At a glance
| Chemical family | Pyridine alkaloid; primary nicotine metabolite |
| CAS number | 486-56-6 |
| IUPAC name | (S)-1-Methyl-5-(pyridin-3-yl)pyrrolidin-2-one |
| IARC classification | Not classified by IARC. Cotinine is a metabolite biomarker, not a hazard in its own right. The carcinogenic compounds in the third-hand smoke story are the nitrosamines NNK and NNN; the addictive and cardiovascular agent is the parent nicotine. |
| Half-life | ~16 hours in adults (Benowitz, Hukkanen & Jacob 2009) — versus ~2 hours for nicotine. The longer cotinine half-life is what makes it the practical exposure biomarker over the past 1–4 days. |
| Where it appears | Urine, serum/plasma, saliva, and hair of anyone exposed to nicotine — including third-hand smoke exposure from contaminated indoor environments |
| Sleep micro environment relevance | Child urinary cotinine correlates strongly (rho 0.65–0.81) with bedroom pillowcase nicotine (Quintana et al. 2023), establishing the chain from contaminated bedding to measurable internal dose. The biomarker that proves the third-hand smoke pathway is delivering exposure to the sleeper. |
| Activated carbon capture | Out of scope — cotinine appears in body fluids, not on bedding. The exposure-reducing actions target the upstream nicotine and TSNA reservoirs. |
What it is
Cotinine is the principal metabolite of nicotine in humans, formed in the liver by the cytochrome P450 enzyme CYP2A6. Approximately 70–80% of absorbed nicotine is converted to cotinine; smaller fractions are excreted unchanged or metabolized to other compounds. The pharmacology, metabolism, kinetics, and biomarker use of nicotine and cotinine are comprehensively reviewed in Benowitz, Hukkanen, and Jacob 2009 in the Handbook of Experimental Pharmacology, which remains the standard reference for the field. Peer-reviewed
Cotinine itself is not classified by IARC as a carcinogen. It is a downstream marker of nicotine exposure, not the hazardous compound. The carcinogenic compounds in the tobacco-smoke story are the nitrosamines NNK and NNN (both IARC Group 1), classified in Monograph 89. The addictive, cardiovascular, and developmental effects are the parent nicotine. Cotinine is included in the Atlas because it is the most useful tool for quantifying the exposure these other compounds produce — not because it is itself a hazard.
The half-life is what makes cotinine the practical biomarker. Nicotine clears from plasma with a half-life of approximately 2 hours, which means it can document exposure only within hours of the event. Cotinine clears with a half-life of approximately 16 hours, which extends the detection window to 1–4 days after exposure ends. For third-hand smoke research, where exposure is intermittent and may happen anywhere in a child's day, cotinine is the time-window that catches it. Peer-reviewed
How cotinine is used in research
Population biomonitoring
Pirkle et al. 1996, in JAMA, established the foundational reference for US population cotinine biomonitoring using the Third National Health and Nutrition Examination Survey (NHANES) data from 1988–1991. The paper documented detectable serum cotinine in approximately 88% of nonsmokers, demonstrating that environmental tobacco smoke exposure was nearly ubiquitous at the population level in that era. Subsequent CDC NHANES updates have tracked the decline in nonsmoker cotinine through smoking bans and shifts in public smoking policy, but Pirkle 1996 remains the methodological reference. Peer-reviewed Regulatory
The conventional cut-point distinguishing active smokers from nonsmokers in NHANES-style analyses is approximately 3 ng/mL serum cotinine (or equivalent urinary concentrations). This cut-point is calibrated to identify active tobacco use, not to identify whether a nonsmoker has been exposed to second- or third-hand smoke. For third-hand smoke research, the relevant signal is any detectable cotinine in a nonsmoker — typically well below the active-smoker cut-point but above population background.
The third-hand smoke chain of evidence
The most direct demonstration that third-hand smoke produces real biological dose in the exposed person comes from Quintana et al. 2023 in the Journal of Environmental Exposure Assessment. The team deployed cotton pillows as passive samplers in 35 children's homes with mixed smoking exposure and measured pillowcase nicotine alongside the child's urinary cotinine. Pillowcase nicotine correlated strongly with both air nicotine (Spearman rho 0.76–0.88) and with child urinary cotinine (rho 0.65–0.81, all p < 0.001). The pillow tracked the room's nicotine chemistry, and the child's urinary cotinine confirmed that the surface chemistry was producing real internal dose. Peer-reviewed
Matt et al. 2017, in Tobacco Control, measured urinary cotinine alongside surface nicotine in the homes of former smokers six months after smoking cessation. Both markers remained measurable in many homes long after smoking stopped — establishing the third-hand-smoke exposure pathway at the biomarker level, not just at the chemistry-on-surfaces level. Matt et al. 2020 in Preventive Medicine Reports extended this finding into multiunit housing, where surface contamination and biomarker-detectable exposure persisted across tenancy changes. Peer-reviewed
Northrup et al. 2016, in Tobacco Control, applied the same biomarker framework to NICU environments — measuring nicotine surface contamination and infant biomarker exposure in hospital settings where vulnerable patients were being cared for. The combination of surface chemistry and biomarker confirmation is the standard third-hand-smoke evidence package. Peer-reviewed
Children's exposure assessment
Merianos et al. 2023 in the Journal of Exposure Science & Environmental Epidemiology measured surface nicotine and the nitrosamine NNK on indoor surfaces in children's homes alongside child cotinine. They detected NNK on surfaces in 48.8% of homes sampled — many in households with no reported active smoking, indicating prior contamination or external sources delivering measurable exposure to children. Peer-reviewed
Bahl et al. 2014 in PLOS ONE, while not a biomarker paper per se, established the developmental exposure multiplier that gives cotinine measurements in infants and toddlers their interpretive weight: toddlers received approximately 6.8× and infants approximately 24× the adult-equivalent dermal dose from contact with contaminated fabric. A given surface nicotine concentration produces a higher internal dose — and a higher urinary cotinine — in the child than in the adult sharing the same environment. Peer-reviewed
What helps reduce the exposure cotinine measures
Tier 1 — most effective. Cotinine is the downstream marker; the upstream targets are the nicotine and TSNA reservoirs that produce the exposure. Eliminate active smoking and vaping in the bedroom. For inherited contamination, replace soft furnishings (mattresses, pillows, upholstered furniture, carpet) in homes with significant prior smoke exposure. The cotinine drops when the exposure drops; the surface-targeted actions described in the nicotine entry and the flagship The Smoke That Stays are the actions to take.
Tier 2 — measurement as feedback. Periodic cotinine measurement (especially in children's exposure assessment) can document whether remediation is reducing real internal dose, not just measurable surface concentration. The biomarker is the audit trail for the cleanup. Discuss with a pediatrician or environmental health provider whether biomarker testing is useful in a specific situation.
Tier 3 — research participation. The cohort studies that have built the third-hand-smoke biomarker evidence (Matt, Quintana, Merianos, Northrup) are an ongoing area of research. Households navigating significant third-hand smoke exposure may benefit from connecting with research programs in their region that can provide both biomarker measurement and remediation guidance.
What does NOT reduce cotinine
"Detoxing" practices do not affect cotinine clearance. Cotinine is metabolized by hepatic enzymes on a fixed pharmacokinetic schedule. The 16-hour half-life is what it is; sweating, fasting, or "cleanse" products do not accelerate it. The way to reduce measured cotinine is to reduce ongoing exposure; the body does the rest in 1–4 days.
Cotinine testing itself does not reduce exposure. The biomarker measures what the exposure is producing; the action is on the upstream chemistry.
Negative cotinine does not prove zero exposure. A measurement below the detection limit at one time point only proves that exposure in the previous 1–4 days was below the assay's sensitivity. Intermittent or low-frequency exposure (a weekly visit to a smoke-affected environment, for example) may not be captured by a single test.
Open research questions
- The dose-response relationship between cotinine concentrations in nonsmoker third-hand smoke exposure and quantified downstream health outcomes (respiratory, developmental, cardiovascular) has been characterized at the population level but is still under-characterized at the bedroom-specific exposure level. Inferred
- The accuracy of consumer-grade cotinine tests at the low concentrations characteristic of third-hand smoke exposure has not been benchmarked against research-grade LC-MS/MS in published head-to-head studies. Spec
- The CYP2A6 polymorphism that affects individual cotinine clearance rates means that cotinine concentration alone does not perfectly index exposure — slow metabolizers register higher cotinine for a given nicotine exposure. The implications for individual interpretation versus population research are an active area. Peer-reviewed
Citations
- Benowitz NL, Hukkanen J, Jacob P III. Nicotine chemistry, metabolism, kinetics and biomarkers. Handbook of Experimental Pharmacology. 2009;192:29-60. doi:10.1007/978-3-540-69248-5_2 Peer-reviewed
- Benowitz NL. Cotinine as a biomarker of environmental tobacco smoke exposure. Epidemiologic Reviews. 1996;18:188-204. doi:10.1093/oxfordjournals.epirev.a017925 Peer-reviewed
- Pirkle JL, Flegal KM, Bernert JT, Brody DJ, Etzel RA, Maurer KR. Exposure of the US Population to Environmental Tobacco Smoke: The Third National Health and Nutrition Examination Survey, 1988 to 1991. JAMA. 1996;275(16):1233-1240. doi:10.1001/jama.1996.03530400021033 Peer-reviewed
- Quintana PJE et al. Cotton pillow samplers for assessment of thirdhand smoke in homes of smokers and nonsmokers with children. Journal of Environmental Exposure Assessment. 2023;2:23. doi:10.20517/evcna.2023.34 Peer-reviewed
- Matt GE et al. When smokers quit: exposure to nicotine and carcinogens persists from thirdhand smoke pollution. Tobacco Control. 2017;26:548-556. doi:10.1136/tobaccocontrol-2016-053119 Peer-reviewed
- Merianos AL, Mahabee-Gittens EM, Hoh E, Quintana PJE, Matt GE. Contamination of surfaces in children's homes with nicotine and the potent carcinogenic tobacco-specific nitrosamine NNK. Journal of Exposure Science & Environmental Epidemiology. 2024;34(4):727-734. doi:10.1038/s41370-023-00629-8 Peer-reviewed
- Northrup TF et al. Thirdhand smoke contamination in hospital settings: assessing exposure risk for vulnerable paediatric patients. Tobacco Control. 2016;25:619-623. doi:10.1136/tobaccocontrol-2015-052506 Peer-reviewed
- Bahl V, Jacob P III, Havel C, Schick SF, Talbot P. Thirdhand Cigarette Smoke: Factors Affecting Exposure and Remediation. PLOS ONE. 2014;9(10):e108258. doi:10.1371/journal.pone.0108258 Peer-reviewed
- Matt GE et al. Persistent tobacco smoke residue in multiunit housing: Legacy of permissive indoor smoking policies. Preventive Medicine Reports. 2020;18:101088. doi:10.1016/j.pmedr.2020.101088 Peer-reviewed
- Sleiman M et al. Formation of carcinogens indoors by surface-mediated reactions of nicotine with nitrous acid, leading to potential thirdhand smoke hazards. PNAS. 2010;107(15):6576-6581. doi:10.1073/pnas.0912820107 Peer-reviewed
Frequently asked questions
What is cotinine?
Cotinine — CAS 486-56-6 — is the primary metabolite of nicotine in the human body. About 70–80% of inhaled or absorbed nicotine is converted to cotinine in the liver via cytochrome P450 2A6 (CYP2A6). Cotinine has a half-life of approximately 16 hours in adults (vs nicotine's ~2 hours), which is what makes it the standard biomarker for documenting tobacco-smoke exposure across the past 1–4 days, including third-hand smoke exposure.
Is cotinine itself a carcinogen?
No. Cotinine is not classified by IARC as a carcinogen. It is a metabolite — a downstream marker — not the hazardous compound itself. The hazards associated with tobacco-smoke exposure come from the parent nicotine (addiction, cardiovascular, developmental effects), the nitrosamines NNK and NNN (IARC Group 1 carcinogens), and the broader combustion-product mixture. Cotinine is included in the Atlas because it is the most useful tool for quantifying that exposure, not because it is a hazard in its own right.
Why is cotinine the biomarker instead of nicotine itself?
Nicotine has a half-life of ~2 hours in adults — too short to reliably document exposure unless you happen to measure within hours. Cotinine's ~16-hour half-life means it can document exposure over the previous 1–4 days, making it the practical biomarker for both active smoking, secondhand smoke, and third-hand smoke contexts. Benowitz, Hukkanen, and Jacob 2009 review the pharmacokinetics of both compounds in the Handbook of Experimental Pharmacology.
How is cotinine measured in studies?
Urinary cotinine via LC-MS/MS is the gold standard for population studies. Serum cotinine via the same method is used in clinical and CDC NHANES contexts (Pirkle et al. 1996 JAMA established the foundational US population reference). Salivary cotinine is a non-invasive alternative used in pediatric research. All three correlate strongly; urinary is most sensitive for low-level (third-hand smoke) exposure and is the standard in research-grade studies.
What cotinine concentration is used to distinguish smokers from nonsmokers?
The conventional cut-point in adults is roughly 3 ng/mL serum cotinine (or equivalent urinary levels), set by NHANES analyses including Benowitz et al.'s reference work. Nonsmokers with second- or third-hand smoke exposure typically have detectable cotinine well below this cut-point — meaning the cut-point distinguishes active smokers, not exposed-versus-unexposed nonsmokers. For third-hand smoke research, any detectable cotinine in a nonsmoker is the relevant signal.
How does cotinine link a contaminated bedroom to actual exposure?
The classic demonstration is Quintana et al. 2023 in the Journal of Environmental Exposure Assessment. They deployed cotton pillows as passive samplers in 35 children's homes and measured pillowcase nicotine alongside child urinary cotinine. Pillowcase nicotine correlated strongly with air nicotine (rho 0.76–0.88) and with child urinary cotinine (rho 0.65–0.81, all p < 0.001). The pillow surface concentration tracks the room's tobacco-smoke chemistry, and the child's urinary cotinine confirms the surface chemistry produces real internal dose.
Can I get my own cotinine measured?
Yes — commercial labs offer urinary, serum, and salivary cotinine testing, often used in life insurance underwriting, smoking-cessation programs, and occupational health contexts. For third-hand smoke exposure assessment, the consumer-grade tests are less sensitive than research-grade LC-MS/MS used in published cohort studies, but a positive result in a nonsmoker is meaningful evidence of exposure. Discuss the clinical relevance with your healthcare provider.
Related compounds
This page describes documented chemistry and the biomarker science that quantifies exposure. It does not provide medical advice. Anyone considering cotinine testing or biomarker assessment of their own exposure should consult their healthcare provider for clinical interpretation.
Last reviewed May 31, 2026.