the air molecules surrounding them, and as they dance, they end up colliding with and being captured by the material the respirator or face mask is made of. Because of the way diffusion-based capture like this works, the finer the fibers are that the material is made of, the more effective they are at collecting these fine particles. So much so that some filter materials are made of a web of incredibly thin fibers that are held in place by a matter of much coarser ones. And then you have the middle-sized particles, the ones that are too small to be captured because of their inertia, but too large to be captured through diffusion. These particles are usually a little smaller than a hundredth the width of a human hair, and they are the hardest to remove. This is why respirator and face mask manufacturers test the efficiency of their products with particles in this size range, those particles that are just the right size to slip through and make it to the other side. In a well-designed respirator or face mask, layers of non-woven materials create sufficiently convoluted pathways that the vast majority of these middle-sized particles are removed from the air. An additional trick that some respirators and face masks use, including N95 respirators, is to incorporate filter material that has a permanent electrostatic charge. Because most airborne particles are also charged, this technique can be used to increase collection efficiency, especially at small and medium particle sizes. But collection efficiency is only half of the story here. As well as blocking airborne particles, respirators and face masks also need to allow the user to breathe freely, pack the fibers too tightly and not enough air can flow. On the other hand, pack them too loosely and most of the particles just pass straight through. To make matters worse, because airflow follows the path of least resistance, the smallest hole or gap in a respirator or face mask can lead to it losing most of its effectiveness. This is why it's so important to have a good seal between a respirator or face mask and your face. It's also why wearing one if you have facial hair is not that effective as it's impossible to get a good seal. This tendency for air to take the path of least resistance also means that some materials are much better than others at filtering out particles from the air. For instance, woven materials are not that good at it. The individual fibers within the threads are far too tightly packed together to allow air to flow between them. And the holes between the threads mean that a single sheet of woven textile quite literally does leak like a sieve. Of course, it's possible to create makeshift face masks with woven materials but they tend to be somewhat inefficient. Unless that is, you use multiple layers of material or you combine woven materials with an unwoven one like felt or even disposable floor dusters. The bottom line is that effective respirators and face masks work not because they act like sieves but because they use the physics of airborne particles to capture them while allowing the wearer to breathe freely. That is, as long as they don't leak. For more information, please do check out the links in the blog below and as always, stay safe.