Third-hand smoke (THS) is the invisible residue left behind by tobacco use that clings to indoor surfaces and dust. It is not the smoke itself, but rather the chemicals that persist and accumulate in the environment. This residue is concerning because the compounds can react with common indoor pollutants, such as nitrous acid, to create a more toxic mixture over time. These chemical remnants become embedded in materials like carpets, upholstery, and walls, where they can be released back into the air or transferred through contact. Exposure is especially concerning for young children, who frequently touch contaminated surfaces and then put their hands in their mouths, potentially leading to ingestion of the toxic compounds.
Chemical Markers Used to Identify Third-Hand Smoke
Testing for third-hand smoke centers on identifying specific chemical compounds deposited from tobacco use. Nicotine, a primary component of tobacco smoke, is the most reliable marker for residual smoke because it adheres strongly to surfaces and is rarely found in indoor environments from other sources. Scientists also monitor cotinine, which is the primary metabolite of nicotine.
The presence of these tobacco-specific compounds is concerning because they are precursors to more dangerous chemicals. Nicotine residue can undergo a chemical reaction called nitrosation, combining with indoor air pollutants to form toxic substances. These secondary pollutants include tobacco-specific nitrosamines (TSNAs), which are known carcinogens. One specific TSNA, NNA, is particularly relevant as it forms when nicotine ages and reacts with nitrous acid, highly indicating third-hand smoke contamination.
Environmental Testing of Surfaces and Dust
Assessing contamination relies on collecting samples from surfaces and dust, the primary reservoirs for chemical residues. For hard, non-porous surfaces like glass or metal, surface wipe sampling is the most common method. Researchers use a standardized protocol involving a pre-moistened wipe to collect residue from a measured area, often one square meter. The collected sample is then sealed and sent for laboratory analysis.
For porous materials like carpets and upholstery, dust collection is the preferred method. Specialized high-volume vacuum cleaners equipped with collection filters are used to gather settled dust from floors and other surfaces over a defined period. Nicotine and TSNAs are concentrated in this collected dust, providing an integrated measure of contamination. Analyzing the dust is particularly important because children are more likely to come into contact with floor surfaces.
Once collected, samples are subjected to laboratory techniques. The gold standard for identifying and quantifying chemical residue is mass spectrometry, often coupled with liquid chromatography (LC-MS/MS) or gas chromatography (GC-MS). This sophisticated instrumentation separates the chemicals and measures their unique molecular weights, allowing scientists to confirm the presence and concentration of nicotine, cotinine, and various TSNAs. Data is typically reported as micrograms of nicotine per square meter of surface area (µg/m²) or per gram of dust.
Assessing Human Exposure Through Biological Samples
While environmental testing measures external contamination, researchers assess internal exposure by analyzing biological samples from individuals. This biomarker testing measures the presence of tobacco-specific chemicals and their metabolites in bodily fluids to determine if a non-smoker has absorbed third-hand smoke chemicals.
Urine and saliva are the most common biological matrices used, as they are non-invasive and contain measurable concentrations of nicotine’s breakdown products. The most widely used biomarker is cotinine, the major metabolite of nicotine, which has a half-life of around 16 hours, making it an excellent marker for recent exposure. Urinary cotinine levels are often four to five times higher than in blood or saliva, providing a more sensitive indicator for detecting low-level exposure.
In addition to cotinine, researchers look for NNAL, a metabolite of the carcinogenic TSNA, NNK. NNAL remains in the body much longer than cotinine, with a half-life of about three weeks, allowing for the assessment of longer-term exposure. The NNAL to cotinine ratio is sometimes used to help distinguish third-hand smoke exposure from low levels of secondhand smoke. Alternative samples, such as hair or fingernails, can also be collected to assess cumulative exposure.
Accessibility and Limitations of Current Testing Methods
Professional third-hand smoke testing remains largely inaccessible to the general public. The techniques described, such as mass spectrometry analysis, require specialized equipment and trained personnel, resulting in high costs that limit their use primarily to research studies. Currently, there are no widely validated, reliable do-it-yourself (DIY) home testing kits available that can accurately measure third-hand smoke contamination.
Commercial kits claiming to test for nicotine often lack the necessary sensitivity to detect contamination levels found in typical homes. A negative result from such a kit may create a false sense of security, as detection limits are often significantly higher than levels identified in scientific studies.
A major scientific challenge is standardizing environmental sampling protocols across different materials and indoor settings, making comparison difficult. Another limitation involves the difficulty in definitively distinguishing third-hand smoke exposure from very low levels of secondhand smoke, although the NNAL to cotinine ratio offers a promising pathway for differentiation.