Humidifiers add moisture to dry indoor air, preventing skin irritation, nasal congestion, and respiratory issues during winter or in arid climates. They maintain 40-60% relative humidity for optimal comfort and health. Types include ultrasonic (vibration mist, quiet), evaporative (fan-blown filter, self-regulating), impeller (cool mist for kids), steam vaporizers (boiled water, antibacterial), and central HVAC systems. Medical humidifiers attach to ventilators or oxygen therapy, warming and moistening gases to protect airways in ICUs and homecare. Features include auto shut-off, hygrostats, antibacterial tanks, and variable mist output.