system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can you measure someone’s blood alcohol level just from their breath?
gpt
The alcohol is in your blood. The blood runs through the lungs in order to exchange oxygen and carbon dioxide, but alcohol is also small and light enough to transfer out of the blood and into the air in the lungs. Breath out, especially a long, full breath that will expel as much air as possible from the deep in the lungs, and alcohol vapor comes out with it.