HOW BREATHING FILTERS WORK
Filters are an essential part of any anti-pollution mask to trap fine particles. They can be sewn in or removable. Let's see how they are made, how they filter and when they need to be replaced.
How are they made ?
In general, masks are made up of multiple layers of filter material and may optionally include an exhalation valve. The illustration below shows the different layers. They perform a variety of functions, which include primarily filtration but also comfort or tear resistance.
The filters are made up of several layers of non-woven fabric. There may be a layer of activated carbon and the last layer of microfibres is mainly dedicated to softness for a more pleasant contact with the skin.
A non-woven textile is a textile whose fibres are randomly arranged at the time of manufacture. These textiles are often classified according to their field of application or their technical characteristics. Its oldest form seems to be felt but for masks it is Polypropylene.
Polypropylene (PP) is the lightest fibre. It is 50% lighter than cotton for the same volume and much cheaper. There are three main methods of manufacturing Polypropylene, each with its own advantages depending on the use chosen
- Meltblown is a classic manufacturing process for micro- and nanofibres where a molten polymer is extruded at high speed through small nozzles.
- Spunbond is a non-woven fabric made of 100% polypropylene continuous filaments, randomly arranged and thermally welded.
- Spunlace is a non-woven fabric with an isotropic property (which gives it equal strength in all directions). Multiple rows of high-pressure water jets perforate the fabric and entangle its crossed fibres.
Activated carbon is used to trap volatile organic compounds such as odour-causing molecules. As such, it is widely used in kitchen extractor hoods. It is made from carbonised wood, bamboo or coconut.
Machine assembling several layers of PP Meltblown
How do they filter ?
The different layers are arranged from least to most filtering, just as a sieve first retains the pebbles before filtering the sand. The more layers there are, the more filtration there is, the less easily air will pass through them.
The filter materials are electrostatically charged. This electrostatic charge ensures that particles that can physically pass through the material are retained by adhering to the material.
What is the difference between a filter and a valv ?
The more efficient a mask is in terms of the size of particles filtered, the more difficult it is to breathe in. For this reason, some masks have exhalation valves for better breathing comfort, provided they are sensitive enough and placed as close to the mouth as possible. A valve is a piece of plastic containing a very thin silicone washer.
Example of valves on the Biker mask
A valve only allows air to pass in one direction, on exhalation, and closes on inhalation, so it does not filter the exhaled air and therefore does not protect those around the wearer if they are carrying a virus.
How to know when to change them?
The service life of a filter depends mainly on the length of time it is used and the air quality index (AQI). See the article on fine particles for an explanation. A mask used in Beijing in the middle of winter will be full in a few hours, whereas it will take several weeks in a European city. The brands claim an average of 15 days for daily use on a commute in the city.
Unfortunately there are no filters that change color when full. The main indicator is a higher resistance to inspiration when the filter is full. Very often the filter will be changed because it is dirty rather than full.
There are applications on mobile phones that allow you to follow the optimal use of a filter according to the duration of use, the areas traveled through and the level of pollution found in these. However, you have to remember to start the application each time you use the mask... tedious isn't it?