About masks—are they really just a few layers of cloth?

This is the 3535th article of Da Yi Xiao Hu

When the epidemic just emerged, everyone began to hesitate whether to buy masks; as the epidemic gradually spread, every household began to follow the advice of experts to buy masks; and now, masks have almost become a necessity for everyone going out... Most of us have gone through the process of being unfamiliar with masks, and now have a general knowledge of the types and uses of masks; it is also very likely that we have had the experience of having nowhere to buy masks and looking for alternatives everywhere.

The epidemic is not over yet, and masks may accompany our daily lives for a long time, so let us take a deeper look at all aspects of masks!

1. The mask itself

First, let’s take medical surgical masks as an example.

Medical surgical masks are divided into three layers: the outermost and innermost layers are spunbond non-woven fabrics, and the middle layer is meltblown cotton. Although you may not be familiar with these two materials, please note that these two "cloths" are not made of cotton, but plastic polypropylene (PP).

Although the raw materials are the same, the functions of these two "cloths" are different: the inner and outer layers of non-woven fabrics can only block droplets, while the middle layer of meltblown cotton is the one that actually filters the bacteria. The well-known N95 mask can protect people from viruses because of this layer of meltblown cotton.

I believe everyone has heard of these basic knowledge during the epidemic. Next, let us take a deeper look at the meltblown cotton that plays a decisive role, as well as the exquisite craftsmanship behind the mask, which determines the characteristics of the mask.

1. What is so special about meltblown cotton? [1]

Compared with spunbond non-woven fabrics with the same raw materials, the filtering effect of meltblown cotton mainly comes from its process characteristics. Simply put: the fibers of medical meltblown cotton are very fine, so the filtering efficiency is very high.

The diameter of the finished meltblown cotton fiber is approximately between 0.5-10 microns, while the diameter of the spunbond layer fiber is around 20 microns.

The fiber diameter of meltblown cotton is extremely small, which means that meltblown cotton has a large surface area and can absorb various particles. At the same time, it is relatively breathable, making it the only choice for making mask filters. But at the same time, it is easy to imagine that the production process of this material is relatively complicated. In fact, meltblown technology is currently the only technology used for large-scale production of micron-level fibers.

The machine generates a high-speed hot air flow, which sprays the molten polypropylene from a tiny nozzle like a spray, and then gathers it on a roller or flat plate to form a non-woven fabric.

However, even though the manufacturing process of meltblown cotton is very sophisticated, the filtration efficiency of meltblown cotton itself is only about 25%. So how does the filtration efficiency of N95 masks reach 95%? The answer is electrostatic charging treatment.

(ii) Electrostatic charging treatment[1]

The term "electrostatic treatment" may be unfamiliar to many people, but the concept is relatively easy to understand. We have all come into contact with static electricity in our daily lives. When our hair has static electricity, it is easy to stick to walls, balloons, etc., and it is also easy to stick to some small pieces of paper. The static electricity used here is the same principle.

In industry, engineers use electric fields to charge tiny particles, which are then attracted by electric grids. When dealing with smog, some regions have also adopted the practice of using static electricity to attract tiny particles.

As for masks, Cai Bingyi invented a method to charge plastic, so that the meltblown cloth can be charged with static electricity and permanently charged, becoming what we call "electret". This treatment is also what we call "electret treatment".

After this treatment, this layer of meltblown cloth can also attract small particles with a diameter of about 100 nanometers, such as the new coronavirus.

(III) Summary of characteristics

Based on the structure, materials, technology, etc. of the masks we have learned above, we can summarize the three characteristics of masks:

1. Waterproof (the inner and outer layers of spunbond non-woven fabrics have good waterproof effect)

2. Non-flammable (the raw material polypropylene melts easily when exposed to fire, but is not easy to catch fire)

3. There is static electricity (the inner layer of meltblown cloth has been treated with electrostatic electret)

We can have a deeper understanding of masks based on these three characteristics, and we can also make judgments about the masks we have already bought home.

2. How to judge whether the mask is qualified

1. Conduct basic inspections on masks that have been received

Based on the characteristics of masks we know, there are roughly three basic and simple inspection methods.

1. Water spraying: After pouring clean water on the mask, it will not get soaked quickly, or even leak. (This mainly depends on the non-woven fabric, so it can only prove that the non-woven fabric on the outer layer of the mask is qualified.)

2. Fire: Use a lighter flame to pass over the edge of the mask, it will not burn. (But please note that meltblown fabrics and non-woven fabrics are both made of polypropylene, so this method cannot be used to distinguish between ordinary non-woven masks and medical surgical masks.)

3. Adsorption: Put the mask close to the scraps of paper, the small pieces of paper will be adsorbed on the mask, and you may feel the mask sticking to your hand when you hold it in your hand. (This method is for testing meltblown cotton, which is the most important point.)

(ii) Inquiries before purchase[2]

We can select "Domestic Medical Device Products" or "Imported Medical Device Products" on the website of the State Food and Drug Administration, enter the relevant information, and confirm that they are regular products.

III. Reuse and disinfection of N95 masks in ordinary households

Although the price of masks is gradually returning to normal levels as the epidemic situation gradually eases, the issues of disinfection and reuse of masks still receive a lot of attention.

(I) Situations where masks must be discarded[3]

1. Currently, it is not recommended to reuse non-N95 masks after disinfection because there is not enough research support for relevant disinfection.

2. N95 masks need to be discarded when it becomes difficult to breathe.

3. When the N95 mask is damaged, it needs to be discarded.

4. When the N95 mask is contaminated by human body fluids, it needs to be discarded.

5. When the N95 mask does not fit the face well, it needs to be discarded.

2. Reuse and disinfection of N95 masks

There is currently no unified and clear best method for mask disinfection during the new coronavirus epidemic, but based on current research, some experts have given some suggestions.

For ordinary families, many feasible methods currently developed are impossible to implement. At the same time, many incorrect disinfection methods or misunderstandings should also attract our attention.

1. Drying N95 masks in the sun will not sterilize them.[4]

As of April 21, 2020, studies have proven that UV-C in sunlight has a certain inactivation effect on the new coronavirus, but the sunlight that actually reaches the ground after passing through the atmosphere does not contain UV-C. Although the UV-B contained in it can have a disinfecting effect to a certain extent, it is too weak to disinfect masks. Therefore, sunlight exposure cannot achieve the effect of disinfecting masks.

2. It is feasible to leave the N95 mask in a suitable environment for more than 7 days before reuse[5]

This is the simplest and lowest-cost disinfection method, and it is also the method currently recommended by the Centers for Disease Control. Storing N95 masks separately in a clean, breathable container at room temperature for 7 days before use can greatly reduce the risk of contact with the new coronavirus on the N95 mask.

But at the same time, we must pay attention to hand hygiene, pay attention to limited reuse, and be sure to discard the mask immediately when it is damaged or contaminated.

3. We cannot use medical alcohol or any water-based method to clean or disinfect N95 masks.[6]

Experiments have confirmed that 75% alcohol immersion, 2% sodium hypochlorite solution spraying and water vapor treatment will greatly reduce the filtration efficiency of meltblown cloth, thereby affecting the function of N95 masks. Therefore, simply put, do not wet the mask when disinfecting it.

4. It is not recommended to simply use hot air (dry heat) or moist heat conditions to disinfect N95 masks [3]

Although some current research data show that a certain combination of high temperature and humidity may have an effect on the purification of N95 masks, there is not enough experimental work to support this view, and the mechanism of action of humidity in inactivating viruses is still uncertain. At the same time, this disinfection method cannot kill many other pathogens, such as bacterial spores. In short, whether it is dry heat or wet heat disinfection methods, there is a lack of more in-depth research and verification, so it is not recommended to rely entirely on related methods for disinfection.

In summary, because research on the new coronavirus is still ongoing, many methods are promising but have not yet been concluded.

At present, our most recommended method for making N95 reusable is to leave it alone for more than 7 days. This method is also easy to achieve in ordinary households. It is a very good choice for those of us who will need masks for a long time in the future!

Finally, although the domestic epidemic is no longer so serious, wearing masks is still essential for us. Such a "delicate" mask is an indispensable layer of protection for ourselves and others! Let's take a look at the situation of droplets splashing when coughing without a mask. Wear a mask when going out, don't relax!

References:

[1]https://mp.weixin.qq.com/s/BiWjC9rowTCr7yU7WTsZ1A

[2]http://app1.sfda.gov.cn/datasearchcnda/face3/dir.html?type=ylqx&WxUg5ztDmi=1588590124906

[3]https://www.n95decon.org/faq

[4]https://www.n95decon.org/uvc

[5]https://www.n95decon.org/time

[6]https://www.medrxiv.org/content/10.1101/2020.04.01.20050443v2?utm_source=wechat_session&utm_medium=social&utm_oi=1004449602778562560

Author: Student of Shanghai Jiao Tong University School of Medicine

Xu Mengmeng

Student ID: 319710910024