The Illinois State Police owns and services breath test instruments used by law enforcement for measuring breath alcohol content of motorists suspected of driving under the influence of alcohol. The instruments are supposed to test deep alveolar breath - breath coming from the alveolar regions of the lungs. The alveolar are very thin tissue where oxygen mixes with blood. This deep lung air is supposed to have a good relationship to blood, and by testing it, the instrument is intended to provide an accurate measurement of blood alcohol content. If you have a basic understanding of human physiology, you may be suspect already, but just wait.