hi there
i may be wrong but i always thought that a humidifier moistens the air
a de-humififier drys out the air like the difference with a steam room and sauna.
my kids dont have ashma but i do and ive always had a humidifier, as i find it easier to breathe with one on, especially if i get a chest infection
i bought one in argos for about 80 euro last year, expensive but it is well worth it the investment as it reduces the times ive had to go to the gp. it has a small vent in the top and you can add essential oils which i find brilliant. i use tea tree for its antiseptic properties and eucalyptus oil to help when i get congested.
i hope that helps you clear that up – and my heart goes out to your kids who suffer from ashma as its a horrible ailment to have. 🙁