Objective Many states have implemented laws prohibiting tobacco smoking in indoor public places. However, private settings remain a major source of secondhand smoke (SHS) exposure for many people. We assessed the association between current asthma and SHS exposure in vehicles among adult never-smokers in Indiana, Kentucky, Louisiana and Mississippi.

Methods Data came from the 2011 Behavioral Risk Factor Surveillance System, a state-based telephone survey of US adults aged ≥18 years. Analyses were restricted to states (n=4) that administered an optional SHS module. Prevalence of self-reported asthma and past 7-day SHS exposure in vehicles was calculated by demographics, voluntary smoke-free vehicle rules and SHS exposure in homes, public places and workplaces. Logistic regression was used to assess the adjusted association between asthma and SHS exposure in vehicles.

Results Among 17 863 never-smoking adults, 7.4% reported having current asthma, whereas 12.3% reported past 7-day SHS exposure in vehicles. Among adults with asthma, SHS exposure in vehicles was lower among those with voluntary smoke-free rules compared with those without voluntary smoke-free rules (9.5% vs 56.7%, p<0.0001). Following adjustment, adults exposed to SHS in a vehicle had a higher odds of having current asthma compared with unexposed adults (OR=2.01, 95% CI 1.18 to 3.40).

Conclusions Never-smoking adults recently exposed to SHS in a vehicle had higher odds of having current asthma compared with unexposed adults. Efforts are warranted to warn about the dangers of SHS and to encourage voluntary smoke-free rules in vehicles, especially among adults with asthma.