Many websites indicates that excessively dry air may cause epistaxis (i.e., nose bleeding).
Dry air is the most common cause of nosebleeds. Living in a dry climate and using a central heating system can dry out the nasal membranes, which are tissues inside the nose. This dryness causes crusting inside the nose. Crusting may itch or become irritated. If your nose is scratched or picked, it can bleed.
How humid should the air be to avoid epistaxis?