The respiratory filter is the essential element of any anti-pollution mask in order to trap fine particles . It can be sewn into the mass or removable. Let's see how respiratory filters work? How they are made, how they filter and when should they be replaced.
How is a breathing filter made?
Filters are usually made up of multiple layers of filter materials. The illustration below shows the different layers. They perform various functions, which include above all filtration but also comfort or tear resistance.
Breathing filters are made of several layers of non-woven fabric. There may be an activated carbon layer to filter odors and the last one in microfibers is mainly dedicated to softness for a more pleasant contact with the skin.
A non-woven textile is a textile whose fibers are randomly arranged at the time of its manufacture. These textiles are often classified according to their field of application or according to their technical characteristics. Its oldest form seems to be felt but for respiratory masks, it is Polypropylene, therefore plastic.
Polypropylene (PP) is the lightest fiber. It is 50% lighter than cotton at the same volume and much less expensive. There are three main methods of manufacturing Polypropylene, each with its advantages depending on the chosen use.
- Meltblown Translated from English – Melt blowing – is a classic manufacturing process of micro- and nanofibers where a molten polymer is extruded at high speed through small nozzles.
- Spunbond is a nonwoven fabric made of 100% polypropylene continuous filaments, arranged randomly and thermally welded.
- Spunlace is a nonwoven 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 cross-fibers.
Activated carbon is used to trap volatile organic compounds such as odor-causing molecules. As such, it is widely used in kitchen extractor hoods. It is made from carbonized wood, bamboo, or coconut shells.
How do respiratory filters work?
The different layers of respiratory filters are organized from the least filtering to the most filtering, like a sieve first retains the stones before filtering the sand. The more layers there are, the greater the filtration, but the less easily air will pass through them. This is why breathing is more complicated on an FFP3-labeled mask, which is complicated to use over time.
Filter materials are electrostatically charged. This electrostatic charge ensures that particles that are able to physically pass through them will be retained by adhering to the material.
What is the difference between a filter and a valve in a mask?
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 valves or exhalation valves for better breathing comfort provided that they are sufficiently sensitive and well placed as close as possible to the mouth, this is rarely the case! A valve is a plastic part containing a very thin silicone washer. This is a source of complication in terms of after-sales service and plastic pollution.
A valve only lets air pass in one direction, when exhaling, and closes when inhaling, so it does not filter the exhaled air and therefore does not protect people around the wearer if the wearer is carrying a virus.
When to change the respiratory filter of a mask?
The lifespan of respiratory filters depends mainly on their duration of use and the index of air quality (AQI). A mask used in Beijing in the middle of winter will be full in a few hours, while it will take a few weeks in a European city. Brands announce on average a fortnight for daily use on a commute to work in the city.
Unfortunately, there are no filters that change color when they are full. The main indicator is a higher resistance to inhalation when the filter is full. As a general rule, the filter will be changed more often because it is dirty than because it is full.
There are applications on mobile phones that allow you to track the optimal duration of use of a filter based on the duration of use, the areas traveled and the level of pollution detected in these areas. You still have to remember to start the application each time you use the mask... Tedious, right?
More generally, removable filters in reusable masks such as Frogmask masks should be replaced every 15 days to 3 weeks depending on the frequency of use and the level of pollution. They generally get dirty faster than they are full. Note that it is not at all advisable to wash them for possible reuse.
Frogmask filters are now made in Italy after having been in China for the first two years.
