delivery to the user. the speed of nicotine salts uptake into the body is close to the speed of nicotine uptake from traditional cigarettes. nicotine salts are less harsh and less bitter, and as a consequence e-liquids that contain nicotine salts are more tolerable even with high nicotine concentrations. traditional cigarettes provide high levels of nicotine, but with the bad taste of smoking. pod mods, however, can provide high levels of nicotine without the negative smoking experience. power variable power and voltage devices variable devices are variable wattage, variable voltage or both. variable power and/or variable voltage have an electronic chip allowing the user to adjust the power applied to the heating element. the amount of power applied to the coil affects the heat produced, thus changing the vapor output. greater heat from the coil increases vapor production. variable power devices monitor the coil's resistance and automatically adjust the voltage to apply the user-specified level of power to the coil. recent devices can go up to 8 v. they are often rectangular but can also be cylindrical. they usually have a screen to show information such as voltage, power, and resistance of the coil. to adjust the settings, the user presses buttons or rotates a dial to turn the power up or down. some of these devices include additional settings through their menu system such as: atomizer resistance meter, remaining battery voltage, puff counter, and power-off or lock. the power source is the biggest component of an e-cigarette, which is frequently a rechargeable lithium-ion battery. smaller devices contain smaller batteries and are easier to carry but typically require more repeated recharging. some e-cigarettes use a long lasting rechargeable battery, a non-rechargeable battery or a replaceable battery that is either rechargeable or non-rechargeable for power. some companies offer portable chargeable cases to recharge e-cigarettes. nickel-cadmium (nicad), nickel metal-hydride (nimh), lithium ion (li-ion), alkaline and lithium polymer (li-poly), and lithium manganese (limn) batteries have been used for the e-cigarettes power source. pv with variable and regulated power offering battery protection. temperature control devices temperature control devices allow the user to set the temperature. there is a predictable change to the resistance of a coil when it is heated. the resistance changes are different for different types of wires, and must have a high temperature coefficient of resistance. temperature control is done by detecting that resistance change to estimate the temperature and adjusting the voltage to the coil to match that estimate. nickel, titanium, nife alloys, and certain grades of stainless steel are common materials used for wire in temperature control. the most common wire used, kanthal, cannot be used because it has a stable resistance regardless of the coil temperature. nickel was the first wire used because it has the highest coefficient of the common metals. mechanical pv with a rebuildable atomizer. the temperature can be adjusted in celsius or fahrenheit. the evolv's dna40 and yihi's sx350j are control boards used in temperature control devices. temperature control can stop dry wicks from burning, or e-liquid overheating. mechanical devices mechanical pvs or mechanical "mods", often called "mechs", are devices without integrated circuits, electronic battery protection, or voltage regulation. they are activated by a switch. they rely on the natural voltage output of the battery and the metal that the mod is made of often is used as part of the circuit itself. the term "mod" was originally used instead of "modification". users would modify existing hardwares to get better performance, and as an alternative to the e-cigarettes that looked like traditional cigarettes. users would also modify other unrelated items like flashlights as battery compartments to power atomizers. the word mod is often used to describe most personal vaporizers. mechanical pvs have no power regulation and are unprotected. because of this ensuring that the battery does not over-discharge and that the resistance of the atomizer requires electric current within the safety limits of the battery is the responsibility of the user. e-cigarette liquid composition various bottles of e-liquid. an e-liquid containing a mixture of cannabinoid concentrates. e-cigarette liquid, e-cig liquid, e-liquid, juice, vapor juice, vape juice, smoke juice, vaping fluid, vaping juice, e-juice, e-fluid, or vape oil is the mixture used in vapor products including e-cigarettes. since e-cigarettes are not regulated in many countries, the composition of the liquid can change often. there is a great amount of variability in e-liquid formulations due to fast growth and changes in the manufacturing designs of e-cigarettes. the composition of the e-liquid for additives such as nicotine and flavors vary across and within brands. e-liquids come in many variations, including different nicotine strengths and many different flavors. the main ingredients are propylene glycol, glycerin, and flavorings; and most often, nicotine in liquid form. the liquid typically consists of a combined total of 95% propylene glycol and glycerin, and the remaining 5% being flavorings, nicotine, and other additives. the most frequently used e-liquid solvents are propylene glycol and glycerin. flavorings may contain menthol, sugars, esters, and pyrazines. flavor components include eucalyptol, camphor, methyl salicylate, pulegone, ethyl salicylate, cinnamaldehyde, eugenol, diphenyl ether, coumarin, diacetyl, acetoin, 2,3-pentanedione, cyclohexanone, benzaldehyde, cresol, butyraldehyde, and isoamyl acetate. sugars are frequently used in e-liquids to provide a sweet flavor. diacetyl, acetoin, and 2,3-pentanedione are used for buttery flavoring. camphor and cyclohexanone are used for minty flavoring. benzaldehyde is used for cherry or almond flavoring. cinnamaldehyde is used for cinnamon flavoring. cresol is used for leathery or medicinal flavoring. butyraldehyde is used for chocolate flavoring. isoamyl acetate is used for banana flavoring. e-liquids named coffee, tea, chocolate, or energy drinks, typically contain caffeine at levels considerably less than in comparison with dietary products. e-liquids are available with vitamins or cannabis flavors. specific e-cigarettes (mods) are available that allow for not only liquids but also herbs, oils, or fruits to be vaped. dual-function devices handle both concentrates and e-liquids using multiple cartridges. e-liquid can be made with or without nicotine, with more than 90% of e-liquids containing some level of nicotine. the most regularly used base carrier chemical is propylene glycol with or without glycerin. e-liquid containing glycerin and water made without propylene glycol are also sold. there are e-liquids sold without propylene glycol, nicotine, or flavors. e-liquids containing thc or other cannabinoids is also sold. specific kinds of e-liquids contain a tiny amount of alcohol. the amount of alcohol in e-liquids vary, and there are cases where it has not been disclosed as an ingredient. it is uncertain whether the nicotine used in e-liquid is manufactured using a us pharmacopeia grade nicotine, a tobacco plant or tobacco dust extract, or a synthetic nicotine. most e-cigarette liquids contain nicotine, but the level of nicotine varies depending on user-preference and manufacturers. although some e-liquid is nicotine-free, surveys demonstrate that 97% of respondents use products that contain nicotine. about 3.5% of users use liquid without nicotine. an e-cigarette user used approximately three flavors. a 2016 study showed that measurable amounts of arsenic, nickel and other metals were in e-liquids. over 80 chemicals such as formaldehyde and metallic nanoparticles have been found in the e-liquid. e-liquids typically contain nicotine, propylene glycol, glycerin, 1,3-butanediol, 1,3-propanediol, ethylene glycol, menthol, safrole, ethyl vanillin, camphor, α-thujone, coumarin, and diethylene glycol, according to a 2017 review. e-liquid can contain a range of toxicants and can contain impurities. a 2013 study found the e-liquids tested had as high as five times the upper threshold permitted levels of impurities. e-liquids have been found to contain low levels of some of the toxicants found in tobacco smoke, as well as small concentrations of carcinogens. in 2009, the fda analyzed e-liquid cartridge samples which were found to contain tobacco-specific nitrosamines (tsnas), diethylene glycol (detected one e-cigarette cartridge), cotinine, anabasine, myosmine, and beta-nicotyrine. the tsnas n-nitrosonornicotine (nnn), 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (nnk), n-nitrosoanabasine, and nitrosoanatabine have been detected in five e-liquid cartridge samples from two companies in levels comparable to nicotine replacement products, according to the results of the fda's analysis. tsnas were found in a broad range of levels. tsnas present in tobacco smoke, were also found in e-liquids, at different levels, in trace amounts. 2013 studies of other e-liquids had not detected diethylene glycol. the majority of the e-liquids analyzed contained nnn from 0.34 to 60.08 μg/l and contained nnk from 0.22 to 9.84 μg/l. the fda issued warnings to several e-cigarette companies for selling e-cartridges and refill solutions containing active pharmaceutical ingredients such as rimonabant (zimulti) for the purpose of losing weight and reducing smoking addiction, and tadalafil (the active ingredient in cialis) for the purpose of increasing sexual capacity. fda analyses of these e-cartridges and solutions showed the presence of amino-tadalafil and not tadalafil, and the presence of an oxidative product of rimonabant, as well as rimonabant. the e-liquid often contain other substances unknown and/or undisclosed to the user. the specific origin of the e-liquid ingredients is often unclear. when content information is given on the packaging, it is usually incomplete. contamination with various compounds in e-liquids is a result of poor quality control. some nicotine and tsnas have been found in e-liquids labelled as 'no nicotine'. nicotine content information on labels for some e-liquid companies may be vague, inaccurate or absent. e-liquid were found to contain low levels of anthracene, phenanthrene, 1-methyl phenanthrene and pyren. diethylene glycol, ethylene glycol, hydrocarbons, ethanol, terpenic compounds and aldehydes, particularly formaldehyde and acrolein were found in the e-liquid. diethylene glycol is a potential byproduct of propylene glycol. a 2014 study showed that e-liquids from a specific manufacturer contained greater amounts of ethylene glycol than glycerin or propylene glycol, which was likely a result of improper manufacturing methods. some liquids contained residual solvents such as 1,3-butadiene, cyclohexane, and acetone. some e-liquids contain tobacco alkaloids such as nornicotine, anabasine, or anatabine, and tsnas, such as n-nitrosonornicotine (nnn), 4-(methylnitrosamine)-1-(3-pyridyl)-1-butanone (nnk), nitrates, and phenol. tobacco alkaloids that were identified in some e-liquids were not found on the ingredient list. small quantities of volatile organic compounds (vocs) such as benzene, toluene, xylene, and styrene have been found in the e-liquid. diethyl phthalate and diethylhexyl phthalate have been found in e-liquids. some e-liquids contain tin "whiskers," microscopic crystals that originate from tin in the solder joints. levels of aldehydes in e-liquid aldehydes (in μg/g) in bottles of e-liquids∗ company code formaldehyde acetaldehyde propionaldehyde crotonaldehyde butyraldehyde benzaldehyde hexaldehyde lod 0.060 0.030 0.043 0.053 0.077 0.035 0.036 janty h60339 0.497 0.728 0.043 <0.053 <0.077 <0.035 <0.036 ecigexpress h60346 0.161 1.74 <0.043 <0.053 0.186 0.160 <0.036 vapor4life h60349 0.776 0.507 0.089 <0.053 0.217 40.0 <0.036 totally wicked h60352 0.532 0.129 <0.043 <0.053 <0.077 0.821 <0.036 sedansa h60355 0.813 1.25 0.167 <0.053 0.164 <0.035 <0.036 johnson creek h60360 0.356 2.58 0.122 <0.053 <0.077 0.291 <0.036 tecc h60364 0.467 0.235 <0.043 <0.053 <0.077 0.078 <0.036 intellicig h60369 0.114 4.05 0.083 <0.053 <0.077 0.581 <0.036 e-cigarettes.fr h60370 0.257 0.413 <0.043 <0.053 <0.077 0.104 0.068 ciglib h60373 0.274 0.421 <0.043 <0.053 <0.077 0.035 0.089 v2 cigs h60374 0.411 0.332 0.045 <0.053 <0.077 0.146 0.115 e-liquide.com h60375 9.00 3.14 <0.043 <0.053 <0.077 0.145 0.100 tasty vapor h60376 3.52 2.37 <0.043 <0.053 <0.077 305 0.532 e-cig.com h60379 0.226 0.393 0.047 <0.053 <0.077 0.062 0.132 ∗a 2013 analysis tested a total of 42 bottles of e-liquids. contents the e-liquid is sold in bottles, pre-filled disposable cartridges, or as a kit for consumers to make their own e-liquids. e-liquids are made with various tobacco, fruit, and other flavors, as well as variable nicotine concentrations (including nicotine-free versions). the standard notation "mg/ml" is often used on labels to denote nicotine concentration, and is sometimes shortened to "mg". some flavors are created to resemble the flavors used in traditional cigarettes such as tobacco and menthol-tobacco. adults in general also preferred sweet flavors (though smokers like tobacco flavor the most) and disliked flavors that elicit bitterness or harshness. young adults overall preferred sweet, menthol, and cherry flavors, while non-smokers in particular preferred coffee and menthol flavors. in surveys of regular e-cigarette users, the most popular e-liquids had a nicotine content of 18 mg/ml, and the preferred flavors were largely tobacco, mint and fruit. men tend to favor flavors with tobacco, while women tend to favor chocolate or sweet flavors. the most favorite flavors among regular e-cigarette users reported in a 2017 uk survey were fruit, tobacco, and menthol/mint. the survey also found 2.6% regular e-cigarette users used no flavors.