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I am very concerned about particles / dust / fibers from assorted bass trapping materials getting into the air. I am generally quite allergic to airborne particles.

Most folks wrap bass trapping materials / insulation with tight fabric which likely holds in most of the fibers / particles. As well, unless you're beating your bass traps with a stick every day, you shouldn't really have a problem anyway.

However, I am still neurotic about keeping the air quality in my room 100% fiber-free. Occasionally I'll have fans or an air conditioner blowing hard while rehearsing, I'm sure having a fan blow against a fabric wrapped trap could potentially push out some fibers into the air, etc.

When making a custom bass trap out of 703 / 705 / mineral wool etc, one thought is to first cover the insulation with a sheet of plastic (like 3 mil, like a plastic trash bag), and then cover that with decorative fabric. The plastic should positively hold in all fibers from reaching the air... even if you beat it with a stick. I like that level of "protection" from airborne particles.

Realistically, is a 3 mil sheet of plastic over your mineral wool yet under your decorative final layer of fabric going to harm acoustics in any way?

Maybe the plastic is just not necessary, but it would give me good peace of mind knowing that zero fibers can reach the air inside my room.

One thought I had for one of my very small rehearsal / tracking rooms...

Against one entire wall, floor to ceiling, install a row of 2"X3" studs on 24" centers, 6 inches out from the main wall... then add some 1"X4"s or furring strips to the back side of the new studs, horizontally... then add 703, 705 or mineral wool between the studs against the furring strips so it's all 6" out from the main wall. Next, staple plastic sheeting over this entire new wall of insulation. Final treatment, add a big hanging curtain made of decorative fabric in front of this new wall of insulation. Then the entire wall, floor to ceiling would be one huge bass trap.

The advantage of using a hanging "curtain" in front is that the fabric could then be removed easily for periodic washing. If the fabric was stapled to the frame as with most bass traps, this makes cleaning more difficult.

Behind the curtain would be the plastic sheeting which would seal out 100% of any fibers that otherwise might make it into the air (even if just a few).

I realize that this would be an extreme level of bass trapping, but since this room is quite small with a low ceiling, having one wall treated like this might be a good thing. In this scenario, would the plastic sheeting have any sonic drawbacks?

Thanks