Some abstract nouns for mildew are blight, growth, or
mustiness.
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A noun for damp, stale air is must or mustiness; the adjectives
are musty or dank.
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Yes it is dangerous. Mustiness isn't so bad (caused by
hot/cold/damp) but mold has been known to make people ill. If the
mold is in the walls and you rent then get out. If you own the home
then you have to consider getting to the source of the problem and
making repairs.
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In a cave, you may be able to taste the dampness or mustiness of the air. Additionally, if there are any mineral formations or water sources within the cave, you may be able to taste the mineral content in the water.