Abstract Introduction Secondhand smoke (SHS) constitutes a significant health risk for children. This study examined whether knowledge of SHS harms to children and concern about setting examples for children and are associated with intentions to quit among adults who smoke in Japan, a country where child health and parental role as models for their children are highly valued. Methods Data came from respondents of the 2021 ITC Japan Wave 4 Survey who smoked cigarettes at least weekly (n = 2586). Weighted logistic regression analyzed whether respondents’ knowledge of SHS harms to children (Knowledge Index), concern that one’s smoking would influence children to smoke (Influence Index), and support for banning smoking in cars with children (Policy Index) were associated with intentions to quit. Results 39.3% of respondents intended to quit smoking. 85.5% of all respondents supported a law to ban smoking in cars with children. The Influence Index was most strongly associated with quit intentions (OR = 2.21, 95% CI = 1.94, 2.53), and this association was stronger for respondents living with children at home than for respondents who did not (interaction p=.039). Greater knowledge of SHS harms (OR = 1.39, 1.27, 1.54) and support for banning smoking in cars with children (OR = 1.39, 1.27, 1.54) were also associated with quit intentions, but that association was no different among those who lived with children. Conclusions Both knowledge about SHS health harms on children and concerns that one’s smoking would influence children to smoke were significantly related to quit intentions. This suggests that interventions to increase awareness among adults who smoke may reduce children’s exposure through increased quitting, especially among those with children. The very high support among those who smoke for a law to ban smoking in cars with children suggests that this is a feasible policy option in Japan.
Goh et al. (2026) studied this question.