It is not usually necessary to buy a dehumidifier and there is no evidence that using a dehumidifier will improve symptoms. If you own a dehumidifier remember that it should be cleaned regularly to avoid a build up of moulds which could trigger symptoms. Paint Paint itself is usually only a problem when it is drying. Some paints claim to kill dust mites. There is no published medical evidence that these are shown to reduce symptoms and it is soft furnishings that harbour house dust mites not walls.
Some people prefer to wear a mask during decorating and even during everyday cleaning.
If you find that large amounts of dust are being thrown up when you vacuum it may be better to buy a vacuum cleaner with a more efficient exhaust rather than wear a mask which will only protect you in the short term. Most DIY shops and cycling shops sell masks of various types. Given the lack of evidence regarding medical benefits, if you decide to buy a mask, it is best to start with the cheapest and most comfortable as this may well be sufficient for everyday needs. Various chemical sprays and powders are available which kill dust mites or modify their allergen. It is worth noting that dust mites return very quickly even if their population is virtually wiped out.