Chemical constituents of e-liquid and e-cigarette aerosols
| Amount in e-liquids/e-cigarette aerosol | Amount in conventional tobacco cigarette | Permissible limit | Toxic effect | Molecular mechanism of toxicity | |
| Acetaldehyde | 0.10–15.63 mg·L−1 (refill solution) [68] or 2.0±0.1–13.6±2.1 µg per 150 puffs [69] | 680 µg per cigarette [70] | 45–270 ppm for 1 h (AEGL and NAC) [71] | Eye, skin and respiratory tract irritation on acute exposure Coughing, erythema, pulmonary oedema and necrosis on higher exposures Probable carcinogen based on animal and human cancer studies [72, 73] | Readily binds to protein and DNA, forming damaging adducts and impairing normal function and enzyme activity [74, 75] |
| Acetone | 2.9 mg·m−3 [76] or 0.16 ppm per 38-mL puff [77] | 287 µg per cigarette [70] | 750–1000 ppm per 8-h work shift as per OSHA guidelines [78] | Respiratory irritant in small quantities; nausea, CNS depression and cardiorespiratory failure in large amounts [78, 79] | Metabolism in high amounts is not possible, leading to its accumulation and toxicity [78] |
| Acrolein | ND to 41.9±3.4 µg per 150 puffs [69] | 60–140 µg per cigarette [70, 80] | 0.1 ppm per 8 h as per OSHA guidelines [81] | Highly toxic even in small quantities Respiratory and cardiovascular toxicant Possible carcinogen [79, 81] | Highly reactive, leading to DNA and protein adduction, endoplasmic reticulum stress, membrane damage, mitochondrial disruption, oxidative stress and immune dysfunction [82] |
| Cadmium | ND | 0.103 µg per cigarette [70] | 5 µg·m−3 of air for 8 h as per OSHA guidelines [83] | Chronic inhalation causes pulmonary changes with obstructive damage Toxicity leads to renal dysfunction Associated with teratogenicity in animals, but limited data on humans [84, 85] Classified as a carcinogen, showing association with lung and prostate cancers [86] | Has a long biological half-life (∼15–20 years) Interacts with DNA repair machinery, acts as a catalyst for ROS production, increases lipid peroxidation and induces apoptosis in cellular systems [87, 88] |
| Chromium (Cr) | 0.007 µg per 10 puffs [28] | 0.0042 µg per cigarette [70] | 5 µg·m−3 of air [89] | Acute exposure causes fever, nausea, vomiting, diarrhoea; chronic exposures may cause irritation, nasal ulcers and perforations Lung cancer is a major long-term effect [70, 79] | Most of the toxicity is attributable to chromium(IV). Chromium(IV) is readily soluble and under physiological conditions, can produce reactive intermediates, hydrogen peroxide and GSH, which can attack DNA, protein and membrane lipids [90] |
| Formaldehyde | 0.02–10.09 mg·L−1 refill solution [68] or 3.2±0.8–56.1±1.4 µg per 150 puffs [69] | 20–100 µg per cigarette [70, 80] | 0.75 ppm per 8-h work shift as per OSHA guidelines [91] | Potent respiratory stimulant causing irritation, cough, wheezing, significant respiratory inflammation, pneumonia and bronchitis on exposure Has been recognised as a carcinogen Has more toxic effects on children compared to adults Known to cause neurological symptoms upon high-level exposure [70, 79, 91] | Highly reactive electrophilic reagent that can easily attach to neutrophilic biological targets, leading to formation of harmful adducts and ROS [92] |
| Nicotine | 0–87.2 mg·mL−1 refill solution or 0.3±0.2–8.7±1.0 mg per 150 puffs [93] | 1.0–1.5 mg per cigarette [80] | 0.5 mg·m−3 skin [94] | Nausea, vomiting, increased salivation, bronchorrhea, hyperpnoea, hypertension, tachycardia, vasoconstriction, headache, restlessness, etc. Causes developmental defects Addictive [94, 95] | Mechanism not fully understood, but toxicity attributed to oxidative damage, lipid peroxidation and DNA adduct formation [96] |
| N-Nitrosamines | ND to 28 µg per 150 puffs [69] | 0.019–72 µg per cigarette [70] | 0.3 ng·m−3 [97] | Carcinogen [70] | Forms diazonium or oxynium ions which cause alkylating DNA Activates various oxidative damage and radical scavenging pathways [98] |
| Toluene | ND to 6.3±1.5 µg per 150 puffs [69] | 72.8 µg per cigarette [70] | 200 ppm (300–500 ppm for a maximum 10 min per 8-h shift, as per OSHA guidelines) [99] | Neurotoxicity including headache, nausea, euphoria, depression, cognitive impairment, etc. [100] | Rapidly absorbed, has high affinity for lipids and crosses the blood–brain barrier Metabolises to form hippurate ions resulting in metabolic acidosis and hypokalaemia Has been found responsible for increased production of dopamine [101] |
| Lead | 0.03±0.03–0.57±0.28 µg per 150 puffs [69] | 0.00128 µg per cigarette [70] | 50 µg·m−3 per 8 h (NIOSH) [102] | Neurotoxin, cardiotoxin, behavioural changes and developmental changes [103] | Causes oxidative stress and ionic imbalance [104] |
AEGL: acute exposure guideline level; NAC: National Advisory Committee; OSHA: Occupational Safety and Health Administration; CNS: central nervous system; ND: not determined; ROS: reactive oxygen species; GSH: glutathione; NIOSH: National Institute for Occupational Safety and Health.