When we breathe, oxygen enters the lungs and then crosses into the blood. Hypoxemia occurs when the lungs cannot transfer enough oxygen into the blood. In children, breathing problems are the most common cause of hypoxemia.
Some heart problems can cause hypoxemia, but unknown heart disease in school-aged children is rare (because the majority of children with significant heart disease are diagnosed as infants).
High altitude (above 10,000 feet) certainly contributes to hypoxemia, especially during exercise. However, at Denver’s altitude, hypoxemia during exercise (specifically an oxygen saturation under 92%) is not normal and requires evaluation.
Parents and other caregivers need to be aware of the symptoms of low blood oxygen levels. If your child experiences these symptoms, contact your healthcare provider. They can review your child's overall medical condition and decide what treatment is necessary.