Dust Allergies and bunk beds

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New to mumsnet.. so Hello.... My first post too.
I'm just curious really on this..
Due to space I've my sons in bunk beds. My older son has breathing problems and my younger son has dust allergies. Have anyone ever noticed that their breathing is more pronounced on the bottom bunk rather that the top. I'm looking maybe to find a mattress protector to go on the underside of the mattress (the side that the son on the bottom bunk would be looking at), has anyone ever found one that covers the whole mattress, or how would you get it to stay on?
Thank you

This isn't something I'd even considered so I'm subbing as my ds2 is asthmatic and il b putting my boys in bunks over the summer. Would 2 mattress protectors work? Like one one the top as normal and then one on the underside of the same mattress? Or would this bulk out too much to fit back in the frame?

We often sleep in bunk beds when youth hosteling. We know how dusty they are .
Yuck!
Unless it was an absolute necessity I wouldn't dream of it for any child with dust mite allergy.
You can get the smaller size beds, or use a bunk that forms a right angle-with the head end out.

Waltonswatcher good idea but its not an option at the moment. I've ordered a waterproof mattress online and I'm going to put it on the underside of the top bunk now I need to find some thick elastics to hold it in place or sew it.
Unfortunately I have no choice but to keep them in bunk beds so I'm hoping it works out.