Comment by Lazare
3 years ago
From the same Wikipedia page:
> The specification used in the European Union: European Standard EN 1822-1:2009, from which ISO 29463 is derived,[4] defines several classes of filters by their retention at the given most penetrating particle size (MPPS): Efficient Particulate Air filters (EPA), HEPA and Ultra Low Particulate Air filters (ULPA).
The Wikipedia citation goes to a specification which is, annoyingly, not freely available, but Google seems to confirm that Wikipedia is correct. Many sources confirm that EU standards measure penetration at the MPPS, I can find nothing suggesting they use 0.3 microns for anything.
I think the introduction paragraph for that Wikipedia page is simply wrong, and it's conflating two different standards. The actual section on HEPA specifications, however, is much clearer.
Technically, if retention at the most penetrating particle size is 99.95%, then retention at the 0.3 micron level is at least 99.95%.
But obviously measurement is not actually done at the most penetrating particle size, because retention at that size would be indistinguishable from zero. It isn't clear what they mean when they say "most penetrating particle size".
> defines several classes of filters by their retention at the given most penetrating particle size
If the MPPS is given... what is it?
> But obviously measurement is not actually done at the most penetrating particle size, because retention at that size would be indistinguishable from zero. [...] If the MPPS is given... what is it?
I'm not entirely sure what you mean. Retention is a U shaped curve, with the local minima (aka the MPPS) around (as near as I can google), the 0.2 to 0.3 micron range. As a practical matter, because the MPPS seems to not vary much between filters, and is always quite close (although apparently always slightly smaller than the 0.3 micron US standard?) nobody really seems to stress about the exact MPPS size, and it doesn't seem to ever really be reported.
(Implicitly this entire discussion is taking place in the context of airborne particulates. Obviouslly no air filter is going to work very well against, say, gamma radiation. Or a car.)
Which does mean it is possible that you could have a filter that doesn't qualify as HEPA under the EU standard if, say, it hits 99.94% at a MPPS of 0.2 microns, but does qualify as HEPA under the US standard (if it hits 99.97% at 0.3 microns). Or conversely, it could hit 99.95% at an MPPS of 0.25 microns (thus being HEPA by the EU standard), but only 99.96% at 0.3 microns (thus not being HEPA by the US standard).
Which is interesting, except when you calculate the math as applied to actually filtering room air, they're all going to be effectively identical.
> Implicitly this entire discussion is taking place in the context of airborne particulates. Obviously no air filter is going to work very well against, say, gamma radiation.
I'm perfectly willing to consider gamma radiation "not particulate". But if we restrict ourselves to considering particles that have mass, the retention rate at the MPPS of any usable air filter is still going to be zero. The diameter of a water molecule is 0.000265 μm. This is significantly smaller than the diameter of an oxygen molecule at 0.000346 μm. Of course, I don't expect an air filter to remove 99.95% of the oxygen in the air. But I do expect the fact that oxygen molecules are airborne particles to be taken into account when we talk about the retention rate of airborne particles. Another commenter has stated that "particles" must be identifiably liquid or solid, but you can easily have an ice crystal of 20 molecules, which -- if it were a straight line measured along its length -- would still have a "diameter" of less than 0.01 μm.
So I don't understand how appealing to "but we're only talking about particles" is supposed to save this. Particles are, at their lower reaches, very small, much too small for an air filter to address without killing the user. It seems clear to me that any workable filter standard must define a minimum particle size below which filtration is neither measured nor desired. What is that size, for European filters?
>> But obviously measurement is not actually done at the most penetrating particle size, because retention at that size would be indistinguishable from zero. [...] If the MPPS is given... what is it?
Something's gone wrong... you've run together two completely separate comments as if they were related to each other.
>>> If the MPPS is given... what is it?
You quoted a standard that specifies a certain level of retention "at the given Most Penetrating Particle Size".
That wording explicitly states that the Most Penetrating Particle Size is defined by the standard ("given"). So, what is the MPPS?
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