ULPA filters

Let’s clarify what I have understood about filters and, in particular ULPA filters.

At the end of March 2020, SAGES and EAES published joint recommendations about “Resources on smoke & gas evacuation during open, laparoscopic and endoscopic procedures“. On April 16, they updated the document.

At the end of the recommendations they published a table with a summary of commercially available smoke evacuation systems. It is a working list of products that could potentially be used to filter CO2 gas or smoke evacuated during open, laparoscopic, and endoscopic procedures. In the table, among the listed characteristics, you can find an item called “micron filtration”. That value is, of course, posted as specified by the manifacturer and is commonly 0.1 micron. Actually, the standard indication is that a Ultra-Low Particulate Air (ULPA) filters can remove a minimum of 99.99% of airborne particles with a particle penetration size of 0.1 micron.

I am afraid that the interpretation of that value might be misleading to most surgeons. Therefore a lot of the basic information about the topic can be found in the excellent and very sound video published by Mark Soliman; I will not repeat here those information. Instead, I tried to get a more in-depth insight about filters and I will share what I found.

As most surgeons, at the beginning I thought that value corresponds to the “minimum” sized particle that the filter would be able to catch. Just imagine a fish net: every fish larger than the meshes will be caught, but the smaller fish are not. But, we all know that the size of the COVID-19 is about 0.06-0.14 micron. Therefore a “net” with meshes of 0.1 micron would not block most particles. Fortunately, this is not true because the language used by surgeons often does not match that of engineers, and let’s see why.

Filters do not act as fish nets. At least four filtration methods are active in a filter:

Filtration mechanisms EMWfiltertechnik ULPA filters fabio cesare campanile
Figure 1. The filtration mechanisms (thanks to EMW Filtertechnik)
  • Interception (more or less the fish net mechanism)
  • Inertial impaction a large particle is unable to adjust to the change in air direction near a filter fiber. Its inertia ensures that it continues along its original path instead of circumventing the fiber, resulting in its capture.
  • Diffusion relies on the Brownian motion of gas particles and allows to trap smaller particles (typically 0.1 μm or less)
  • Electrostatic attraction
ULPA filters MMPS fabio cesare campanile laparoscopia laparoscopy civita castellana
Figure 2. Capture efficiency of a HEPA filter

The combination of those methods allows the filter to trap particles of very diverse sizes, but its efficiency is not the same for all sizes: it is possible to identify the diameter for which the filter is least efficient. This size is defined as MMPS (Most Penetrating Particle Size), in other words the MMPS is the size that is more difficult to catch for the filter. The MMPS for a ULPA filter is about 0.1 micron (actually, to be precise 0.12 micron which is very close to the HIV virus). In the figure, you can see the behaviour of a filter with a 99.97% efficiency at the MMPS of 0.3 micron (actually, a HEPA filter, but the concept is the same).

You can see that 99.97% of the 0.3 particles (the more difficult to catch) are captured; but the smaller particles are filtered with an even greater efficiency. In fact, particles smaller than μm 0.1 are easily trapped by diffusion while particles larger than 0.4 microns are captured by inertial impaction. Particles between 0.1 and 0.4 μm are too large for diffusion, too small for inertial impaction and efficient interception; so, the filter is less efficient to get at them. The same applies for ULPA filters, but their MMPS is μm 0.1.

The ISO standard 29463 has been issued to harmonize the European Standard EN 1822 and the U.S. MIL-STD-282. To make a long story short, these regulations define the classification of the filters and the methods to assess their efficiency. According to the standards, ULPA filters are specified to have a ≥ 99.9995% efficiency at an MMPS of 0.12 micrometer. It means that all ULPA filters are so efficient that will let pass only far less than 5 for every million COVID-19 particles (that are actually smaller than 0.12 micron).

Back to our EAES/SAGES table: the “micron filtration” box most likely reports the MMPS for the filter used in the device as reported by the manifacturer. Anyway, the ULPA filters must be tested according to the ISO standard 29463 and remove all but 5 parts per million. The “micron filtration” data is probably reported from the manifacturer declaration but, unless some specific tests are done (additional to what is required by the ISO 29463 regulations) it might mislead us to think that the smaller the number the better the machine is. This is unlikely to be true.