A continuous positive airway pressure machine or CPAP is a device that uses room air delivered through a mask system to keep the airway open during sleep. A CPAP is generally used to treat mild obstructive sleep apnea by preventing the airway from collapsing during sleep. CPAPs are typically used with a heated humidifier that uses heat to turn water into a mist delivered with the air to prevent nasal passages and the mouth from getting dried out due to the airflow. Distilled water is advisable to use in the humidifier to avoid mineral build-up and deposits on the metal tank in the humidifier. Another comfort feature on CPAPs is called a ramp. This means the pressure will start lower and slowly build or ramp up to the prescribed level. A CPAP can either be a standard CPAP set at a single pressure or an Auto that has a range of pressures that adjust automatically based on the user's breathing. All pressures or ranges must be set according to a prescription from a doctor and cannot be modified without a prescription.