Humidifiers are devices that increase humidity, improving indoor air quality and preventing health problems caused by dry air. Examples of products include ultrasonic humidifiers, which use vibration to produce a fine mist, and evaporative models, which heat water and release steam. Humidifiers often have adjustable humidity levels, timer functions and automatic shut-off for added safety. They are particularly suitable for people suffering from dry air, such as allergy sufferers, asthmatics and babies' rooms.