Friends complain about their healthy small children get nightly asthma-like symptoms lately - They are living in 3 different states. Please help to find the reasons!
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Edit December 24 2012:
Thank you for all of your helpful posts.
The problem solved at one of the children by a more thorough cleaning of the humidifier and she filled it up with apple-cider vinegar solution.
Next night went without any symptoms, and so it go since.

The other 2 kids will try apple cider vinegar, too.

For those with similar problem who find this thread by search:
Google Spongea and Allergene besides the methods described here.