Rachel, we have all got dust allergies and so on at chez Boffin. I may have some answers for you - local NHS allergy clinic taught us how to do it and it seems to have made a big difference.
It's all to do with developing an anti-dust and anti-mould regime. See it as an ongoing war. First up, as far as weaponry is concerned, you need to get a hoover with a HEPA filter if you haven't got one, because that doesn't recirculate the bastard dust back into the air as you clean (which sounds like what is going on at yours). You also need to hoover strategically under the beds and hoover your mattresses at least once a month as an absolute minimum, but preferably once a week. Another weapon in your arsenal is to wash bedding at 60C once a week to kill the dustmites, aka dust footsoldiers, and use anti-dustmite mattress and pillow covers (you probably don't need any other special merchandise). To continue the attack, you also need to open all the windows in a room for an hour after cleaning it to let the place air. Regarding mould, this time of year is terrible because of condensation on the window frames in the mornings, so airing for a couple of hours every morning and wiping the windows down with a dilute solution of bleach once a week will stop things getting too bad. I even have a steam cleaner to do them particularly thoroughly 2 or 3 times a year, but this is a refinement because my mum bought me the gadget. The steam cleaner is the nuclear warhead of housewifing.
If you do all this I would be heartily surprised if your asthma didn't improve rapidly.
If you fancy a holiday from your asthma, choose an Alpine holiday at a resort higher than 500m, because nasty evil dustmites do not exist at these altitudes. There are an enormous amount of seriously family friendly hotels in the Alps.
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