Cigarette smoking leads to measurable levels of nicotine in the blood. Nicotine is broken down into cotinine, which remains in the blood longer than nicotine and is a better indication (marker) of whether a person is an active smoker. Most people have cotinine in their blood, especially if they live with a smoker and are exposed to secondhand smoke. Saliva, blood, or urine can be used to measure cotinine levels.
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