Secondhand smoke produced by traditional cigarettes has many effects on human health, including increased risk of respiratory tract diseases, lung cancer, infectious diseases, acute cardiovascular diseases, and cerebral apoplexy . The secondhand smoke of traditional cigarettes is mainly the side-stream smoke of the cigarette; however, the E-cigarette does not produce side-stream smoke. As a result, the second-hand smoke of the E-cigarette is composed of the exhaled fog of the E-cigarette user. We have evaluated the safety of secondhand smoke produced by E-cigarettes. Schripp, T. et al. placed subjects in a closed chamber to smoke E-cigarettes and exposed them to second-hand smoke. It has been shown that aerosol particles in the electronic smoke enter the lung tissue from the respiratory tract, cycle in the lungs and become smaller particles after being exhaled. In indoor environments, people can also “inhale the steam passively”. The analysis of air components showed that formaldehyde, acrolein, isoprene, acetaldehyde and acetic acid were present, but the chemical compounds produced by traditional cigarette burning were 5 to 40 times than those of the electronic ones . The study results of Goniewicz, M.L. et al. found that the release of smoke from E-cigarettes can increase the amount of nicotine deposition on windows, walls, floors, wood and metal surfaces. Therefore, they believe that smoking E-cigarettes may expose humans to the risk of nicotine indirectly, and future studies should explore the potential risks of carcinogens formed by the nicotine that is released from E-cigarettes . Schober, W. et al. conducted indoor research 6 times. It was found that after 2 h suction of E-cigarettes, the content of PM2.5 (particles less than 2.5 microns), propylene glycol, glycerol and nicotine in the indoor air increased significantly. In addition, the total polycyclic aromatic hydrocarbon (PAHs) content, which may cause cancer, increased by 20%, and the concentration of aluminum in the environment increased by 2.4 times . The results of the 5 experiments conducted by Czogala, J. et al. showed that after the use of E-cigarettes, the level of nicotine in the environment was 10% (3.3 compared to 31.6 g/m3) that of traditional cigarettes, and the PM2.5 concentration was about 18% of that of traditional cigarettes . In the study of Flouris, A.D. et al. and colleagues, 15 non-smokers were exposed to secondhand smoke from traditional cigarette or smoke from E-cigarettes for an hour, and then the cotinine levels in their serum were tested. The results showed that the cotinine levels in the serum of non-smokers were similar after inhaling the secondhand smoke of traditional cigarettes and E-cigarettes (2.6 to 2.4 ng/mL) . As a result, nicotine levels, along with levels of some potentially toxic substances in the body, will increase when they are exposed to the secondhand smoke of E-cigarettes. Although current studies have shown that the risk of secondhand smoke generated by E-cigarettes is expected to be lower than that of traditional cigarettes, there is a high possibility that non-smokers will be exposed to certain doses of nicotine and other toxic substances. With more and more E-cigarettes used in public spaces, a greater diffusion of these toxic and harmful substances will be released into the environment.