**4. Conclusions**

Although traditional tobacco smoking has been in decline since the 1980s, newer generations of NDS have been steadily increasing in popularity ever since they were introduced into the market in 2013. This accelerated growth, together with their recent appearance, has led to an impendent need for studies to be developed measuring the e ffects of such.

The present study allowed the evaluation of the concentrations of smoke pollutants, more specifically, the particulate matter and gases originating from di fferent types of NDS in real life scenarios where smoking is still common among electronic nicotine delivery systems users, which consider these a safer option than TCs.

The results showed that although the levels of pollutants emitted by e-cigarettes and HNBT are substantially lower compared to those from TCs, the new smoking devices are still a source of indoor air pollutants. All smoking options are avoidable sources of indoor pollutants, and to protect the health of smokers and non-smokers, they should not be used in homes and cars.

The presented results pertain to a single brand of HNBT and specific brands of e-cigarettes and may not represent the possible variability among di fferent brands or manufacturers. Additionally, the configurations of the equipment as well as the e-liquid charges used for each e-cigarette may not represent other brands or configurations of these devices.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1660-4601/17/10/3455/s1, Table S1: Concentrations of air pollutants measured in the home for traditional cigarettes (TC1 and TC2), e-cigarettes (JUUL and Vape), and HNBT, Table S2: Concentrations of air pollutants measured in the car for traditional cigarettes (TC1 and TC2), e-cigarettes (JUUL and Vape), and HNBT.

**Author Contributions:** Conceptualization, J.S., N.C. and S.M.A.; methodology, J.S., N.C. and S.M.A.; validation, N.C. and S.M.A; investigation, J.S., N.B., N.C. and S.M.A.; resources, S.M.A.; data curation, J.S., N.C. and S.M.A.; writing—original draft preparation, J.S.; writing—review and editing, N.C. and S.M.A.; supervision, N.C. and S.M.A.; project administration, S.M.A.; funding acquisition, S.M.A. All authors have read and agreed to the published version of the manuscript.

**Acknowledgments:** This work was supported by LIFE Index-Air project (LIFE15 ENV/PT/000674). This work reflects only the authors' view and EASME is not responsible for any use that may be made of the information it contains. Authors also acknowledge the support of the Portuguese Foundation for Science and Technology (FCT, Portugal) through the Postdoc Grant SFRH/BPD/102944/2014, the contract IST-ID/098/2018, and the strategic projects UIDB/04349/2020+UIDP/04349/2020 and UIDB/50017/2020+UIDP/50017/2020.

**Conflicts of Interest:** The authors declare no conflict of interest.
