system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does a breathalyzer accurately tell how much alcohol is in your blood, from your breath?
gpt
Your lungs are essentially a big permeable membrane separating your blood from the outside air. Smaller molecules like carbon dioxide, oxygen, water vapor and indeed alcohol are free to move through this membrane. So the alcohol in your blood will evaporate into your lungs and you breathe it out. The higher the concentration of alcohol in your blood the higher the concentration of alcohol in your breath.

It is very hard to chemically analyze air for the presence of a specific molecule like alcohol. It can be done but require a lot of advanced equipment. So most police breathalyzers do not actually do this. Instead they contain a sensor which detect any volatile organic compounds. Basically anything that will burn. There is a small heater which will heat up the air and then sensors will detect how hot the air gets. If it gets hotter then expected then something in the air must be contributing with energy, such as burning alcohol. The breathalyzer does correct for things like airflow, humidity and temperature which it can measure with other sensors.

This does mean that it is very hard to fool a breathalyzer into reading low. Anything you may put in your mouth will either have no effect or cause the air to be even more flammable. There are extremely few substances which are harmless to humans and reduces the flammability of the air. So unless you happens to carry around some fluorketone just for this occasion and manages to take it without the police officer noticing it you are going to trigger the breathalyzer if you are drunk.

On the other hand since the breathalyzer is not actually measuring blood alcohol levels but use proxies it can often be defeated in court. So police do not use this as their only piece of evidence, or even the main evidence of drunk driving. But it can give probably cause to conduct more invasive searches such as looking for open or empty alcohol containers in the vehicle or taking a blood or spit sample for further laboratory testing.