Talking about wearing masks in prevention and control of popular science

There are a variety of masks on the market. It should be said that in an emergency, which kind of mask is better than none, but not all of them are qualified for epidemic prevention. Let’s focus on two mainstream masks.

N95, which is a famous mask in the first World War after the fight against SARS in 2003, is made of multi-layer non-woven fabric with polyurethane and polypropylene as the substrate. As the name suggests, non-woven fabrics are not woven from warp and weft yarns, but are made by special processes, in which fibers are randomly arranged and tangled. N represents non oily particulate matter, such as cooking fume is not included in the protection. 95 means that the filtration efficiency of 0.3 micron particles is not less than 95%. It is necessary to examine the special significance of 0.3 μ M. according to the discovery made by American scientist Langmuir when designing the anti radiation dust filter for Manhattan Project, 0.3 μ m is the particle size with the most difficult filtration and strongest penetration. Particles smaller than 0.3 μ m will deviate from the air streamline due to their light weight and weak inertial force, and diffuse in a Brownian motion mode, so as to be more tolerant It is easy to be adsorbed and captured.

novel coronavirus has an average diameter of 0.1 microns, just under the N95 mask. What’s more, viruses are not “streaking” alone in the air, they are “riding” patients’ droplets around. The smallest droplet diameter is 1-5 microns, so it can not break through the dense net of N95 mask.

it is worth mentioning separately that the core material of N95 mask is melt blown layer. The concept of meltblown originated from volcanic eruption. According to the different Reynolds number and Weber number of magma, Bailey hair and Bailey tears are formed in the air. The silk reeling of silkworms and spiders in the life world can also be regarded as the “melt blown non-woven fabrics” of nature. When polypropylene, the raw material of N95 mask, escapes from the spinneret hole in the molten state, forms non-woven fabric on the screen curtain and roller through cooling and drawing, and then becomes electret after high-voltage electric field treatment, which carries with it long-lasting static electricity. When particles in the air pass through the mask, they are absorbed by static electricity and fall into the net one after another, which greatly improves the filtration capacity of N95 mask.

N95 mask looks so simple, but it requires correct and standard wearing methods. Whether it’s bowl shaped or folded, first buckle it on your mouth and nose with a clean hand, stretch the rubber bands at the bottom and top in turn, and cover the neck and head. Then, press the aluminum nose clip with the index fingers of both hands from inside to outside to make it consistent with the undulation of the face, and fine adjust the edge to make the mask fit perfectly with the face to achieve the airtight state without leakage. When we breathe and speak, we can see the corresponding small fluctuation of the mask, which indicates that we are wearing it correctly. N95 mask is a proven “loyal guard” for medical staff who are exposed to the virus and have close contact with patients.

but we must make an important reminder that although N95 masks are efficient and reliable, they are not suitable for all people at all times. Let’s start with the principle of breathing.

our lungs are bellows that pull for life. When the diaphragm contracts and moves down, the intercostal muscles contract and lift the ribs up, bringing about the expansion of the chest. According to Boyle’s principle, when the air pressure of the lung drops below atmospheric pressure, external air will flow into the lung, and then inspiration occurs; on the contrary, when the diaphragm and intercostal muscles relax, our lung tissue will recover to its original size by elasticity, and “breathe out” the air in the lung passively. This cycle ensures the exchange of oxygen and carbon dioxide.

as a physical barrier on the respiratory channel, respirator will inevitably increase respiratory resistance. Studies have shown that when wearing N95 masks, inspiratory and expiratory resistance increases by 126% and 122%, respectively. This means that it is difficult to “breathe freely” and that we need to “breathe harder” to maintain normal pressure gradient and air flow. The data showed that after wearing N95 mask for 4 hours, the air exchange volume decreased by 37%. The increased partial pressure of carbon dioxide and the decrease of blood oxygen saturation can also cause fatigue, dizziness, headache and heat stress, which is more likely to cause dyspnea under the condition of physical activity. Therefore, for people with weak cardiopulmonary function, N95 mask should not be used carelessly.

we see a N95 mask with an air window, which is a “back door” for direct discharge of exhaled air without filtration. If we only consider the prevention of haze and dust, it would be a clever design to reduce the burden of breathing. However, in the prevention and control of the epidemic, this kind of mask that makes the virus “only out of the door” has no protective effect on others, which is inevitably too “selfish” and highly despised.

now we want to talk about another “main type of mask”, which is a surgical mask with low cost, strong comfort and high popularity. It consists of three layers of non-woven fabric. The outer layer is hydrophobic, which can effectively prevent the infiltration of water and body fluids. The middle layer is the melt blown filter layer, which is the main barrier to intercept particles in the air. The inner layer is soft, absorbent and comfortable to wear. Surgical masks must be non combustible. We often use water and fire to check whether the mask material is true or not and whether it is of high quality.

Although surgical masks do not meet the protection standards of N95 masks, they still have a good barrier effect on droplets and micro particles. It is true that the fit between the surgical mask and the face is relatively loose, and the air will enter the respiratory tract through the gap with the least resistance. However, those droplets are difficult to turn around, and they will “run aground” and “anchor” at various turning points, leaving them on the mask fiber and the face skin. Therefore, surgical masks should be adequate for daily defense in public places. Besides, in addition to the benefits of breathing easily and being cheap and light, we should also leave the N95 mask to those who really need it.

correct wearing is the key to the effectiveness of surgical masks. Wash your hands, hold the left and right ties and hang them on your ears. Let the dark side of the mask face outwards, and the side with the aluminum nose clip should face up. Do not reverse inside and outside and upside down. Press the nose clip in place with two index fingers, stretch the bottom to cover the chin, and half of the face is protected by the mask. It is also an important function of the mask to prevent our hands from accidentally touching the face and causing pollution.

as we breathe, a large amount of dust, droplets and microorganisms will adhere and chime in the fiber, so the mask becomes a place to hide dirt, so it must be replaced in time. The general use time of surgical masks is 4 hours, and then discard the old easy to new. The changed masks should not be thrown at random. They should be disinfected by spraying alcohol and then put into the garbage bag.

almost any kind of epidemic prevention guidelines do not advocate the reuse of masks. However, in the case of a shortage of resources, ordinary people will throw away a mask when they go out to make a turn in their home life, and it is inevitable that they will abandon it and be difficult to continue. Therefore, masks that have been used for a short period of time in daily environment should be allowed to be reused. It is best to go through the year of sun, oven, ozone or ultraviolet rays. Spring is destined to be an unforgettable time. Maybe there is only one mask between our safety and danger. The cultural traditions of many European and American countries are said to be incompatible with wearing masks. It is recognized that the “standard etiquette” is to lower the head and raise the elbow when coughing or sneezing. At this critical juncture of the new coronavirus pandemic, despite the huge “cultural reluctance”, they all chose “personal freedom to give way to the public interest”, changed customs and habits, identified opportunities, formulated laws and regulations, accepted masks, and encouraged people to sew with cotton cloth at home. A new study shows that wearing a mask can reduce the risk of infection from 17.4% to 3.1%. “When we both put on masks, I protected you and you protected me. We are all safe for each other, “called on Czech actress konova, demonstrating the responsibility, obligation and virtue of wearing masks in the epidemic. Fashion brands from all over the world have also heard the wind, looking for huge business opportunities and art space from masks.

as the highest form of life on earth, human beings have waged the most tragic war with the virus of the lowest life form. We need to reflect on too many things, including the opinions and public opinions on wearing masks. The snowy mask is the most familiar and unfamiliar landscape in our world. We’ll take off our masks and smile again. But in the years to come, masks will undoubtedly become the daily necessities of every family, deeply embedded in human culture, and have a profound and lasting impact on the global health pattern. 08/16/2020