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Aerosol composition is affected even by the solvent employed, being vegetable glycerin (VG), propylene glycol (PG) and their mixture the most widely used. They can also influence particle-size distribution, therefore the region of deposition in the respiratory system51. A higher percentage of propylene glycol seems enhancing flavour and strengthening the so-called “throat hit”, whereas a higher percentage of vegetable glycerine may increase vapor production52. Vegetable glycerine exposure has been associated with irritation of eyes, lungs, and oesophagus mucosa53. Likewise, its higher boiling point requires the heating element to reach higher temperatures, resulting in a greater risk of toxicants emission49. The highest yield of aldehydes occurs in devices containing propylene glycol54, also related to upper respiratory infection-like symptoms55. It has been documented that a PG/VG mixture produces more ROS than each component alone56 .
A vast amount of studies aimed at characterizing e-cigarettes emissions and variously reported measurable amounts of ethanol, volatile organic compounds (VOCs), polycyclic aromatic hydrocarbons (PAHs), silicon, lead, nickel, air pollutant, formaldehyde, acetaldehyde, isoprene, acetic acid, 2-butanodione, acetone, and propanol56–58. Volatile organic compounds can provoke eye and respiratory tract irritation, neurological impairment and liver damage59. Polycyclic aromatic hydrocarbons demonstrated carcinogenic, respiratory, immunological, neurological and reproductive effects60. Reactive carbonyls such as aldehydes and acrolein (product of glycerol constituents vaporization61) elicit airway constriction, direct damage to airway epithelium and alterations in gene expression, in addition to neutrophils activation, degranulation and apoptosis62.
The extraordinarily wide variety of flavorants available amplifies the heterogeneity in E-cigs aerosol composition. These chemical components are generally employed in food industry and recognized as safe additives but this does not imply their harmlessness when inhaled63. Some are known allergens (e.g. cinnamaldehyde for cinnamon aroma)64, others may provoke ocular and airway irritation (e.g. benzaldehyde for fruity aromas)65. Pre-clinical studies demonstrated that flavoring chemicals elicit pro-inflammatory responses in lung epithelial cells and fibroblasts and decrease transepithelial resistance in bronchial epithelial cells66. Likewise, diacetyl and acetyl propionyl (butter flavoring volatiles) seem to underlie bronchiolitis obliterans, as seen in microwave pop-corn producing factory workers 67,68. Besides, flavored e-cigarettes are misleadingly considered less harmful than those with tobacco flavor, therefore used carelessly by youth69.
An additional concern is represented by the inconsistencies between declared nicotine levels and actual nicotine content, which have been detected even in liquids supplied from the same company70.
Moreover, e-cigs nicotine derives from tobacco plant, thus e-liquid can include other tobacco-related toxicants, such as tobacco-specific nitrosamines (TSNAs)71.