Comment by thaumasiotes
3 years ago
> Second, and a minor point, the 0.3-micron standard related to the US HEPA standard, not the EU one. It is true that acording to IKEA it doesn't meet the EU standard for HEPA (barely), but we don't know whether it meets the US one.
Wikipedia cites this statement:
> Common standards require that a HEPA air filter must remove—from the air that passes through—at least 99.95% (ISO, European Standard) [...] of particles whose diameter is equal to 0.3 μm
to "European Standard EN 1822-1:2009, "High efficiency air filters (EPA, HEPA and ULPA)", 2009". Have they made a mistake? ( https://en.wikipedia.org/wiki/HEPA )
The same sentence goes on to note that the American standard is similar, but more strict, requiring filtration of 99.97% of 0.3 μm particles. As such, it is not possible to meet the American standard while failing to meet the European standard, so there's no need to discuss the American standard separately.
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.
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