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The recommended humidity level is 40-60%.

Dry mucus membranes makes you more susceptible to a number of respiratory problems. If you have mold, everyone does to some extent, drying it out will just make the health problems worse as it aerosolizes the mold. Mold aside, for optimal health a level between 40-60% is advised. Allergy sufferers are especially sensitive to this.

This is very climate dependent. New Orleans or Miami will have much more natural humidity than Boulder or even Boston in the winter.