When you take a breathalyzer exam, it’s likely to measure inaccurately your actual BAC (blood alcohol content) level. There are several reasons why breathalyzers are unreliable, but primarily they are inaccurate because they only indirectly measure an individual’s BAC level (whereas a blood test is a direct measure). Secondly, breathalyzers ignore the physical variables from person to person; the test assumes that all of us have the same health, body temperature, and weight, and that none of us use prescription medications. If you’re charged with DUI in California, an experienced DUI defense attorney will challenge the results of any breathalyzer exam.

Breathalyzers work on a principle that scientists call Henry’s Law, one of the gas laws formulated by William Henry in 1803. Henry’s Law holds that if we know the amount of alcohol in the blood, and if we know the temperature of the blood, then we can use a formula to determine how much of the alcohol in the blood has evaporated in the air. Breathalyzers actually use the reverse principle; if we measure the alcohol in the breath, we can estimate the alcohol in the blood. However, a breathalyzer assumes that everyone has the same blood and breath temperature. In California’s state laboratories, the assumption is that the breath temperature is 34 degrees Celsius.

Henry’s Law states that at 34 degrees Celsius, a person’s breath will contain 82.7% of the alcohol that is in the person’s blood. The research that arrived at this finding was conducted in 1950, and only seven subjects were tested. More recent research, conducted by several different scientists using hundreds of subjects, has discovered that the actual human breath temperature is closer to 35 degrees Celsius. However, for every one degree increase in breath temperature, a breathalyzer will overstate a person’s true BAC level by about 6 percent.

A good DUI defense lawyer can probably discredit any breathalyzer test being used as evidence against you. If you’re charged with a DUI in California, it’s a serious matter. Get the legal help you need and contact an experienced DUI defense attorney immediately.