Adult population (> 25 years)
Among US adults, the number of e-cigarettes users increased from 1.8% in 2009 to 3.4% in 2010 [
6]. The use of these products continued to increase in 2011 with 5.8% of U.S. adults reporting the use of e-cigarettes in that year [
6]. Data from National Center for Health Statistics (NCHS) in 2015 showed that 3.7% of the U.S. adult population were current e-cigarette smokers. According to National Health Interview Survey of the United States in 2017, approximately 2.8% of the U.S. adult population were current e-cigarette smokers. According to Reuters Health, one in every 20 US adults has tried e-cigarettes. At present, approximately 10.8 million American adults use e-cigarettes [
7]. Therefore, it is interesting to note that the number of current adult e-cigarette smokers showed an upward trend from 2010 until 2014 [
8] followed by a decline in 2017 [
9].
Young adult population (18–25 years)
According to the National Health Survey, the percentage of young adults who use e-cigarettes daily or some days increased from 2.4% in 2012 to 2013 to 5.2% in 2015. In addition, 40% of young adults who use e-cigarettes daily or some days were never smokers before trying e-cigarettes. In comparison with adults aged 25 years and older, young adults (18–25 years) are more likely to try e-cigarettes and report having used e-cigarettes in the past 30 days. Hence, we can see an increase in the number of e-cigarette users over the years among young adults aged 18 to 24 years.
Description of e-cigarettes and how they vary from c-cigarettes
Research has provided substantial evidence that control of nicotine addiction is beneficial for the health of smokers regardless of age. Over the years, tobacco control programs and interventions have demonstrated much success in decreasing initiation in non-smokers and cessation in smokers. However e-cigarettes many reverse this success in smoking cessation.
E-cigarettes are popular battery-operated devices used as an alternative for c-cigarettes. E-cigarette devices produce aerosol (gaseous mist) from a nicotine-containing liquid (e-liquid) and a heating element. The user then inhales the aerosol through the mouth and lungs from which it is absorbed into the bloodstream, and the remaining aerosol is exhaled. Many common terms are used for this device other than e-cigarettes, such as Juul (resembles a USB flash drive), e-vaporizer, vapes, electronic nicotine delivery systems (ENDS), and e-hookahs. E-cigarettes come in different shapes, including pen and regular cigarette shapes, portable pocket device, pen vaporizer, and box mod. There are two main types of e-cigarettes: (1) disposable and (2) cartridge models. The disposable type is an inexpensive alternative for beginners. On the other hand, the cartridge models are rechargeable and contain cartridges that can be refilled or exchanged.
E-cigarettes differ from c-cigarettes in their basic design, nature of the smoke, and the way the vapor is produced. The basic components of e-cigarettes include an aerosol generator, a flow sensor, a battery, and a nicotine-containing solution storage area. The effectiveness of e-cigarettes as a nicotine delivery system can be controlled with interchangeable parts, enabling users to modify the character of the delivered aerosol by changing the voltage [
10]. E-cigarettes produce a gaseous substance/mist at a temperature that is lower than its point of combustion by heating the liquid in the cartridge, which consists of nicotine, propylene glycol, glycerol, or a mixture of water, flavor, and other toxic substances. Research has indicated the presence of trace levels of toxic compounds, namely, tobacco-specific nitrosamines (TSNAs), carbonyl compounds [
11], and polycyclic aromatic hydrocarbons (PAHs) [
12] in the e-liquids or aerosols of some commercially available products although most were found at much lower levels than in c-cigarette smoke. On the other hand, c-cigarettes produce smoke by burning tobacco with fire. This smoke consists of recognized carcinogens, such as the carbonyl compound, formaldehyde, organic compounds, such as benzene, tobacco-specific nitrosamines, free radicals, toxic gases, and heavy metals. There are more than 7000 chemicals present in c-cigarette smoke, among which 70 have been recognized as carcinogens [
13,
14].