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Here's how I think about this:
Just in my personal experience, HEPA filters seem to help with allergies, and visibly help filter out smoke when it's detectable by smell.
But let's ask The Science. What do we see? The first question to ask - does it remove microscopic particles from the air? If it doesn't, we can just stop. This is something I'd expect good data on, it's a (relatively) hard-sciencey question. Looking at this and this, HEPA filters seem to reduce particle concentration (of pm2.5 and bigger particles, which google says is how big allergy-related particles are) by around 50%. Which is substantial, but intuitively seems like less than you'd want. Of course, though, this depends on exactly what HEPA filters they used, how much ventilation there was, what kind of particle, etc. The first study says "ventilation was through the door" - does this mean through gaps in the door, or was the door open? idk. The second study also compares having multiple HEPA filters to one, and three filters on 'medium flow' seems to (eyeballing a very poorly constructed graph) reach 75% reduction in particles. Just intuitively, by having a good model on a (somewhat noisy) high setting, and either having it in a small room without too much air exchange or having multiple for your home, you'd get meaningful (80%+) reductions.
The second study does note some increases in ion concentration (not particle concentration) in HEPA on scenarios. But the increases are only present for some ions, in some particle size categories, so I think it's just noise.
I'm not really sure how much to trust random studies like this anyway. One has authors from "Dr. Bhimrao Ambedkar University" and "Savitribai Phule Pune University, Pune" and the other from Taiwan. The Indian study mislabeled several of their tables to seemingly show that air purifiers double particle concentrations,.
Okay, just from personal experience I'm pretty sure it removes particles, and the science seems to confirm it removes the right ones. But inferring that it stops allergies from that isn't particularly justified. What about studies on allergy symptoms? Well, there are a lot of positive results on google scholar. But they're all positive results on some measure but not others, and they're all either n=20-40 RCTs or meta-analyses of 10 n=20-40 RCTs. I don't think one should believe any of these, either as evidence for or against.
So, uh, my guess is if you pick the right ones and use it correctly, they're effective, mostly because some people I know claim they are. It looks like good ones are $100-$150 on amazon, so if it doesn't work it won't hurt too much. I'm not sure how to match the airflow rate or w/e to how big your room/home is, there's probably a guide on that somewhere.
But if you need increased airflow, that'll significantly reduce the effectiveness of the air filters. Certainly airflow from outside, and if the AC is introducing new particles from itself or outside, as opposed to just recirculating them, that'll hurt too. I guess it'll probably still help, but idk either way.
Awesome, thanks for this - a lot of it is info I'd sussed out already, but looking for that study sent me down the rabbit hole of conspiracy theories and I emerged in a sea of insanity. Taking on your post, I bought an Electrolux with carbon filtering, so even if it doesn't help my allergies it will at least help my place smell nicer.
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