This is to me one of the reasons for CPAP - you get filtered air all night long!

I do wish the filters were better though, they're really not very good, just basic cellulose. They're mainly to catch big stuff like pet hair. Nowhere near a HEPA.

That's a very fair point - I still find that they've made a difference for me, even with the limitations.

Yeah, I'm sure they do. I've got so many low-moderate level allergies that it's kinda hard to establish what normal even really feels like. I got a round of shots as a teen that helps, but it's still some level of congestion at the best of times. Actually the biggest difference I found when I first started on it (almost 10 years ago now), besides the obvious sleep stuff, is that the humidifier keeps me from getting nosebleeds about 3 times a week in the winter.

Unless you have a Phillips Respironics, in which case it helpfully adds bits of foam (pre-recall) or formaldehyde (post-recall) to your air stream. I realize that Phillips says it's under limits, but they aren't really trust inducing right now…

I have, unfortunately, found "low speed" mode on air purifiers to be not very effective at improving air quality.

Measured with an Airthings sensor, in my 1970s house, low-speed mode just clears temporary indoor-source pollutants marginally faster than having no air purifiers. In the case of exterior-source pollutants (i.e. wildfire smoke), low-speed mode is insufficient to keep PM2.5 in an acceptable range.

And this is with five air purifiers throughout the house that are sized for a significantly larger space.

This really depends on the rate at which air inside the house gets exchanged with air from the outside.

In my 2008 house, two air purifiers on low are sufficient to keep AQI at 0 and in the single digits during wildfires. They're rated for about 350 square feet each, while the house is about 1500 square feet.

Yes, completely true - if you had a home with positive pressure and a filtered HRV/ERV with sufficient volume, one wouldn't need internal air purifiers at all.

I've long wished for "smart" inwards-blowing window fans that took filters and conditionally only turned on when the air outside was an appropriate temperature - wouldn't be appropriate in all climates/seasons/times, but in my geography would pump clean air in into the house for months out of the year.

https://naturescoolingsolutions.com/ecobreeze-2-1-smart-wind... exists, but is pricey and isn't from a major-enough manufacturer to make me confident of after-sales support.