During an asthma attack, a person may have a lower blood oxygen level, as asthma can reduce the capacity to take in oxygen. Asthma can cause airways in different parts of the lung to become occluded, resulting in impaired exchange of oxygen between the air sacs in the lung and blood flowing through the lungs. As a result, less oxygen enters the bloodstream for circulation throughout the body. However, during most asthma attacks, oxygen levels remain normal or at least in the normal range. This is because air can still get in, and oxygen can get past obstructed airways and travel to alveoli. From there, it crosses over to the blood system, and in this way, oxygen levels stay in the normal range even during most asthma attacks.

It is important to note that oxygen saturation is not a good indication of how bad an asthma flare-up is. Asthmatics are good oxygen compensators and can have near-normal or completely normal oxygen stats during the midst of a severe asthma attack. Asthma is a disease of the airways, not the little air sacs (alveoli) where the gas exchange itself takes place. So even if the airways are very constricted and tight, often enough oxygen is able to pass through to the blood and therefore show a normal or near-normal oxygen saturation.

If a person with asthma experiences low oxygen levels, they should contact a medical professional as soon as possible. The goal of treatment should be to maintain oxygen saturation at >92% . A pulse oximeter can be used to measure oxygen saturation levels in the blood. However, it is important to note that there are no data that demonstrate that monitoring oxygen levels with an oximeter will help a person manage their asthma.