Introduction:
Exposure to second-hand tobacco smoke (SHS) has serious adverse health effects, causing more than 600,000 deaths among non-smokers each year worldwide. Also exposure to electronic cigarette aerosol has been shown to have potential adverse health effects in bystanders passively exposed.

Methods:
Within the TackSHS Project (www.tackshs.eu), in 2016-2018 we conducted a face-to-face cross-sectional survey in 12 European countries (Bulgaria, England, France, Germany, Greece, Ireland, Italy, Latvia, Poland, Portugal, Romania, and Spain). The survey included a specific section on exposure to SHS and on electronic cigarette use and passive exposure to its aerosol in selected private and public indoor places. In each country, the sample consisted of around 1,000 participants representative of the general population aged ≥15 years.

Results:
Of 11,909 participants, 8,834 (74.2%) were non-smokers. Among them, 31.2% were daily exposed to SHS in indoor places, ranging between 24.3% in Ireland and 68.0% in Greece. This prevalence was 13.1% at home, 11.4% in indoor workplaces, 4.7% in public transportations, 5.7% in private transportations and 14.7% in other indoor places. Overall, 2.4% were current electronic cigarette users. This prevalence ranged between 0.6% in Spain and 7.2% in England. Among 11,570 electronic cigarette non-users, 16.2% were daily exposed to electronic cigarette aerosol, with the highest exposure rate found in England (29.8%) and the lowest one in Spain (4.6%). The exposure rate was 5.8% at home, 6.4% in indoor workplaces, 4.0% in public transportations, 3.2% in private transportations and 8.3% in other indoor places.

Conclusions:
Almost one out of three European non-smokers was daily exposed to SHS. We recognize the dramatic fall in SHS exposure due to the comprehensive smoke-free legislations adopted in various countries over the last decade. Notwithstanding, SHS exposure is still far to be negligible, particularly in homes and in indoor workplaces. Despite current use of electronic cigarette is still relatively low in most European countries, our data suggest that, in indoor settings, more than 54 million Europeans are daily exposed to the aerosol exhaled by less than 8 million electronic cigarette users.