Carbon dioxide tolerance test?

The Carbon Dioxide Tolerance Test is used to measure an individual's capacity to engage the calming aspect of their nervous system and to consciously control their diaphragm. Andrew Huberman explains the steps to perform this test:

1. Begin in the morning after waking up, avoiding electronic devices.
2. Perform four breath cycles:
   • Inhale deeply through the nose and exhale fully.
   • Repeat four times.
3. On the fifth cycle, perform a maximal inhale through the nose.
4. Start a timer and try to exhale slowly, as if through a tiny straw, until you can exhale no more. The duration of this exhale is measured and indicates your ability to offload carbon dioxide 1.

The test is useful for both athletes and non-athletes and can be a measure of the overall systemic health related to breathing efficiency and nervous system balance.