Many flavored vaping juices use a combination of propylene glycol (PG) and vegetable glycerin (VG) as carrier solvents for liquid nicotine. Some people fear that the combustion process for these two chemical compounds will result to the formation of formaldehyde and acetaldehyde in the haze.
But, this fear is insubstantial. The complete combustion of PG and VG in the juice usually breaks down the compounds into carbon dioxide and water molecules. Those are the same particles you get from fizzy water. Put in some flavor and color, and you’re practically breathing in the vaporous equivalent of carbonated soda.
Stackexchange user thomij briefly explains the chemical process in his answer to the original poster’s question “How does propylene glycol/glycerin form formaldehyde/acetaldehyde when vaporized with a vaporizer?”
“What happens is that the hydrocarbons in the propylene glycol and glycerin molecules are partially oxidized – oxygen reacts with them to ‘steal’ electrons. If the combustion was complete, you would wind up with just carbon dioxide and water. When combustion is incomplete, you can get any number of compounds, two of which happen to be formaldehyde and acetaldehyde. Carbon monoxide is another one I would be concerned about.”
Meanwhile, only minimal traces of the atomized nicotine have been found in the hazy plumes exhaled by vapers. In fact, most of the nicotine has already been absorbed by the body before the vapor exits the lungs.
“To date, there is no evidence that nicotine is present in the vapor from liquids with or without nicotine. Inhaled nicotine in cigarette smoke is over 98% absorbed, and so the exhaled mist of the e-cigarette is composed of propylene glycol, and probably contains almost no nicotine; and no CO [carbon monoxide].”
The rest of the ingredients, namely natural flavors and distilled water, are basically harmless. Water turns into steam as the e-juice is exposed to high heat emanating from the coils. And, the electronic cigarette user sucks the vapor into his or her mouth without fear of exposure to huge amounts of carcinogenic substances.
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