1. What are the main ingredients in vape juice?

Vape juice contains four primary ingredients: propylene glycol (PG), vegetable glycerin (VG), nicotine (either freebase or salt form), and flavoring compounds. PG and VG typically make up 90% or more of the liquid by volume, with nicotine and flavorings comprising the remainder.

2. Is propylene glycol safe to inhale?

Propylene glycol is FDA-approved as Generally Recognized as Safe (GRAS) for ingestion in food, but this designation does not extend to inhalation. A 2024 review in Nicotine and Tobacco Research concluded that GRAS status cannot serve as an indicator of toxicity when PG is aerosolized and inhaled. When heated above 350°C, PG can produce formaldehyde, acetaldehyde, and methylglyoxal.

3. What is acrolein and why is it in vape aerosol?

Acrolein is a toxic aldehyde classified as a hazardous air pollutant by the WHO. It is not an added ingredient but a thermal decomposition byproduct that forms when vegetable glycerin (VG) is heated above approximately 280°C. Acrolein is the same toxic compound responsible for much of cigarette smoke's cardiovascular damage.

4. Does vape juice contain diacetyl?

Some e-liquids have been found to contain diacetyl, a butter-flavoring chemical linked to bronchiolitis obliterans (popcorn lung) in occupational settings. Not all e-liquids contain diacetyl, and many reputable manufacturers have removed it. However, consumers cannot easily verify this since flavoring ingredients are often listed under general categories rather than individual chemical compounds.

5. What heavy metals are found in vape aerosol?

Heating coils made of nickel, chromium, and iron can release metal particles into the aerosol. A 2024 study found that at low pH (acidic e-liquids), nickel reached 2,649 parts per billion and chromium exceeded inhalation safety thresholds. A 2025 UC Davis study found some disposable e-cigarettes emitted lead levels equivalent to smoking nearly 20 traditional cigarettes.

6. What is methylglyoxal and why is it concerning?

Methylglyoxal (MGO) is a toxic aldehyde byproduct of PG thermal decomposition. A 2026 UC Riverside study found that MGO was 10 to 100 times more abundant in used disposable devices compared to fresh e-liquid. At low concentrations, MGO damaged mitochondria and weakened the actin cytoskeleton in human airway tissue.

7. Are FDA-authorized vape juices safer?

FDA-authorized products have undergone PMTA review, which requires ingredient disclosure and manufacturing standards. However, authorization does not mean safe. The FDA evaluates whether a product is "appropriate for the protection of public health" — not whether it is harmless. All vaping products carry some risk from thermal decomposition byproducts.

8. What is the safest PG/VG ratio?

A 2024 study by Sun et al. found that a 25:75 PG/VG ratio produced the lowest carbonyl release among tested ratios. Higher VG content generally means thicker vapor but also more acrolein formation when heated. There is no completely safe ratio, as all ratios produce some thermal byproducts when heated.

9. Can flavorings in vape juice be harmful?

Yes. A 2022 systematic review found that cinnamon, strawberry, and menthol flavors showed greater cytotoxicity than other flavors. Some flavoring compounds like vanillin and sucralose can catalyze the thermal degradation of PG and VG, increasing formaldehyde and acrolein formation.

10. What caused the EVALI outbreak in 2019?

The 2019 EVALI outbreak was caused by vitamin E acetate contamination in illicit THC vaping products, not by legal nicotine e-liquids sold through regulated channels. Vitamin E acetate is not a recognized ingredient in FDA-authorized nicotine e-liquids. The CDC identified it as the primary contaminant in products associated with EVALI cases.