Some abstract nouns for mildew are blight, growth, or mustiness.

A noun for damp, stale air is must or mustiness; the adjectives are musty or dank.

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.

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.

To be obliged; to be necessitated; -- expressing either physical or moral necessity; as, a man must eat for nourishment; we must submit to the laws., To be morally required; to be necessary or essential to a certain quality, character, end, or result; as, he must reconsider the matter; he must have been insane., The expressed juice of the grape, or other fruit, before fermentation., Mustiness., To make musty; to become musty.