Relative humidity

Relative humidity is the amount of moisture currently in the air expressed as a percentage of the maximum the air could hold at that temperature. Most comfort guidance targets roughly 30 to 50 percent indoors; below that range, air feels dry and irritates skin and sinuses, and above it, moisture starts to feel sticky and encourages mold and musty odors.

Small apartments are especially prone to humidity problems because they often have less airflow, fewer windows, and appliances like showers, stovetops and drying racks concentrated in a small area, all of which add moisture to the air faster than a larger home would experience relative to its size.

A hygrometer is the only reliable way to know your actual humidity level rather than guessing from how a room feels. Once you know the number, a dehumidifier addresses excess moisture and the musty smell that often comes with it, while a humidifier does the opposite in overly dry conditions, and neither is the right tool if you're guessing which problem you actually have.