Inclusion criteria
* Have a photo identification (ID)
* Male or female individuals age between 18 and 40 years old
* BMI between 19 and 24.9 kg/m2
* Having capacity to consent to the study Exclusion criteria
* Evidence of any physical, mental, and/or medical conditions that would make the proposed studies relatively more hazardous
* Prior high altitude pulmonary edema (HAPE) or high-altitude cerebral edema (HACE) diagnosis
* Born at altitudes greater than 2,100 m (\~7,000 ft)
* Systemic disease with or without any functional limitation; including
* controlled hypertension
* controlled diabetes without systemic effects
* Pregnancy determined by urine pregnancy test, detecting presence of human chorionic gonadotropin (hCG), or less than six weeks postpartum
* Women who are not willing to receive urine pregnancy tests
* Active smoking. Volunteers may be enrolled if they quit smoking for more than 1 year.
* Excess alcohol use: more than ½ L/day of wine consumption or equivalent
* Any current medication use except oral contraceptives.
* Living in areas that are more than 1,200 m (\~4,000 feet), or have traveled to areas that are more than 1,200 m for more than four days within the last 2 months
* Tobacco chewers
* Abnormal hemoglobin or hematocrit levels or presence of hemoglobin S
* Evidence of apnea or other sleeping disorders
* Evidence of asthma
* Lower respiratory infection within the last 30 days
* If applicable, unwilling to refrain from using energy drinks or other caffeinated beverages for 7 days prior to and during the study
* If applicable, unwilling to refrain from use of all over-the-counter oral medications, herbal remedies, and nutritional supplements for 7 days prior to and during the study
* Not willing to have blood drawn from an arm vein each test day of the study
* Claustrophobia (inability to wear a facemask) or other active psychiatric conditions or not willingness to cooperate with the investigators and the other medical team
* Currently enrolled in another research study
* Facial abnormalities that would preclude proper use of a face mask
Pregnancy Prevention/Testing: Women using oral, subdermal or injectable contraceptives, and those using other means of birth control may participate. A urine pregnancy test will be conducted as part of the screening process for study participation no more than 7 days before starting the study. The test result will be read by a female staff member who will keep the result confidential. If a woman declines to have a pregnancy test, she will not be able to participate.