Biomarkers of secondhand smoke exposure (SHSe), characteristics and cut-off points for distinguishing smokers from non-smokers
| Biomarker | Half-life | Invasiveness | Cut-off point | Pros | Cons |
| Cotinine | Reflects recent SHSe | ||||
| Urine | 16 h (average) | Non-invasive | 50 ng/ml for higher SHSe | Higher concentrations than other matrices (higher sensitivity) | Need of facilities with privacy during collection Difficulty for population-based or children studies Need for creatinine clearance adjustment Collect data on renal disease and some prescription drugs |
| Blood | 16 h (average) | Invasive |
| No adjustment required for hydration | Pregnant women have increased clearance rate Difficulty for infants and young children Lower sensitivity |
| Saliva | 16 h (average) | Non-invasive | 14 ng/ml for higher SHSe | Good for multiple measurements over a limited period of time | Potential issues with age, gender, race, oral pH, type of diet, dehydration, or drug treatment Lower sensitivity |
| Nicotine/cotinine | |||||
| Hair | 1 cm of hair proximal to the scalp is approximately equal to the last month's exposure | Non-invasive |
| Easy to collect, ship and store (room temperature ≤5 years) Less affected by daily variability (fluctuating exposure, varying metabolism and nicotine elimination) Represents longer exposure | Scarcity of hair in infants and adults Chemical hair treatments can reduce concentrations by 9% to 30% Age, gender and race may play roles in determining hair nicotine concentrations |
| Toenails | 1 mm is approximately equal to last month's exposure | Non-invasive | Not available | Easy to collect, ship and store (room temperature ≤20 years) Overcomes day-to-day exposure variability Represents longer exposure | Need for further research and population concentrations |
| NNAL* | |||||
| Urine | Up to 3 weeks | Non-invasive | Not available | Related to a lung carcinogen Represents longer exposure than cotinine (urine/blood/saliva) | Analytical expertise Costly equipment NNAL is carcinogenic and mutagenic, special lab handling Further research needed |
↵* NNAL (4-[methylnitrosamino]-1-[3-pyridyl]-1-butanol).