>HEPA is not right for a purifier that sits in a room
Why not? I don't know anything about HEPA, or quality, air flow, etc.
Why not? I don't know anything about HEPA, or quality, air flow, etc.
Here’s a thought experiment: Take a 1000 cubic feet room and a purifier that processes 100 cubic feet of air per minute. (I follow Wirecutter in using vulgar imperial units.) Assume pessimistically that all particles are the worst-case size. If you run that purifier with an E12 filter, the fraction of particles that will remain after one minute is
.1 × (1-.995) + .9 = 0.9005.
Meanwhile, if you run that purifier with an H13 filter instead then the fraction of particles that remain will be
.1 × (1-.9995) + .9 = 0.90005.
The moment I read that I checked out on the rest of the authors opinions.
It's a 10x difference.
The author's "I'll just times .1 by the percent of flow, and produce very small numbers that look fine! See! The numbers are so small!" trick is just … wrong.
The author implies that the difference can be made up by the volume of air being processed, but that would only be true of a sealed environment, where no new pollutants are added to the air.
Setting aside the basic misunderstanding of probability, and ignoring that home purifiers don't operate in sealed environments, the IKEA unit does not process 10x the amount of air as the other units, so the point is mute.
In this case they are measuring the % of particles captured (an amount), not the likelihood a particle is captured (a probability). The parent is right, it’s a tiny difference.
An H13 filter filters out 99.95% of particles above 0.3 microns.
Assuming a volume of 10000 particles above 0.3 microns:
An E12 filter will leave 50 particles.
An H13 filter will leave 5 particles.
The "rootlocus" filter would leave 0.5 particles.
So yes, I would say your filter is 100x better because it literally is.
That volume is not the same volume processed by all filters in the same amount of time.
In the first minute:
E12 filters 10000 particles @ 99.5% performance -> removes 9950, leaves 50
H13 filters 10000 particles @ 99.95% performance -> removes 9995, leaves 5
RLv1 filters 10 particles @ 99.995% performance -> removes 10, leaves 0
RLv2 filters 1000000 particles @ 99% performance -> removes 990000, leaves 10000
By your method, RLv2 is 2000x slower than H13, but in the same ammount of time filtered 99x more particles. RLv1 needs to run 99000 minutes to filter the same amount of particles RLv2 does in one minute.
The example is meant to show air flow totaly dominates performance, and it's not "a trick" to multiply by it. I also want to point out that comparing the amount of particles "left" (50 vs 5 vs 0 vs 10000) is nonsense and absolutely no indication of performance in any way.
(With apologies to Wayne Gretzky)
it is. that's one minute of filtration and the difference is minuscule. over time, this would trend to zero. in 10 minutes you'd expect to be near the steady state of the room. (obviously not completely steady state since you are filtering some already filtered air and probably introducing more particulates but close enough for an approximation)
In a sealed environment, you're right, you'd eventually end up with all particles filtered.
But homes are not sealed environments.
> but homes are not sealed.
Correct, but neither are they ultra high throughput (at which point any filter sitting in the room would be useless anyway, since you never get the filtered air). So "not sealed" is too vague to make any conclusion.
It's a 0.049997% difference, not a 10x difference.
In an unsealed environment, the steady state will be related to amount filtered * % filtered / amount exchanged for any given time period. The difference in % filtered is not a significant factor in the above ratio.
An H13 filter filters out 99.95% of particles above 0.3 microns.
Assuming a volume of 10000 particles above 0.3 microns:
An E12 filter will leave 50 particles.
An H13 filter will leave 5 particles.
That's a 10x difference.
That's it. Yes, one filter is 10x as efficient. It doesn't matter because in this example they aren't moving enough air relative to the room size/leakiness for it to matter.
Using small numbers to make the difference look small doesn't hide that.
If you are taking air, running it once through a filter, and using the air that comes out for an application that needs very few particles, then a 99.99% filter is “10x” as efficient as a 99.9% filter in the sense that the air coming out will have 1/10 as many particles. For example, a 99% efficient face mask is “10x” as efficient as a 90% efficient mask (assuming both fit perfectly, which they don’t, although a PAPR approximates a perfect fit).
But an air purifier doesn’t do this at all. It continuously sucks in air, removes particles from it, and sends the filtered air right back into the room to mix with all the other air. The performance of a 95% filter in this context is barely distinguishable from that of a hypothetical 100% filter. Your characterization would have the 100% air purifier being “infinitely” more efficient.
Air purifiers operate on a fraction of available air. That air supply is continually being cycled, refreshed and mixed. Particulate matter within that air is not evenly dispersed.
That, for a single minute, as a percentage of total air, a 99.5% and a 99.95% purifier produce a minor difference in total air quality is deeply irrelevant to the overall performance of the purifier over any length of time. The 10x difference, however, will matter over time.
This is why the tests, which the author dismissed without any reasoning beyond "looks wrong!", in the original WireCutter article showed such stark differences between the performance of the Förnuftig and the Levoit Core 300, over a 30 minute span.
If you were correct, over those 30 minutes, the amount of particulate in the test room would have been roughly equal for both purifiers. It wasn't. The Förnuftig removed only 64.5% of the particulate while the Levoit removed 97.4%.
Can you point to a test which shows dramatically different results than the ones the WireCutter reported?
> The idea that the difference between 0.9005 and 0.90005 is "small" is … weird.
When you use the author's numbers, 0.9005 and 0.90005, the implication is that you're taking the parameters of they hypothetical as given. You then go on to say that the difference between those numbers is significant. Remember that in this abstract, idealized scenario, the air filters are only able to process 1/10th of the air in the room (hence the shared 0.9, the dominant portion of the magnitude). Perhaps the room reciculates with its environment at the rate of one room volume per day, and the filters can only process 1/10th of the room per day. Given that, do you still think the difference is significant? Or are you just outright refusing to participate in the thought experiment at all? Because that's what it seems like now that you're trying to broaden the scope of your contention to the other sections of the article.
Those numbers represent percentages (90.005% and 90.0005%) and those two inputs, especially when applied to a chaotic system, will produce outsized differences over time.
And the data shows that the two filters produced outsized differences over time.
I'm not broadening the scope of my contention. I'm pointing out that my contention (there is a large difference in those numbers that is hidden by the way the author presents them) is confirmed by the data.
Note that you are talking about the 0.3 micron measurements: if we look at larger particles the difference is smaller. But that's fine!
There are two big ways that that comparison is different from what we're talking about here:
* Those two purifiers have very different capacities: 135 CFM (CADR) for the Levoit, 82 for the Förnuftig
* The filter on the Förnuftig is much less effective against very small particles. The math above is comparing filters that are 99.5% vs 99.95% effective, while in this case it's more like 70% vs 99.97%.
The author claimed the difference between the purified air, as a percentage of total air volume, was small. He used percentages expressed as a decimal to make that difference look small (0.9005 vs 0.90005). But a clever observer would translate those numbers back into their percentages (90.05 vs 90.005), start applying some math (i.e. 100000 x 0.9005 vs 0.90005), see the 10x difference, understand how that 10x different is going to multiply over time in a chaotic system, check the data to see if that's true, and then throw away the author's point.
If your goal is to play with numbers, you could raise them both to a large power. You would discover that the ratio between them increases exponentially, but this would pale in comparison to the fact that both results would exponentially approach zero much faster than the ratio would increase.
Meaning that the first filter left 5 particles vs the second filter leaving .5 particles.
A 10x difference.
The goal isn't to "play with numbers" but to understand why/if the relative effectiveness of a filter results in a substantive difference in air quality.
The data shows it does.
As noted in the article, the Wirecutter does not explain its methodology or give particularly complete data, and what explanations they do give about filtration make no sense.
I don't understand why you think more of an explanation is required?
Your comment is like observing that car A burns 87 octane gasoline and another burns 89 octane gasoline and claiming, without explanation, that one of them accelerates faster because (90-octane) is 3x lower.
hint: the bigger purifier wins because it has a more powerful, more power hungry fan pushing air through it. Its performance might be further improved (depending on the fan and motor characteristics) by putting a less efficient, lower pressure drop filter in because more air would go through it per unit time.
Meanwhile, two IKEA filters will outperform it in every measure, including cost, noise, and power consumption. But their efficiency will still be lower.
As I described above [1] the data show that the difference between 70% and 99.95% matters, not that the difference between 99.5% and 99.95% does. (And that's ignoring difference in flow rates, which is also very large.)
This is very straight-forward.
[1] (1−.995)÷(1−.9995)
[2] (1−.7)÷(1−.9995)
Can you explain, with actual math, what you’re trying to say?
There are plenty of plausible explanations for Wirecutter’s unexpected results. They could have messed up (quite likely). The difference in the behavior of the fans could be circulating the air differently (also seems reasonably likely). The conditions of the test could be such that the difference in CADR was relevant (possible but doesn’t seem likely). They could have failed to set up the IKEA filter correctly (I once failed to set up a Conway filter correctly — it was somewhat embarrassing). Or, by pure magic, the fact that the extremely clean outgoing air from the IKEA filter was less extremely clean than the extremely clean outgoing air from the other filter made a difference (seems very unlikely).
Since the air purifier intakes and exhausts in the same space (meaning filtered air gets re-filtered), all the slightly worse filter means it that you'd need to run it for a couple more minutes to get the room down to a similar concentration of particulate per unit volume... So the difference in particulate concentration would likely not be anywhere near 10x at steady state, it would be much smaller (but depends how much air leaks into the room from outside, the particulate content of the outside air, the volume of air you're getting through the purifier per unit time, etc.)
Using one small number or produce another small number, so the difference looks small, doesn't hide the 10x change.
In an operating room or chip fab, the room would be over pressure and the new air coming into the room would be filtered. The cleanliness of that air would be determined by the quality of the filter.
Also, if you need air that clean, you need to have strategies for all sorts of things besides filtering.
The point is, you need to be very careful when you put numbers on the internet, and when you read numbers on the internet. Numbers make things feel more real than they are.
For me to actually trust the numbers here, I would need to see the graphs for multiple runs of each filter.
> The idea that the difference between 0.9005 and 0.90005 is "small" is … weird.
We aren't talking about a situation where both filters are processing all the air in the room. We're talking about a situation where the filters are only processing 10% of the air in the room. That's the defining characteristic of the hypothetical.
You're describing a situation where the filter is on the intake, but this thread and article are about purifiers within rooms. I agree that the math is really different in your situation.
The genuine HEPA filter in a cleanroom [0] is not sitting in front of a fan in the middle of the room. It’s very carefully installed such that all the air coming into the clean area goes through it once. The calculation is entirely different. (A medical or industrial HEPA filter may well be in the exhaust, in which case the considerations are again different.)
[0] There’s none of this “true HEPA” stuff in a cleanroom. There is a filter that meets a specific standard, and that filter will have a gasket that seals with considerable force against the air handling equipment. The “true HEPA” filter in a Wirecutter-approved air purifier achieves nowhere near 99.97% due to the lack of the aforementioned gasket regardless of how amazing the filter media may be.
It is (hopefully) easy to see that e.g. a filter that removes 99.5% of particles, but moves twice as much air per minute will remove almost twice as many particles per minute as a filter that removes 99.95% of particles.
Using the numbers from TFA (20% of the room for the 99.5 rather than 10%):
.2 × (1-.995) + .8 = 0.801
.1 × (1-.9995) + .9 = 0.90005
Thanks for teaching me the name for this principle!
Or the articles about how Python is causing climate disaster while the author continues to drive an oversized SUV.
(0.9005 × .1) × (1-.995) + (0.9005 × .9)
It doesn't show: the difference is too small to notice at every stage: https://www.jefftk.com/remaining-particles-by-minute-995-vs-...
Even comparing 95% vs 99.95% it's barely noticeable: https://www.jefftk.com/remaining-particles-by-minute-95-vs-9...
Sheet: https://docs.google.com/spreadsheets/d/1wVIdWR0lWgZRt4gbnVaH...
This is utterly false. HEPA filters are measured at the efficiency of what’s known as the MPP (the Most Penetrating Particle size). It’s the hardest particle size to capture as it can get by the two methods used to capture large particles (impaction), and smaller particles (diffusion).
Considering almost none of the air in a room is passing through the filter at a given moment, the efficiency of the filter is less important than how much air it moves through the filter media per minute, which IKEA have favoured here.
Essentially this filter performs close to par with more expensive units, while using less energy, and having dramatically lower costs for filter replacements when due.
What they don’t do is give reviewers either kickbacks or basic physics lessons.
To be fair, it took a pandemic for me to go to the literature of mask effectiveness and finally found the "on the filtration efficiency of fiberous filters" paper that showed the u shaped curve. it's not something that they scream from the hills about in their product brochures. That said it should be screamed from the hills.
If X% is 99.95% or higher and below 99.9995%, it's technically HEPA; 99.9995% and above it's ULPA, and below 99.95% (but 85% or higher) it's EPA.
So let's say you've got an E12 filter that removes 99.6% of particles. Technically not HEPA, but after one pass through the filter and you've got 0.4% of the particles left. Two passes and you've got 0.0016% left, three passes and it's 0.0000064% left.
An H13 filter might remove 99.97% of particles. One pass and you've got 0.04% left, two passes and you've got 0.000016% left.
Or in other words, one pass through a technically HEPA filter might leave you with 25 times more particles than two passes through a technically non-HEPA filter. So if the air is cirulating back through the filter (as it would in a closed room), what matters is both how good the filter is and how much air it can filter. And since at the high end the filters are all so good, the volume of air processed dominates. A filter that processes twice as much air is vastly better than one that filters out an extra fraction of a percent of particles.
(US standards are similar, but the cutoff for HEPA is 99.97% of 0.3 micron particles, not 99.95% of "whatever the filter is worst at". But the difference is generally irrelevant.)