The ideal relative humidity level is between 30 and 50%. Outside this range is unhealthy and can feel uncomfortable, too. Lower than this, bacteria, viruses, and irritants thrive. Higher than this, mold and mildew are likely to grow.
Keeping the humidity in the ideal range will lower energy bills. In dry regions or times of the year, you may need a humidifier. This will add water to the air. In wet regions or times of the year, you may need a dehumidifier. This will remove water from the air. To know the relative humidity level, obtain a humidity sensor and adjust the humidifier and/or dehumidifier until relative humidity is in the30-50% range.