Breathalyzers work by measuring a person's blood alcohol content, or BAC level. A personal breathalyzer can be a useful tool that may help you determine your BAC, but it is important to know that your BAC level can change quickly and is influenced by factors that vary from person to person. Individual factors like genetics and ethnicity can affect how quickly your BAC levels rise and how many drinks it takes for your body to reach a certain BAC level. Your BAC level can also continue to climb even after you finish drinking.