Impact of banning smoking in cars with children on exposure to second-hand smoke: a natural experiment in England and Scotland

England banned smoking in cars carrying children in 2015 and Scotland in 2016. We used survey data from 3 years for both countries (N(England)=3483–6920, N(Scotland)=232–319) to assess effects of the English ban using logistic regression within a difference-in-differences framework. Among children aged 13–15 years, self-reported levels of regular exposure to smoke in cars for Scotland were 3.4% in 2012, 2.2% in 2014 and 1.3% in 2016 and for England 6.3%, 5.9% and 1.6%. The ban in England was associated with a −4.1% (95% CI −4.9% to −3.3%) absolute reduction (72% relative reduction) in exposure to tobacco smoke among children.