Vaporizers and humidifiers are based on the old notion that mist tents were beneficial for people with CF. Years ago, that was thought to be the best way to help CF patients breathe better. We now know better. As late as 1972, the mist tent was debunked as a viable option for CFers. "No evidence of improvement in ventilatory function was found in these patients during the use of a mist tent; instead there was a small but significant decline in their ventilatory function." (1)
Whether a vaporizer is safe and beneficial for CF patients depends on the type of vaporizer you're using. Essentially, a warm mist humidifier and vaporizer are the same thing. Both are designed to release moisture into the air, which can help moisten your nasal passages and airways. Vicks makes a waterless vaporizer, so that's one to avoid for us CFers. It uses a scented pad (usually a mentholyptus-based compound) to release vapors. Those are the things that can actually dry you out, and it's best to avoid them.
If you're looking to obtain the maximum benefit from a humidifier or vaporizer, your best bet is to use something that produces a warm mist that results from boiling water. Be careful however to keep the thing extra-ordinarily clean. Bacteria and molds love to grow in warm moist places, and if their spores get mixed in with what you're breathing in, you could have a major problem on your hands.
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(1) Moytoyama, E. K., Gibson, L. E., Zigas, C. J. "Evaluation of Mist Tent Therapy in Cystic Fibrosis Using Maximum Expiratory Flow Volume Curve." PEDIATRICS Vol. 50 No. 2 August 1972, pp. 299-306.