Quantitative reference levels for thirdhand smoke residue from tobacco products in indoor environments: a systematic...

Commercial tobacco use leaves hazardous chemical residues in indoor environments, known as thirdhand smoke. The residue has been found to be highly persistent and can still be detected months and years after indoor smoking has ceased.

This systematic review synthesizes studies that measure surface wipe nicotine, a validated thirdhand chemical marker, to propose reference levels for indoor thirdhand smoke contamination.

PubMed and SCOPUS were searched for field studies published since 1991 across a wide range of indoor settings and with different smoking histories. Twenty-one studies met the inclusion criteria, reporting 3167 nicotine measures across 102 independent groups. Pooled mean and median estimates of surface nicotine were calculated based on the smoking history of indoor environments.

Homes of nonsmokers with long-term bans had the lowest nicotine levels (weighted median = 0.97 µg/m²; 95% CI: 0.47-1.37). Higher levels were found in previously smoked-in homes (4.57 µg/m²; CI: 3.20-8.11), currently smoked-in homes (51.09 µg/m²; CI: 21.64-113.53), and environments with prolonged heavy smoking (1,028.00 µg/m²; CI: 568-2,749). We propose four reference levels indicating increasing thirdhand smoke exposure hazards: Level 1 ( < 2 µg nicotine/m²) reflects low hazard in nonsmoking homes with long-term bans. Level 2 (2 - <20 µg/m²) represents moderate hazard in homes of smokers with indoor bans or former smoking. Level 3 (20 - <200 µg/m²) indicates high hazard from active indoor smoking. Level 4 ( ≥ 200 µg/m²) marks a very high hazard in places with sustained indoor smoking, such as casinos.

Findings show a clear pattern of increasing nicotine surface concentrations, from nonsmoker homes with smoking bans to indoor spaces with sustained indoor smoking. The proposed reference levels offer practical benchmarks for evaluating THS contamination and guiding remediation decisions. Their potential applicability as a health risk metric is discussed.

The chemical residue from tobacco smoke, also known as thirdhand smoke, is an indoor pollutant of growing concern. Starting January 1, 2026, California law requires disclosure of thirdhand smoke in residential real estate transactions. Based on field studies conducted across a wide range of indoor settings and with different smoking histories, this is the first study to show a clear pattern of increasing nicotine surface concentrations, from nonsmoker homes with smoking bans ( < 2 µg/m²) to indoor spaces with sustained indoor smoking ( > 200 µg/m²). The proposed reference levels offer practical benchmarks for evaluating THS contamination and guiding remediation decisions.