An oximeter is a device used by doctors to measure blood oxygen levels. It's often used in hospitals to determine if lungs are functioning properly to oxygenate the body. Oximeters are non-invasive, fit snugly on your finger, and provide a digital reading in seconds. They're available inexpensively for consumers, as well. Under normal conditions, the oximeter reading should be above 97%. But for travelers at high altitudes who are unaccustomed to the elevation, oxygen levels can plummet.
Here's what one traveler experienced on a recent trip to Colorado: