<unk>  (vapor) exhaled by an e-cigarette user.|alt=Aerosol (vapor) exhaled by an e-cigarette user. The chemical composition of the<unk><unk><unk>  varies across and within manufacturers. Limited data exists regarding their chemistry. However, researchers at Johns Hopkins University analyzed the vape clouds of popular brands such as Juul and Vuse, and found "nearly 2,000 chemicals, the vast majority of which are unidentified." The<unk>  of e-cigarettes is generated when the e-liquid comes in contact with a coil heated to a temperature of roughly 100–250 °C within a chamber, which is thought to cause pyrolysis of the e-liquid and could also lead to decomposition of other liquid ingredients. The<unk>  (mist) produced by an e-cigarette is commonly but inaccurately called vapor. E-cigarettes simulate the action of smoking, but without tobacco combustion. The e-cigarette<unk>  looks like<unk>  smoke to some extent. E-cigarettes do not produce<unk>  between puffs. The e-cigarette<unk>  usually contains propylene glycol, glycerin, nicotine, flavors, aroma transporters, and other substances. The levels of nicotine, tobacco-specific nitrosamines (TSNAs), aldehydes, metals, volatile organic compounds (VOCs), flavors, and tobacco alkaloids in e-cigarette<unk> s vary greatly. The yield of chemicals found in the e-cigarette<unk>  varies depending on, several factors, including the e-liquid contents, puffing rate, and the battery voltage. Metal parts of e-cigarettes in contact with the e-liquid can contaminate it with metals. Heavy metals and metal nanoparticles have been found in tiny amounts in the e-cigarette<unk> . Once<unk> ized, the ingredients in the e-liquid go through chemical reactions that form new compounds not previously found in the liquid. Many chemicals including carbonyl compounds such as formaldehyde can inadvertently be produced when the nichrome wire (heating element) that touches the e-liquid is heated and chemically reacted with the liquid. Propylene glycol-containing liquids produced the most amounts of carbonyls in e-cigarette vapors, while in 2014 most e-cigarettes companies began using water and glycerin instead of propylene glycol for vapor production. Propylene glycol and glycerin are oxidized to create aldehydes that are also found in<unk>  smoke when e-liquids are heated and<unk> ized at a voltage higher than 3 V. Depending on the heating temperature, the carcinogens in the e-cigarette<unk>  may surpass the levels of<unk>  smoke. Reduced voltage e-cigarettes generate very low levels of formaldehyde. A Public Health England (PHE) report found "At normal settings, there was no or negligible formaldehyde release." However,this statement was flatly contradicted by other researchers in a 2018 study. E-cigarettes can emit formaldehyde at high levels (between 5 to 15 times higher than what is reported for<unk>  smoke) at moderate temperatures and under conditions that have been reported to be non-averse to users. As e-cigarette engineering evolves, the later-generation and "hotter" devices could expose users to greater amounts of carcinogens.
Composition of electronic cigarette aerosol
Composition of heated tobacco product emissions
Measures of pollutant concentration
Safety of electronic cigarettes
Measured environmental concentration
Instruments used in medical laboratories
Toxic capacity
Ventilated cigarette