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Comment by csours | Hacker News Reader

The author explicitly states that it's small in the home use context. If you're talking about medical or cleanroom manufacturing contexts, yes it's a huge difference.

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The difference between 0.9005 and 0.90005 is not huge in a medical context, or in a chip fab context, or in any other practical context. We're not talking about the difference between 0.0005 and 0.00005. The numbers in question are 0.9005 and 0.90005, and the point being made is that the 0.9 problem dwarfs the 10x efficiency difference way over in the thousandths place.

That difference is from his comments on the toy model of 1000 cubic feet room and 100 cubic feet per minute recirculating air.

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.

Yes, the numbers are from a toy example, one that the author used to make one uncontroversial point in one section of the post. Those are the numbers we are discussing in this subthread, which began with:

> 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.

I did a poor job of being explicit in my first reply to etchalon.

> In an operating room or chip fab, the room would be over pressure and the new air coming into the room would be filtered.

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.

Yes, but in that case you wouldn't be comparing 0.9005 and 0.90005, but rather 0.0005 and 0.00005. No one is arguing that the difference in filters wouldn't matter in a cleanroom context, just that recirculating air in a home the difference in filtration is more like 0.9005 vs 0.90005, and the difference between those numbers is small (in any context to which they apply).

Even in a medical context, the difference, when operated like an air purifier, is negligible.

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.

Small home or not, homes are not sealed environments. A 10x difference is a 10x difference.

Using one small number or produce another small number, so the difference looks small, doesn't hide the 10x change.

How is there a 10x change? I see a 0.1% change.

0.90005 times 10 is 9.0005, not 0.9005 (I.e. the two fractions presented are 90.005% and 90.05%). Even if you look at the complement you get 9.995% vs 9.95% which is small. One could imagine that these differences could also arise from eg obstructions to airflow or positioning in the room or the direction of the wind outside. The point is that the difference is dominated by air flow in a typical environment rather than filtering differences.

Is it a 10x difference? If you used the better filter, you would still have 99.95% of the particles you would have had if you used the worse filter

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