Vaping can expose non-users to aldehydes and it reduced indoor air quality due to their released aldehydes. Since e-cigarettes involve an aerosolization process, it is suggested that no meaningful amounts of carbon monoxide are emitted. Thus, cardiocirculatory effects caused by carbon monoxide are not likely. However, in an experimental study, e-cigarettes increased levels of carcinogenic polycyclic aromatic hydrocarbons in the surrounding air. Passive inhalation of vapor might have significant adverse effects. Though, e-cigarettes exposes non-users to nicotine but not to tobacco-related combustion toxicants. Exposure to e-cigarette vapor can reduce lung function. E-cigarettes do pollute the air in the form of exhaled mainstream aerosol from people using e-cigarettes. Nicotine, ultraﬁne particles, and products of heating propylene glycol and glycerin are increased in the air where e-cigarettes are being used, although, as expected, at lower levels than produced by smoking the same number of traditional cigarettes. As with traditional cigarettes, however, when several people are using e-cigarettes indoors at the same time, the air can become polluted. For example, levels of ﬁne particulate matter (PM2.5) in a large hotel event room (4,023m3) increased from 2–3 μg/m3 to as high as 819 μg/m3 (interquartile range: 761–975 μg/m3) when 59–86 people were using e-cigarettes. This level is comparable to a very (conventional tobacco) smoky bar or casino and dramatically exceeds the US Environmental Protection Agency annual time-weighted standard for PM2.5 of 12 μg/m3. Evidence has also shown that bystanders absorb nicotine when people around them use e-cigarettes at levels comparable with exposure to traditional cigarette second-hand smoke. In a study of non-smokers living with nicotine e-cigarette users, those living with traditional cigarette smokers, or those living in homes where no one used either product, cotinine (a metabolite of nicotine) levels in bystanders' urine were signiﬁcantly elevated in both the people exposed to second-hand e-cigarette aerosol and those exposed to second-hand tobacco smoke compared with people living in aerosol- and smoker-free homes. Interestingly, the levels of elevated urinary cotinine in the two exposed groups were not signiﬁcantly different (although the passive smokers had higher point estimates), despite the fact that the increase in air pollution in the smokers' homes was much higher than in the e-cigarette users' homes (geometric mean air nicotine concentrations of 0.13 μg/m3 in e-cigarette users' homes, 0.74 μg/m3 in smokers' homes, and 0.02 μg/m3 in the control homes).