They have a very small three-phase motor to run the fan and a mechanical 4-position switch to control it. There's no built-in timer or other noisy circuitry needed to run it. It does have an ionizer that makes some electrical noise (and possibly some ozone), but it can be turned off by way of another mechanical switch. (You can tell with an AM radio that the power supply for the ionizer is shut down when the switch is off.)
Mine came with a "HEPA-type" filter, but there are real HEPA filters for it also.
I'd be interested in what other people on here have tried. (The only other purifier I have experience with is a Honeywell 17007-HD, which was loud and made a lot of electrical noise. I wouldn't recommend it.)
I don't think it emits a lot, no DE coming out for sure. Pretty silent.
Let me know if you want me to test anything on it, I can test M/E.. and radio too..
I think it helps me. It has a special "OFS" filter that has a weird smell though, I took it out.. the HEPA part of it is good enough. It does have a control board, so might be too much for you.
I also have a Honeywell similar to the one you posted , sit flat and is round in shape. On silent it's good, it's 360 degrees of filtering, and just a plain fan, nothing else, no board. But I have a feeling that one is worse than this one. I also read that with a laser particle counter, you will find that some of these filters are "fake" and they spread around dust more than filter. Never got to test that though, I am looking at buying a Particle meter.