Substantial harm could result from concurrent cigarette and e-cigarette use (i.e., dual use) were it to undermine smoking cessation. Perceptions of chemical exposure and resulting harms may influence dual use. We conducted a probability-based phone survey of 1164 U.S. adult cigarette smokers in 2014–2015 and analyzed results in 2016. In a between-subjects experiment, smokers heard a hypothetical scenario in which cigarettes and e-cigarettes had the same amount of harmful chemicals or cigarettes had more chemicals than e-cigarettes (10× more, 100× more, or chemicals were present only in cigarettes). Smokers indicated how the scenario would change their interest in dual use and perceived health harms. Few smokers (7%) who heard that the products have the same amount of chemicals were interested in initiating or increasing dual use. However, more smokers were interested when told that cigarettes have 10× more chemicals than e-cigarettes (31%), 100× more chemicals than e-cigarettes (32%), or chemicals were present only in cigarettes (43%) (all p < .001). Individuals told that cigarettes have more chemicals were more likely than those in the “same amount” scenario to perceive that cigarettes would be more harmful than e-cigarettes (79% vs. 41%, OR = 5.41, 95% CI = 4.08–7.17). These harm perceptions partially explained the relationship between chemical scenario and dual use interest. Smokers associated higher chemical amounts in cigarettes versus e-cigarettes with greater health harms from cigarettes and thus expressed increased interest in dual use. The findings suggest that disclosing amounts of chemicals in cigarette smoke and e-cigarette aerosol could unintentionally encourage dual use
