Secondhand smoke (SHS) is an aerosol containing over 7,000 chemicals, including at least 69 known carcinogens. Determining how long SHS remains in the body is complex because exposure involves absorbing this mixture of compounds, not just the visible smoke. These substances are absorbed through the lungs, skin, and mucous membranes. The duration of exposure is traceable based on the body’s metabolic breakdown of these chemical components and their subsequent excretion.
Identifying Biological Markers of Exposure
To measure exposure to secondhand smoke, scientists look for specific chemical byproducts, known as biomarkers, in the body. The primary chemical absorbed is nicotine, which is quickly metabolized in the liver by the cytochrome P450 2A6 (CYP2A6) enzyme. This metabolic process converts most absorbed nicotine into a compound called cotinine.
Nicotine is a poor marker for long-term exposure because it has a very short half-life, with half of the compound eliminated from the bloodstream in approximately two hours. Cotinine is the preferred compound for tracking exposure because its average half-life is much longer, ranging from 16 to 20 hours. Tracking cotinine levels provides a reliable way to determine the extent and duration of secondhand smoke exposure.
Short-Term Clearance in Bodily Fluids
The detection window for cotinine varies significantly based on the biological fluid tested and the intensity of exposure. In blood plasma, cotinine from a single exposure can remain detectable for up to 10 days. Nicotine clears much faster, typically within one to three days. Saliva testing is highly sensitive and can detect cotinine for four to seven days following exposure.
Cotinine concentrations are typically higher in urine than in blood or saliva. For light exposure, the compound is usually cleared from urine within three to four days. However, for individuals with prolonged or heavy exposure, cotinine may remain detectable in urine for up to two to three weeks.
Individual Factors and Long-Term Detection
The clearance timelines for cotinine are general ranges, as multiple individual factors influence the rate of metabolism. Genetic variations in the CYP2A6 liver enzyme significantly affect how quickly nicotine is converted to cotinine and cleared from the body. Individuals with less efficient versions of this enzyme break down cotinine more slowly, resulting in a longer detection window.
Other physiological factors, such as kidney and liver function, body composition, and hydration levels, also contribute to variable clearance rates. For a historical record of exposure, testing can use non-fluid samples that capture metabolites incorporated into the body’s keratin structures. Hair and nail samples can reliably detect cotinine, providing a lookback window of approximately three months. This long-term detection method indicates historical exposure, but it does not measure how long the smoke has actively remained in the system.
The Persistence of Environmental Residue
The persistence of secondhand smoke also relates to Thirdhand Smoke (THS), which describes the residue that persists in the environment. THS is the sticky chemical contamination that settles on indoor surfaces, clothing, carpets, and dust after the visible smoke has dissipated. This residue is a collection of toxins, including nicotine and cancer-causing Tobacco-Specific Nitrosamines (TSNAs).
These compounds become embedded deeply within materials and are difficult to remove with standard cleaning methods. The persistence of THS is measured in months or even years, especially in environments where smoking occurred regularly. Exposure to THS occurs through inhaling re-emitted gases, ingesting contaminated dust, or direct skin contact. This lingering residue means the potential for continued, low-level exposure remains long after biological markers have cleared from a person’s system.