Reverse engineering Xiaomi air purifier NFC stickers

If you don't understand the purpose, it means you don't need it - as so often. The reason they exist is that in some parts of the world the air is so bad it literally has significant negative health effects just breathing it. That's why people use them, and for what it's worth as far as I understand the quality ones do work, in the sense that you can use an air quality meter and detect a noticeable improvement in air quality. They have filters that you need to change regularly.

-Edit- Since so many people are raving about them and recommending you buying an air purifier I want to add this: If you live in a place where the air is truly that bad, my main recommendation would be to move. I have personal experience with this, it's not worth it living like that. You may filter the air in your home but then whenever you go outside you're at risk, if you do sports even more so. Some people wear surgical or cheap cloth masks but these don't actually work.

Don't listen to people who tell you that you can "see" pollution of "feel" the difference. I've lived in some of the most polluted cities in the world and the truth is in most cases for regular healthy people they don't notice anything for years, even at pollution levels far beyond the imagination of folks in Western countries. Most of this is their imagination or placebo like you suspect. For example I've had people complain to me that the air is "so bad" in a place that's naturally foggy and where visibility is often low, but had good air quality. Others were happy about the lovely "clean" skys in a city that gets lots of sunshine but was actually horribly polluted. Also sure, smoke from wildfires is visible (and short lived) but a lot of the pollution from industrial sources isn't. It's a slow and stealthy killer.

Can someone explain what's the result of filter utilization / capacity. Is it efficiency? Does it start spilling out particles?

I feel like a 100% used filter will still do a pretty good job, so unclear how much of the utilization % has commercial reasoning?

My intuition is that the more “full” it is, the less air will flow through it, meaning it just doesn’t do as much.

Not to suggest that what the manufacture decides “100%” is is actually that.

Efficiency in terms of airflow goes down, and static pressure difference goes up - but the filters trap more of the particles going through them. Changing filters on furnaces and AC units is really important, since lack of airflow hurts efficiency.

Wirecutter (which is usually trash) confirmed this in testing; versions of units that had been running for months or longer (ie one they bought a year ago and used in someone's home, vs a new unit bought new with little run time) tended to perform better in terms of how many particles they completely removed.

The unit starts using more electricity to do the same work, has to run at higher fan speeds, etc. So it's a tradeoff between that and the cost and waste generated by buying more filters.

If you live on a main road, there's likely a lot of pollution in the air. How much is actually in your home will depend on a lot of complicated things (prevailing winds, home construction, etc), so it's best and easiest to just measure.

HN's own https://news.ycombinator.com/user?id=ahaucnx has a company with a DIY version: https://news.ycombinator.com/item?id=27124671

I got a Dylos before I learned about AirGradient, and I really like it too. Depending on your budget and tech savviness, it's a great option too.

One important note: these detectors don't do chemistry, and not everything that causes a spike of PM is actually a problem. Showering, for example, will cause a spike for hours, but those salts are probably fine. Running the oven will also cause a spike, and those particles probably aren't fine.