If you are using a breathalyzer to keep track of your BAC level and monitor the amount you are drinking, you may want to reconsider smoking a cigarette. According to a British Medical Journal, smoking slows gastric emptying and therefore causes a delay in alcohol absorption into your body. This means that your results may be temporarily lower than they actually are. If you are depending on the machine to tell you when to stop drinking, this could allow you to drink more than you want to.
Blowing smoke into a keychain breathalyzer could also cause harm to the sensor. This could cause your machine to malfunction. It is important to read the directions and manuals that come with the device in order to get the longest possible life out of it. Most instructions say to wait at least twenty minutes after smoking before using the breathalyzer.