Electronic cigarette-generated aldehydes: The contribution of e-liquid components to their formation and the use of urinary aldehyde metabolites as biomarkers of exposure

Figures & data

Figure 1. Formation of aldehydes as a function of the percentage of propylene glycol (PG) in a mixture with vegetable glycerin (VG). (a) Saturated aldehydes: formaldehyde and acetaldehyde; (b) unsaturated aldehyde acrolein and acetone (ketone for comparison). To generate aerosols from PG:VG, either neat or mix was loaded into a refillable, clear tank (≈0.5 mL) atomizer with a coil resistance of 1.8 Ohm (Mistic Bridge) coupled with a rechargeable bluPLUS+™ (3.7 V) battery (power output 7.6 W; where puff volume = 91 mL, puff duration = 4 s, frequency = 2 puff/min). Values are means ± SD (n = 3). In A, *p < 0.05: 100% VG vs. all % PG; 75, 50, and 25% PG; ⁺p < 0.05: 100% PG vs. 75, 50, and 25% PG. In B, *p < 0.05: 100% VG vs. all % PG; ⁺ p < 0.05 100% PG vs. all % VG.

Figure 2. Aldehyde levels generated from Set I e-cigarettes or from PG and VG. To genereate aerosols, Set 1 e-cigarettes (blu battery, 3.7 V, 4.56 W) or a refillable, clear tank (≈0.5 mL) atomizer with a coil resistance of 1.8 Ohm (Mistic Bridge) for carrier solutions (PG, VG; bluPLUS + battery, 7.6 W; where puff volume = 91 mL, puff duration = 4 s, frequency = 2 puff/min). Values are means ± SD (n = 3). CT: Classic Tobacco; MM: Magnificent Menthol; VV: Vivid Vanilla; CC: Cherry Crush; PG: 100% Propylene Glycol; VG: 100% Vegetable Glycerin. *p < 0.05: Formaldehyde: MM = CC > PG; Acetaldehyde: MM = PG = VG > CT; Acetone: CT = MM = VV = CC > PG = VG; Acrolein: CC = MM = VV > CT = PG; CC > VG.

Figure 3. Aldehyde levels generated from Set II e-liquids or from PG and VG. To generate aerosols from e-liquids e-liquids were loaded into a refillable, clear tank (≈0.5 mL) atomizer with a coil resistance of 1.8 Ohm (Mistic Bridge) coupled with a rechargeable bluPLUS+™ (3.7 V) battery (7.6 W; where puff volume = 91 mL, puff duration = 4 s, frequency = 2 puff/min). Values are means ± SD (n = 3).PG: 100% propylene glycol; VG: 100% vegetable glycerin. *p < 0.05: Formaldehyde: Menthol = Southern Ice > Mocha Café=PG = VG; Acetaldehyde: no statistically siginificant differences; Acetone: Mocha Café=Southern Ice ≥ Menthol = PG = VG; Acrolein: Southern Ice > Menthol = Mocha Café=VG > PG.

Table 1. Urinary metabolites (formate, acetate) of saturated aldehydes (formaldehyde, acetaldehyde) as biomarkers of exposure to tobacco products aerosols in mice.

Prod.:Metab.	Air control (bsl)	Post-exp. 0–3 h	Delta change 0–3h-bsl	Post-exp. 3–16 h
E-cig CT:formate	30.94 ± 3.44	30.77 ± 2.32	7.91 ± 2.13	45.46 ± 3.38
E-cig MM:formate	25.35 ± 4.51	24.99 ± 4.03	2.30 ± 3.12	30.41 ± 1.28
MCS: formate	38.57 ± 6.35	30.65 ± 3.35	–	57.22 ± 7.34
E-cig CT:acetate	72.27 ± 13.31	94.30 ± 8.23	33.37 ± 8.25	316.70 ± 65.77
E-cig MM:acetate	63.49 ± 9.65	176.20 ± 22.05	60.03 ± 26.82	143.55 ± 16.01
MCS: acetate	77.88 ± 8.60	115.18 ± 6.87	–	244.51 ± 39.62

bsl: baseline; Post-Exp.: timed urine collection post 6 h exposure; Delta Change 0–3 h; Post-Exp 0–3 h level minus bsl level; e-cig CT: Classic Tobacco; e-cig MM: Magnificent Menthol; MCS: mainstream cigarette smoke of KY Reference cigarette, 3R4F; –, below zero due to high variability and low n. Values = means ± SE in μg/mg creatinine; n = 5 for e-cig products and n = 3 for MCS. No statistically significant differences obeserved between e-cig CT vs. e-cig MM.

Figure 4. Time-sensitive urinary excretion of metabolites of two unsaturated aldehydes, acrolein (3HPMA) and crotonaldehyde (HPMMA), from mice exposed to e-cigarette aerosols or reference tobacco cigarette smoke. (a) Classic Tobacco (CT); (b) Magnificent Menthol (MM); (c) KY Reference Cigarette, 3R4F. Values are means ± SE (n = 5 different mice per exposure). *p < 0.05 between 0–1 h level – baseline for CT and MM; conditions of 4 h (e-cigarette) or 6 h (3R4F) exposure are shown in Supplementary Figure 1 and Supplementary Table 2.

Figure 5. Time-sensitive urinary excretion of nicotine (NIC) and its metabolites, cotinine (COT), and trans-3-hydroxycotinine (3HC), from mice exposed to e-cigarette aerosols or reference tobacco cigarettes. (a) Classic Tobacco (CT); (b) Magnificent Menthol (MM); (c) KY Reference Cigarette, 3R4F, shown as positive control. Values are means ± SE (n = 5 different mice per e-cigarette exposure and n = 3 for MCS exposure). Conditions of 4 h (e-cigarette) or 6 h (3R4F) exposure are shown in Supplementary Figure 1 and Supplementary Table 2.

Table 2. Sum of urinary nicotine and nicotine metabolites (cotinine, COT; trans-3′-hydroxycotinine, 3HC; ∑_NIC+COT+3HC) and the 3HC/COT ratio (marker of CYP2A6 activity) as time sensitive markers of overall tobacco exposure and metabolism in mice.

Product	0–1 h Post-exposure	1–2 h Post-exposure	2–3 h Post-exposure	3–16 H Post-exposure	0–3 h Post-exposure
E-cig CT: ∑	11.02 ± 0.53	4.86 ± 0.78	2.61 ± 0.60	0.58 ± 0.38	18.48 ± 1.32*
E-cig MM: ∑	14.12 ± 0.78	5.98 ± 1.84	6.08 ± 1.16	0.36 ± 0.18	26.18 ± 2.49*
MCS: ∑	66.06 ± 6.76	47.52 ± 8.86	36.27 ± 4.88	3.13 ± 0.51	–
E-cig CT: 3HC/COT	7.40 ± 0.80	11.13 ± 3.05	12.65 ± 6.17	15.59 ± 6.97	9.71 ± 1.44
E-cig MM: 3HC/COT	8.78 ± 0.41	11.63 ± 4.15	13.98 ± 3.81	NA	11.26 ± 1.40
MCS: 3HC/COT	4.27 ± 0.48	6.47 ± 0.98	17.69 ± 5.01	10.15 ± 2.46	8.89 ± 1.85

e-cig CT: Classic Tobacco; e-cig MM: Magnificent Menthol; MCS: mainstream cigarette smoke of KY Reference Cigarette, 3R4F; – not calculated; NA: not applicable due to zero denominator. Values = means ± SE in ng/mg creatinine (for ∑_NIC+COT+3HC data only); n = 4–5 for all products at all time points (except for product ratios at 3–16 h post-exposure where n = 1, 0, and 4 for CT, MM, and MCS, respectively). *p < 0.05 between e-cig CT and e-cig MM by t-test.

Figure 6. Theoretical thermal- and/or heating coil-induced generation of carbonyl compounds from Propylene Glycol (PG) and Vegetable Glycerin (VG) (compare with data in Figure 1).