How effective are masks? Depends on the material it is made of

Worldwide, the covid-19 epidemic is spreading.

SARS CoV-2 virus did not magically disappear, and in many places, this situation rose again, and it seems that the new reality of closing and resident orders will last longer than most cases. We initially expected.

Given this, according to the CDC guidelines, I recently need to wear reusable masks when shopping online, which I can wear when I have to go out in public.

When shopping online, there are many choices for masks, which have many colors, patterns, shapes, fit, and materials (it is reported that even glass is made).

For functional masks, the gold standard is N95, so named because its nonwoven polypropylene fiber filters out an incredible 95 percent of air-borne substances, including viruses, from the air we breathe.

The reason why N95 gas masks are so effective and why they were so revolutionary at first is their comfort (enough comfort to filter out almost all the air you breathe), but they are breathable. Surgical masks are usually made of the same material, but they are not fit well, so they are usually less efficient than N95 masks.

The nonwoven polypropylene fiber of this material forms a tangled fiber mesh, which can effectively capture even small particles. This property is called filtration efficiency, but it contains large pores, which makes the wearer breathe easily.

The air permeability coefficient is measured by pressure drop, which is another important factor in the effective mask. If breathing creates a large pressure difference between the sides of the mask material, the pressure drop is high and breathing is more difficult.

But since the N95 respirator is designed for medical personnel, current guidelines indicate that people like me usually wear cloth-based masks.

Back to online shopping, although many individuals, companies, and enterprises offer masks for sale, I notice that a large part of these products provides little or no information about their materials and layers. , or other important factors can help me determine whether the mask is really effective and comfortable to wear.

Many websites are selling masks made of lightweight cotton, which are relatively inefficient despite their excellent air permeability. They are always better than none, but studies seem to show that they only block about 20% of the small particles (the results here vary greatly depending on the type of material, thickness, and test procedure).

Studies show that the material is important to consider for effective masks because not all materials are created in equal measure - the high filtration efficiency materials described in this paper, smaller pore sizes usually mean that they have a high-pressure drop and are therefore difficult to breathe through.

Therefore, it is the best combination of filtration efficiency and pressure drop to make the mask efficient and comfortable. So I'm glad to find a study published in nano express in early June that provides more complete data on the properties of a range of commonly used materials.

Using an improved version of the test protocol used to certify N95 masks, * the performance of synthetic and natural household materials (cotton, nylon, polyester, silk, and paper (cellulose)) was compared with that of non-woven polypropylene. N95 materials and surgical masks.

*The researchers used NIOSH protocol to measure the size of an aerosol salt (NaCl) particle with a median diameter of 0.075 ± 0.02 μ m-sars-cov-2 virus is about 0.120 μ m, but the aerosol particles containing the virus that may be inhaled are larger.

The team measured the filter efficiency and pressure drop of various materials and combined these methods into a method called filtration quality factor - a scoring system used to evaluate the efficiency and permeability of each material as a mask.

The team's relatively strict testing standards indicate that almost all tested materials may be surprisingly inefficient compared to N95 masks. Even masks from two different manufacturers scored relatively low.

Material Initial filtration efficiency Initial pressure drop (Pa) Filter quality factor (k/Pa)
Meltblown polypropylene (N95) 95.94% 9.0 162.7
Meltblown polypropylene (surgical mask 1) 33.06% 34.3 5.0
Meltblown polypropylene (surgical mask 2) 18.81% 16.3 5.5
Spunbond polypropylene 6.15% 1.6 16.9
Cotton, pillow cover 5.04% 4.5 5.4
Cotton, T-shirt 21.62% 14.5 7.4
Cotton, sweater 25.88% 17.0 7.6
Polyester baby wrap 17.50% 12.3 6.8
Silk napkin 4.77% 7.3 2.8
Nylon exercise pants 23.33% 244.0 0.4
Paper towel 10.41% 11.0 4.3
Tissue paper 20.2% 19.0 5.1
Copy paper 99.85% 1883.6 1.5


Although most of the test materials are not comparable to the filtering quality of N95 masks, with the exception of copy paper, although the copy paper is very efficient, the pressure drop is very high, and it can not be well maintained in the humid environment caused by breathing - the results show that many common household materials can be made into masks, even if the efficiency is not as good as surgical masks.

When making your own mask, stacking a variety of different materials together may provide the most effective solution.