Personal protection has become a matter of great interest in the last few months due to the COVID-19 pandemic. Different kinds of masks are increasingly used by people when in public areas. A large number of different products are available and each is designed for a specific purpose. In the following a short overview of the different protective devices is presented.
We will consider half-masks only which cover the mouth and nose area. Here, basically three different types of masks can be identified:
- Surgical masks
- Half-mask made from filter material
- Half-mask with exchangeable cassette filters
The last two products are designed as particle filters that protect the wearer from hazardous substances. Surgical masks on the other hand serve a different purpose. They are designed to protect the environment, i.e. the patient or healthcare personal, from the wearers’ exhaled droplets. For this they are mainly tested for bacterial filtration efficiency. Optionally, splash-resistant masks are available to protect its wearer from e.g. blood splashes.
A summary of relevant Standards and properties for common products are listed in the Table below.
|Minimum filtration efficiency / %
|ISO 17420-2 / 16900-3
|NIOSH 42 CFR Part 84
|R100 / P100*
|R99 / P99
|R95 / P95
|Minimum bacterial filtration efficiency / %
|98 (+ splash resistant)
* P-series filter require testing until no further decrease in efficiency occurs
** Type I masks are only for patients to reduce risk of spread of infections
*** Type II and IIR are for healthcare professionals
Lorenz Meßgerätebau’s testing machines, such as the FMP03 or FMP04, can measure the filtration efficiency for all particle filters with liquid aerosols (paraffin oil, DOP, etc.), marked as green in the Table. Additionally, the exhalation and inhalation pressures can be determined as well. They are not stated in the Table as there are numerous different specifications, e.g. with new and loaded filters, different flow rates, masks with or without valve, etc.
More details about e.g. particle size distribution and testing methods can be found in the section below.
So far particle filtration was mentioned but not specified in detail. The goal is always the same: to remove particles from inhaled air that are potentially capable of entering the human respiratory system.
Large particles are filtered by the human body automatically. It’s the very small particles that can reach either the upper respiratory system or even the pulmonary alveoli. If the diameter is smaller than about 10 – 15 µm particles can reach the upper respiratory system. Even smaller particles below 2.5 µm are able to reach the alveoli. Correspondingly these two classes of particulate matter (PM) are called PM10 (ca. 2.5 – 10 µm diameter) and PM2.5 (< 2.5 µm diameter) respectively. For comparison, bacteria typically have sizes between 0.5 and 3 µm while viruses are much smaller with the majority between 0.02 to 0.3 µm.
Filter material consists of many layers of fine fibers in a regular or irregular mesh. Air can easily pass through the mesh while particles have a certain probability of being stopped. For large particles the mesh acts like a sieve while very small particles are stopped by different effects (diffusion, electrostatic effects). In between, there is a particle size for which the filter has its lowest efficiency, the most penetration particle size (MPPS). It’s typically in the range of 100 to 300 nm. For testing it is important to have many particles in this range to check the filter at its weakest spot.
Particle size distributions
Particle filters are tested with challenge aerosols of either liquid or solid particles with certain concentration and size distribution. Different standards require different substances and slightly different size distributions (although always in the region of the MPPS).
Common aerosol generators produce aerosols whose particle diameters are (logarithmically) normally distributed, the well-known Gaussian bell curve. It is characterized by its maximum location and width (standard deviation). The most intuitive way is to state how many particles are present at each diameter (or diameter interval), which is therefore called “particle number distribution”. The curve’s maximum is called the “count median diameter” (CMD).
But there are also other definitions, such as the “mass distribution”. When scientists began to classify aerosols by their particle size there were no optical particle counters available that we have today. The only option was to determine the amount of particles gravimetrically, i.e. by weighing. Therefore, the mass distribution was, and still is, of high interest. It is very important not to confuse these two types of distributions and always clearly state which one is required. Here, the maximum is analogously called the “mass median diameter” (MMD). For the same aerosol the MMD is always larger than the CMD. By how much depends on the width of the distribution.
Filtration efficiencies are measured with a challenge aerosol that is directed through the test specimen. Photometers before and after the filter determine the respective aerosol concentrations which yield the efficiency. Liquid aerosols typically consist of paraffin oil, DEHS, PAO or DOP. Solid aerosols in the described standards always consist of sodium chloride particles.
The inhalation/exhalation resistance is determined with a differential pressure sensor connected to the filter mount. Some devices, such as our FMP04, can measure inhalation and exhalation pressures automatically with integrated pneumatic valves to switch the flow direction.
The bacterial filtration efficiency (BFE) for surgical masks is tested with aqueous droplets with a mean diameter in the range of (3.0 ± 0.3) µm. These droplets contain live Staphylococcus aureus bacteria. Droplets passing the mask are collected on agar plates and incubated. The number of colonies compared to that for control runs give the efficiency. No particle filtration test is performed.