Filtration is a process of removing particulate matter from water by forcing the water through a porous media. This porous media can be natural, in the case of sand, gravel and clay, or it can be a membrane wall made of various materials. Sometimes, large particles are settled before filtration; this is called sedimentation. For information on sedimentation and filtration, in general, see the Conventional Water Treatment: Coagulation and Filtration fact sheet.
The size of materials that can be removed during filtration depends upon the size of the pores of the filter. The chart below summarizes the various separation processes relative to common materials that would be filtered out through each process. Particle filtration refers to conventional media filtration, while the other types are membrane filtrations.
Size of Materials That Are Removed By Various Separation Processes
WHAT IS ULTRAFILTRATION?
An ultrafiltration filter has a pore size around 0.01 micron. A microfiltration filter has a pore size around 0.1 micron, so when water undergoes microfiltration, many microorganisms are removed, but viruses remain in the water. Ultrafiltration would remove these larger particles, and may remove some viruses. Neither microfiltration nor ultrafiltration can remove dissolved substances unless they are first adsorbed (with activated carbon) or coagulated (with alum or iron salts).
WHAT IS NANOFILTRATION?
A nanofiltration filter has a pore size around 0.001 micron. Nanofiltration removes most organic molecules, nearly all viruses, most of the natural organic matter and a range of salts. Nanofiltration removes divalent ions, which make water hard, so nanofiltration is often used to soften hard water.
WHAT IS REVERSE OSMOSIS?
Reverse osmosis filters have a pore size around 0.0001 micron. After water passes through a reverse osmosis filter, it is essentially pure water. In addition to removing all organic molecules and viruses, reverse osmosis also removes most minerals that are present in the water. Reverse osmosis removes monovalent ions, which means that it desalinates the water. To understand how reverse osmosis works, it is helpful to understand osmosis.
病毒刚发现的时候称为“滤过性病毒”，意思是它可以通过任何滤网。无论多小的滤网，哪怕空气和水都通不过，也无法阻止病毒。某些病毒可以穿过滴水不漏的素烧瓷和陶罐。任何口罩只要戴上后还能呼吸不会憋死，就过滤不了病毒。
N95口罩能过滤95%的微小颗粒，可病毒能用颗粒形容吗? 进一步阅读可知那所谓微小颗粒是0.3微米左右，而大部分病毒是0. 1微米。而且病毒也不会被静电吸附。
如果将病毒放大到人的尺寸，N95口罩的空隙就相当普通房子的大门，人可以自由穿过。
所以论证口罩不能过滤病毒，不需要引用专家的话，只用“滤过性”三字即可。
但口罩有用吗？当然有用。因为现实中没有单独存在的病毒，它们都是附在某种物体上。单独讨论过滤病毒毫无意义。
呼吸传染病的病毒靠飞沫和气溶胶传播（当然还包括接触，所以要常洗手）。飞沫是能看到的颗粒比如打喷嚏说话时飞出的口水。气溶胶是看不到的更小颗粒比如呼气时的气流。夸张的说飞沫相当小雨，气溶胶相当雾霾。
雾霾是PM2. 5，即2.5微米。0. 3微米级的N95口罩挡住气溶胶没问题。一般医用口罩可以防飞沫。由于那些微粒可以被静电吸附，有静电效果的多层口罩效果更好。挡住了飞沫气溶胶微粒就相当于挡住了病毒。
所以口罩能不能防病毒，争论一年也说不清楚。“过滤”的定义不明确。
口罩过滤不了病毒? 正确。病毒是滤过性，素烧瓷都挡不住遑论口罩。
口罩能过滤病毒？正确。病毒是附在飞沫等微小颗粒上的，挡住颗粒就是挡住了病毒。
回到那个比喻，如果病毒放大到人，N95口罩就相当房子的门，只能挡住大象挡不住人。但如果人没有行动能力，出门遛弯必须靠骑大象代步，那么挡住大象就等于限制了人的活动。
神圣化了，或跟孙悟空一样。
corv 病毒，单体20nm-200nm，但droplet 或agglomerates 就在um 量级，就仅仅考虑口罩是个physical barrier，一层口罩就能挡住一部分virus，两层就更有效果。
都会agglomerates，或吸附在空气中的各样粉尘上而传播。
病毒可以通过飞沫传播，有些又可以通过气溶胶传播。是不是单个病毒也能在空中飞舞传播呢？不知道。
不过根据常识，飞沫携带的病毒量是最大的，传播的距离也最短
https://finance.sina.com.cn/roll/2020-08-27/doc-iivhuipp0887281.shtml
不但可以防止新冠传染，还可以防止流感传染。
因为新冠期间，中国人民全民带上了口罩，记者就发现有七家主营相关呼吸类治疗药物企业2020年上半年营业收入下滑幅度在10%以上。其中，主营特一止咳宝片产品的特一药业，目前营收下滑幅度最大，出现同比近四成的下滑幅度，公司归属于上市公司股东的净利润下滑七成左右。
由此可见口罩的威力不小啊。
一般都是坐火车，飞机，轮船的
滤过性病毒的称呼，是当初用来过滤细菌的滤膜，不能过滤这种微生物，所以该微生物被称为滤过性病毒。即：细菌不能通过这种滤膜，但病毒可以。
培养过细胞的人都知道这种滤膜，用来过滤血清或一些培养液，这类液体不能用高温消毒，只能用滤膜，达到无菌目的。
这是肯定的。
无论多小的滤网，哪怕空气和水都通不过，也无法阻止病毒。
陈薇院士开发的那个疫苗。莫非多伦多和你那块儿政策不一样？
，室友先阳性，然后，4 人全部是阳性。我分析，就是他把病毒弄回来的。
我猜，二月23 号开始，全国统一政策了。
空气中不是孤立的存在，应考虑附着于液体小颗粒的效应。
WHAT IS FILTRATION?
Filtration is a process of removing particulate matter from water by forcing the water through a porous media. This porous media can be natural, in the case of sand, gravel and clay, or it can be a membrane wall made of various materials. Sometimes, large particles are settled before filtration; this is called sedimentation. For information on sedimentation and filtration, in general, see the Conventional Water Treatment: Coagulation and Filtration fact sheet.
The size of materials that can be removed during filtration depends upon the size of the pores of the filter. The chart below summarizes the various separation processes relative to common materials that would be filtered out through each process. Particle filtration refers to conventional media filtration, while the other types are membrane filtrations.
Size of Materials That Are Removed By Various Separation Processes
WHAT IS ULTRAFILTRATION?An ultrafiltration filter has a pore size around 0.01 micron. A microfiltration filter has a pore size around 0.1 micron, so when water undergoes microfiltration, many microorganisms are removed, but viruses remain in the water. Ultrafiltration would remove these larger particles, and may remove some viruses. Neither microfiltration nor ultrafiltration can remove dissolved substances unless they are first adsorbed (with activated carbon) or coagulated (with alum or iron salts).
WHAT IS NANOFILTRATION?A nanofiltration filter has a pore size around 0.001 micron. Nanofiltration removes most organic molecules, nearly all viruses, most of the natural organic matter and a range of salts. Nanofiltration removes divalent ions, which make water hard, so nanofiltration is often used to soften hard water.
WHAT IS REVERSE OSMOSIS?Reverse osmosis filters have a pore size around 0.0001 micron. After water passes through a reverse osmosis filter, it is essentially pure water. In addition to removing all organic molecules and viruses, reverse osmosis also removes most minerals that are present in the water. Reverse osmosis removes monovalent ions, which means that it desalinates the water. To understand how reverse osmosis works, it is helpful to understand osmosis.
https://www.safewater.org/fact-sheets-1/2017/1/23/ultrafiltrationnanoandro
的时候不可能避免.....之前我看见有人露着头脸穿防护服上飞机觉得可笑至极，那穿法什么也防不了，就跟普通衣服一样
我们这里医学专家们也讨论过口罩的问题，但反对使用的焦点不是口罩本身的（物理）作用，而是口罩的不正确使用带来的负面影响，比如摘口罩时在脸上乱摸、再比如带了口罩后到处乱跑。专家们也对口罩的抗疫角色有些争论，这个类似打疫苗，要有一定的规模才能有效，而当时口罩短缺，需要留给医务人员。
很少逛这坛，也就是热点看到才来看看，发现这里一点基本常识都可以争半天，是不是都是老留父母们都在这里唠嗑啊？
是有意把我当成她的吗？
我一直在此坛出没，偶有一天被这里的一个贴带到了那个白菜坛？遇到了你和她，说了几句话。。。仅此而已。后来发现是误入奇葩坛，就没再去了。谢谢你还记得我，也向你问好！
打阴谋论，打造谣传谣的毫无底线的邪教组织，见一个打一个，如果时间允许的话
我去那里遇到你们俩的时候还没有开始川普拜登之争。
支持你打阴谋论，打击无底线的邪恶。