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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.