【HEPA的概念，有效防疫口罩應該如何設計】
By: Health Ecoforum
Tags: Masks, Covid 19, Virus, Health

HEPA是甚麼樣的概念呢?

What is HEPA?

HEPA全名是High-Efficiency Particulate Air Filter，屬於一種高效空氣過濾濾網，可以用濾紙，不織布，或是玻璃纖維的方式去製成。依據美國能源部的定義，能夠把超過99.7%，大小為0.3微米的懸浮微粒擋下來的濾網，就是HEPA濾網。它是由非常細小的纖維交織而成，可以非常有效的捕捉微粒。

HEPA stands for High-Efficiency Particulate Air Filter, which can be made of filter paper, non-woven fabric, or glass fiber. According to the definition of the U.S. Department of Energy, a filter that can block more than 99.7% of suspended particles with a size of 0.3 microns is a HEPA filter. It is interwoven by very fine fibers, which can capture particles very effectively.

HEPA過濾空氣並不完全是篩子篩雜質一般，用小的網眼去直接過濾這些

粒子。實際上，HEPA濾網是靠纖維的吸附作用來「粘」住PM2.5以下的

微塵。重點是，如何做到高效過濾0.3微米的粒子，卻還是可以用較大較為

透氣的纖維空隙去阻隔比空隙還小的微粒呢?

The way how HEPA filters air is not like sieving particles, in fact, HEPA filters rely on the adsorption of fiber to hold the fine dust below PM2.5. The point is, how to efficiently filter 0.3 micron particles, but still use a fiber that has a bigger and more breathable mesh to block particles that are smaller than the mesh?

我們可以考慮以下的兩種效益：

We can consider the following two benefits:

篩選的效率：基本上顆粒小於網孔的可以通過，大於網孔的就會被阻攔。適用於大粒子的過濾(在空氣介質中的粒子直徑如果是>3μm，也就是細菌的氣溶膠，有時可以大到20μm微粒的過濾)。

Filtering efficiency: Basically, particles smaller than the mesh can pass through, and those larger ones will be blocked. It is suitable for the filtration of large particles (if the diameter of the particle in the air dielectric is more than 3μm, that is consider as bacterial aerosol, sometimes it can be as large as 20μm). 布朗運動的效率：如果粒子夠小(在空氣介質中的粒子直徑如果是<1μm，也就是適用於病毒氣溶膠的大小)的狀態下，我們就無法用粒子的牛頓力學(巨觀的慣性)運動來做考量。這個狀況下，我們必須以氣體動力學裡面的分子碰撞的微觀模型來看待這個問題，這個就是布朗運動的概念。簡單的說，在一般的空氣環境與夠小的微粒情況下，我們對於這些存在於空氣中的微粒，必須得根據氣體動力學中的空氣中的氣體分子與必須被計算過濾效果的病毒氣溶膠微粒之間的碰撞所造成的布朗運動來做思考。

Brownian motion efficiency: If the particle is small enough (the diameter of the particle in the air dielectric is smaller than 1μm, which is the size of the virus aerosol), we cannot use the Newtonian mechanics (macro inertia). Under this situation, we must look at the problem with the microscopic model of molecular collision gas dynamics, which is the concept of Brownian motion. To put it simply, in the case of general air environment and small enough particles, it must be based on the aerodynamics, which is the Brownian motion caused by collisions between gas molecules in the air and virus aerosol particles whose filtering effect must be calculated.

當微粒被空氣中的分子碰撞，而產生了布朗運動的病毒氣溶膠顆粒，朝向口罩移動時，它會呈現一個非直線的隨機運動的方式在空間中移動。這個運動分成直線運動(沒有受到撞擊時)與非可微分的瞬間轉向運動(受到撞擊時)的組合，造成這些病毒氣溶膠粒子，在穿透口罩的過程之中，可以有效的被口罩上的纖維所阻擋而流在口罩的織品之上，即便是口罩織物或是纖維的密度，沒有達到比病毒氣溶膠還小的網格細小度時，也會因為這些隨機運動而沾附到口罩的纖維上面。

When the particles are collided by molecules in the air, and the virus aerosol particles that produce Brownian motion move toward the mask, they will move in the space in a non rectilinear random motion. This motion is divided into a combination of linear motion (when not impacted) and non-differentiable transient reversible motion (when impacted), causing these virus aerosol particles to be effectively blocked by the fiber of the mask, even if the density of the mask fabric or fiber does not reach the mesh size that is smaller than the virus aerosol, it will still stick to the mask fiber due to these random movements.

我們進一步的分析這些病毒氣溶膠微粒在空氣中的布朗運動，可以歸納出以下的四個結論：

When we further analyze the Brownian motion of these virus aerosol particles in the air, we can get the following four conclusions:

● 微粒的質量愈小，布朗運動愈明顯，也就是愈容易被空氣中的分子撞到

產生一種隨機的擾動。

The smaller the mass of the particle is, the more obvious the Brownian motion is, that is to say, the easier it is to be hit by the molecules in the air to produce a random disturbance.

● 微粒的體積愈小，周圍的分子愈不容易產生平衡的碰撞(例如左右各一個

粒子以相同的方向與力道撞上來，力互相抵消，等於沒有撞到)，也就使得

布朗運動愈發明顯。

The smaller the size of the particle, the less likely it is for the surrounding molecules to produce balanced collisions (two particles with the same direction and force collides, and the two force will be offset, which means nothing happens), which makes the Brownian motion more obvious.

● 溫度愈高，則空氣中的分子運動速度愈快，布朗運動愈明顯。

The higher the temperature is, the faster the movement of molecules are, and the more obvious the Brownian motion is.

● 空氣的密度愈低，瞬間撞在微粒上的分子數量較少，也就比較容易造成不平衡的力，形成較為明顯的布朗運動。

The lower the density of the air, the smaller the number of molecules hitting the particles, and it is easier to caused unbalanced force and form more obvious Brownian motion.

簡單的結論是：病毒氣溶膠的 質量越小，體積越小，以及在天氣越熱，

空氣越稀薄的環境下，病毒氣溶膠的布朗運動越劇烈，於是，我們會

發現，病毒越容易被口罩的織物所抓住。

The simple conclusion is, the smaller the mass and the volume of the virus aerosol, and the hotter the weather, the thinner the air, the more intense the Brownian motion of the virus aerosol. So we find that the virus is more likely to be grabbed by the mask fabric.

口罩織物的設計，為了阻隔這些呈現布朗運動的微粒，通常可以看到以下

幾種基本的策略：

In the design of mask, in order to block these particles that’s doing Brownian motion, we can usually see the following strategies:

策略一：使用靜電吸附的方式，病毒氣溶膠微粒，在以布朗運動的模式

接近口罩時，會非常容易被靜電吸附層所吸引過去，黏附在口罩上面。

Strategy 1: Use electrostatic adsorption. When the virus aerosol particles approach the mask in a Brownian motion, they will be very easily attracted by the electrostatic adsorption layer and stick to the mask.

策略二：使用HEPA濾網的各種設計方法，比方說讓濾片表面呈現3D的

紋路，使得微粒的穿透路徑長度增加，提高了病毒氣溶膠微粒被口罩材料

吸附的比例。

Strategy 2: Use various design methods of HEPA filter. For example, let the surface of the filter show a 3D texture, which increases the proportion of virus aerosol particles adsorbed by the mask material.

以上兩種策略，看上去是具有很高的阻隔效果，但是幾個重要的風險，

我們不得不去重視：

The above two strategies seem to have a high barrier effect, but we have to pay attention to several important risks:

病毒微粒留在口罩外層的風險在於手會去觸摸，調整，取下喝水。於是

手上沾了病毒，再去觸摸面部的眼睛或是其他部位，造成各種交叉汙染

的風險。

The risk of virus particles remaining on the outer layer of the mask is that we will touch, adjust, and take off to drink. So if you get the virus on your hand, you can touch the eyes or other parts of your face, causing various risks of cross contamination. 運用布朗運動的原理，以較大的孔隙(透氣但是卻可以把病毒微粒阻擋在纖維上面)去提高透氣率與舒適度，所產生的最終因為重複吸氣與

吐氣，逐漸地把口罩外層的微粒，一點一點的推入口罩內層，最終還是會進入呼吸道之中。只是每個氣溶膠微粒在口罩上停留的時間較久。

Using the principle of Brownian motion, with larger pores

(more breathable, but can block virus particles on the fabric as well) to improve the air permeability and comfort, the particles will eventually enter the respiratory tract due to repeated inhalation and exhalation, gradually pushing the particles into the inner layer, it’s just that each aerosol particles stays on the mask for a longer time.

這邊有個極為重要的關鍵概念，就是讓病毒氣溶膠微粒，因為在空氣中的布朗運動，而在口罩上停留比較久的時間。如果口罩沒有抗菌功能，我們能做的只是被動的等待病毒隨著時間慢慢的失去活性。而病毒可以在氣溶膠中存活的時間，往往長過於一般無論是HEPA還是靜電式的口罩設計，所能做到的微粒暫留在口罩上的時間延長。結論是，

這些方式的防疫效果，其實還是非常有限的。

An extremely important key concept here is to allow virus aerosol particles to stay on the mask longer due to Brownian motion in the air. If the mask has no antibacterial function, all we can do is passively wait for the virus to slowly lose its activity over time. The time that the virus can survive in the aerosol is often longer than the time the particles stay on the mask that has HEPA or electrostatic design. The conclusion is that the epidemic prevention effects of these methods are actually very limited.