Quantitative reference levels for thirdhand smoke residue from tobacco products in indoor environments: a systematic review and meta-analysis

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