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(Source Wikipedia)

Propylene glycol (PG) or propane-1,2-diol, also known as 1,2-dihydroxypropane or methyl glycol, is a diol with the chemical formula CH3-CHOH-CH2OH with many industrial or agropharmaceutical uses, in low doses as a food additive and recently in electronic cigarettes (vaping liquid).

Propylene glycol is obtained from the reaction of propylene oxide with water to form monopropylene glycol (MPG), with subsequent reactions producing di-(DPG), tri-(TPG) and other propylene glycols.

Why Propylene Glycol?

(source propylene-glycol.com)
For over five decades, propylene glycol has enjoyed an unrivalled reputation for safe use in a wide range of consumer products: food, feed, cosmetics and pharmaceuticals, as well as industrial applications.

Common applications

(source propylene-glycol.com)
Propylene glycol is used in applications closely related to sensitive health, such as food, feed, cosmetics and pharmaceuticals. It meets the strict quality standards and hygiene regulations required by health and safety authorities worldwide.

Propylene glycol in vape (electronic cigarette)

Role in the e-liquid

PG is a component of the diluting matrix used in the majority of commercial e-liquids. Historically, it is the main component of e-liquid. Tested and approved by the pharmaceutical industry as a vehicle for active bronchodilators (e.g. ventolin), it appears to be the ideal base for e-liquids due to its physical and chemical properties:

It vaporises at relatively low temperature. In its gaseous state, it condenses into fine droplets (usually in the presence of an air flow) thus trapping some of the neighbouring molecules (i.e. nicotine, aromatic molecules, water, etc.). This rapid phenomenon produces an aerosol that visually mimics smoke.

The liquid particles or "droplets" of PG that form the aerosol serve to diffuse the other molecules they trap. Due to their size, these PG particles (from 0.1 μm to 2 μm [30]) penetrate deep into the airways [31], thus ensuring near optimal delivery and absorption of the nicotine they contain.

Its low viscosity is beneficial in the operation of a personal vaporiser (PV). Indeed, when vaping the e-liquid, the wick at the heart of the resistance can dry out locally (at the heating element of the VP). If the wick is rehydrated too slowly, it can cause overheating, resulting in an unpleasant smell and taste sensation called "dry hit". With its low viscosity (and therefore good capillarity), PG limits the local drying of the wick and therefore reduces the risk of dry hit.

Its chemical properties allow it to perfectly solubilise nicotine as well as many aromatic molecules present in the flavours used to manufacture e-liquids. Its thermal stability means that it does not undergo significant degradation when the e-liquid is vaporised (under normal conditions of use). The few toxic molecules resulting from its degradation (mainly from the aldehyde family) are in much smaller quantities than in tobacco smoke.

The role of PG in the "throat hit" phenomenon is more hypothetical. This contraction of the larynx is sought by the smoker and comes from a response of the trigeminal nerve [32, 33] to an irritating or painful sensation. Nicotine is responsible for this phenomenon. Several assumptions can be made about the role of PG: as a mild irritant, it could promote the sensation of contraction. In addition, due to its physical and chemical properties, PG is more fluid and vaporizes more easily (at lower temperatures) than vegetable glycerine (VG), which is another medium commonly used by the vape industry. Thus, for the same set-up (same material, same vaping behaviour), the consumption of a 100% PG e-liquid will be approximately twice as high as a 100% VG e-liquid (graph 1, experiments carried out at LFEL [41]). Thus, a greater consumption of e-liquid naturally leads to a greater consumption of nicotine, which increases throat irritation and therefore throat hit.

Administration by inhalation

In the scientific literature, it is possible to find studies on the toxicological consequences of PG inhalation on animals and humans.

In animals, two studies can be cited in which PG vapours were inhaled by primates and rats on a daily basis for several months [19,20]. Some minor side effects were reported, such as weight loss or gain, nosebleeds, slight decrease in white blood cell count, etc. Post-mortem examination of the animals shows that no organs were altered. A similar but more recent experiment showed similar findings [39].

Tests on humans are rarer. However, one study [21] on 93 patients with chronic respiratory disorders can be cited. The subjects inhaled an aerosol made from a 40% solution of PG for 15 minutes using a breathing mask. The authors of this study report that the patients tolerated the inhalation of the aerosol without any side effects being reported during the test and in the following days. Better still, these researchers recommend the use of PG as a "vehicle" for the administration of aerosolised bronchodilator drugs.

It is therefore understandable that PG, when inhaled, does not present toxicity to any organ including the lungs. The only negative effect noted is an irritation of the respiratory tract following repeated exposure. This phenomenon would only affect a part of the population, which would highlight its subjective nature.