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There's a history here of discussion of how to make good air purifiers (like this). Today I learned about ULPA filters and found someone's DIY video using one of them.
A ULPA filter can remove from the air at least 99.999% of dust, pollen, mold, bacteria and any airborne particles with a minimum particle penetration size of 120 nanometres.
I recently moved to a place with worse air quality. The fatiguing effect on me is noticeable to me (though I suspect I might have vulnerable physiology). It makes me want to try to update far in the other direction: maybe any level of impurity causes bad effects, but I didn't notice them (or associate it with air quality) cause some impurity was constant even with normal filters.
My current idea is to try to make a system like in the video, plus a tube so I can get the filtered air directly to my face. I don't know how feasible it is to filter a whole room to that level.
Here's a thought experiment meant to construct a situation where one's choice would acausally determine larger background facts about the world they're in, such as physical laws.
Suppose you're in a world which you correctly believe was created by either an 'angel' or a 'devil'.[1] If created by an angel, it will eventually be filled with happy beings. If created by a devil, it will eventually be without life.
Suppose you're a superintelligent agent which controls this world from the inside. If you choose to fill it with happy beings, that would mean it was all along created by an angel. If you choose to destroy all life, that would mean it was all along created by a devil. If you wanted to, you could pick which of those larger background facts is true by doing one or the other.
Next, we can modify this thought experiment by replacing the 'angel' and 'devil' with two sets of physical laws, which either ensure one outcome or another eventually. Then your choice of which outcome to cause would acausally correspond to which of those sets of laws governs the universe you are in.
Inspiration: https://en.wikipedia.org/wiki/Angel_problem