The State of Texas uses a machine called the Intoxilyzer 5000, which is commonly referred to as a breathalyzer. Breath testing is predicated on Henry's law of physics, which states that if temperature, and pressure is held constant the substances in the corresponding gases and liquids will reach equilibrium and will be present in equal proportions. This means that the same concentration of a substance will be present in liquid and gaseous states. The problem is that if someone's temperature is above 98.6 they will probably have a higher breath test result.
In all drunk driving cases, the prosecution must prove that the defendant's blood alcohol concentration at the time of the offense was at or above a statutory limit. In many states, the limit is .10 percent, but in others such as Texas it is .08, and there is a national movement afoot to make that the limit in all states. In order to prove the requisite level of alcohol in the blood of someone arrested for drunk driving, it is necessary to obtain a suitable sample of the arrestee's blood, urine, or hair at the time of arrest. The use of a breath test is by far the most popular scientific method for establishing that drunk driving has occurred. Some defendants, however, have been able to successfully challenge the results of such tests in court, thereby preventing a conviction.