Table 3.
Summary of findings for all daily ECU studies and exclusive ECU studies by study design.
| Outcome | Group1 | ECU exposure | Study design | Number of studies | Sample size | Relative effect2 | Certainty of evidence |
|---|---|---|---|---|---|---|---|
| Cardiovascular health | |||||||
| CV disease | ECUvs. NS | Exclusive | Cross-sectional | 2 | 380,644 | No statistically significant difference One study found, however,variationby CVD outcome, with a significantly higher odds of myocardial infraction, but not stroke or coronary heart disease |
Very low
 
Initial rating: low RoB: −1, Inconsistency: 0, indirectness: 0, imprecision: 0, publication bias: 0 |
| ECU vs. NS | All | Cross-sectional | 5 | 493,064 | No statistically significant difference (4/5 of the studies) |
Very low
Initial rating: low RoB: −1.5, inconsistency: 0, indirectness: 0, imprecision: 0, publication bias: 0 |
|
| Cardiovascular risk factors (No studies for exclusive ECU) | |||||||
| Blood pressure | ECU vs. NS | All | RCT | 3 | 237 | No statistically significant difference |
Very low
Initial rating: high RoB: −1.5, inconsistency: 0, indirectness: −0.5, imprecision: −1, publication bias: −1 |
| ECU vs. NS | All | Cross-sectional | 3 | 376 | No statistically significant difference |
Very low
Initial rating: low RoB: −1.5, inconsistency: 0, indirectness: −0.5, imprecision: −1, publication bias: −1 |
|
| ECU vs. TS | All | RCT | 4 | 335 | No statistically significant difference |
Very low
Initial rating: high RoB: −1.5, inconsistency: 0, indirectness: −0.5, imprecision: −1, publication bias: −1 |
|
| ECU vs TS | All | Cross- sectional | 3 | 510 | No statistically significant difference |
Very low
Initial rating: low RoB: −1.5, inconsistency: 0, indirectness: −0.5, imprecision: −1, publication bias: −1, |
|
| ECU vs DU | All | Cross- sectional | 1 | 88 | No statistically significant difference |
Very low
Initial rating: low RoB: −1.5, inconsistency: −1, indirectness: −0.5, imprecision: −1, publication bias: −1 |
|
| Lipid biomarkers | ECU vs NS | All | RCT | 2 | 237 | No statistically significant difference |
Very low
Initial rating: high RoB: −1.5, inconsistency: 0, indirectness: −0.5, imprecision: −1, publication bias: −1 |
| ECU vs NS | All | Cross- sectional | 2 | 451 | No statistically significant difference |
Very low
Initial rating: low RoB: −1.5, inconsistency: 0, indirectness: −0.5, imprecision: −1, publication bias: −1 |
|
| ECU vs TS | All | RCT | 3 | 914 | Mixed findings: No statistically significant difference (2/3 of the studies). The 3rdstudy showedbetter HDL and overall lipid profile in ECU |
Very low
Initial rating: high RoB: −1.5, inconsistency: 0, indirectness: −0.5, imprecision: −1, publication bias: −1 |
|
| ECU vs TS | All | Cross- sectional | 3 | 642 | Mixed findings: No statistically significant difference (2/3 of the studies). The 3rd study, reported significant higher odds of high triglycerides and HDL-cholesterol (c), but insignificant difference in LDL-c |
Very low
Initial rating: low RoB: −1.5, inconsistency: 0, indirectness: −0.5, imprecision: −1, publication bias: −1 |
|
| Cardiovascular function | ECU vs NS | All | Cross- sectional | 4 | 553 | No statistically significant difference (3/4 of the studies) |
Very low
Initial rating: low RoB: −1.5, inconsistency: 0, indirectness: −0.5, imprecision: −1, publication bias: −1 |
| ECU vs TS | All | RCT | 1 | 40 | Statisticallysignificant betterCV function inECU |
Very low
Initial rating: high RoB: −1.5, inconsistency: −1, indirectness: −0.5, imprecision: −1, publication bias: −1 |
|
| ECU vs TS | All | Cross- sectional | 5 | 411 | No statistically significant difference (4/5 of the studies) |
Very low
Initial rating: low RoB: −1.5, inconsistency: −1, indirectness: −0.5, imprecision: −1, publication bias: −1 |
|
| ECU vs DU | All | Cross- sectional | 1 | 88 | No statistically significant difference |
Very low
Initial rating: low RoB: −1.5, inconsistency: −1, indirectness: −0.5, imprecision: −1, publication bias: −1 |
|
| Immunological health | |||||||
| Inflammation | ECU vs NS | Exclusive | Cohort | 1 | 27 | No statistically significant difference |
Very low
Initial rating: high RoB: −1.5, inconsistency: −1, indirectness: −0.5, imprecision: −1, publication bias: −1 |
| ECU vs NS | All | Cohort | 4 | 299 | No statistically significant difference (3/4 of the studies) |
Very low
Initial rating: high RoB: −1.5, inconsistency: 0, indirectness: −0.5, imprecision: −1, publication bias: −1 |
|
| ECU vs NS | Exclusive | Cross sectional | 3 | 195 | Mixed findings: No statistically significant difference (2/3 of the studies), and statistically significant poorer biomarkers in ECU in the 3rd study |
Very low
Initial rating: low RoB: −1.5, inconsistency: −1, indirectness: −0.5, imprecision: −1, publication bias: −1 |
|
| ECU vs NS | All | Cross- sectional | 20 | 1549 | Mixed findings: No statistically significant difference in 40% of the studies (8/20), andstatistically significant poorer biomarkers in 60%(12/20) of the studies |
Very low
Initial rating: low RoB: −1.5, inconsistency: −1, indirectness: −0.5, imprecision: −1, publication bias: −1 |
|
| ECU vs TS | All | Cohort | 3 | 914 | No statistically significant difference (2/3 of the studies). Poorer biomarkers of inflammation in ECU in the 3rd study |
Very low
Initial rating: high RoB: −1.5, inconsistency: −1, indirectness: −0.5, imprecision: −1, publication bias: −1 |
|
| ECU vs TS | Exclusive | Cross- sectional | 1 | 151 | No statistically significant difference |
Very low
Initial rating: low RoB: −1.5, inconsistency: −1, indirectness: −0.5, imprecision: −1publication bias: −1 |
|
| All | Cross- sectional | 13 | 1152 | Mixed findings: No statistically significant difference in 54% (7/13) of the studies and statistically significant better biomarkers in 46% of the studies (n = 6/13) |
Very low
Initial rating: low RoB: −1.5, inconsistency: −1, indirectness: −0.5, imprecision: −1publication bias: −1 |
||
| Immune response | ECU vs NS | Exclusive | RCT | 1 | 27 | No statistically significant difference |
Very low
Initial rating: high RoB: −1.5, inconsistency: −1, indirectness: −0.5, imprecision: −1, publication bias: −1 |
| All | RCT | 5 | 299 | Mixed findings: No statistically significant difference(2/3 of the studies) |
Very low
Initial rating: high RoB: −1.5, inconsistency: −1, indirectness: −0.5, imprecision: −1, publication bias: −1 |
||
| Exclusive | Cross- sectional | 2 | 97 | No statistically significant differences (1.5/2 of the studies) One study showed insignificant differences and the other showed both statistically insignificant and poorer immune response in ECU |
Very low
Initial rating: low RoB: −1.5, inconsistency: −1, indirectness: −0.5, imprecision: −1, publication bias: −1 |
||
| All | Cross- sectional | 11 | 654 | Mixed findings: No statistically significant difference, (7/11 of the studies) |
Very low
Initial rating: low RoB: −1.5, inconsistency: −1, indirectness: −0.5, imprecision: −1, publication bias: −1 |
||
| ECU vs TS | All | RCT | 3 | 914 | Mixed findings: No statistically significant differences (2/3 of the studies). |
Very low
Initial rating: high RoB: −1.5, inconsistency: −1, indirectness: −0.5, imprecision: −1, publication bias: −1 |
|
| Cross-sectional | 8 | 721 | Statistically significant better immune response (7/8 of the studies, one of which also showed no significant difference) |
Very low
Initial rating: low RoB: −1.5, inconsistency: 0, indirectness: −0.5, imprecision: −1, publication bias: −1 |
|||
| Oxidative stress | ECU vs NS | All | Cohort | 2 | 237 | No statistically significant difference |
Very low
Initial rating: high RoB: −1.5, inconsistency: −1, indirectness: −0.5, imprecision: −1, publication bias: −1 |
| ECU vs NS | All | Cross- sectional | 4 | 512 | Mixed findings: No statistically significant difference in 2 studies and statistically significant poorer biomarkers in ECUin 2 studies |
Very low
Initial rating: low RoB: −1.5, inconsistency: −1, indirectness: −0.5, imprecision: −1, publication bias: −1 |
|
| ECU vs TS | All | Cohort | 3 | 914 | Mixed findings: No statistically significant difference (2/3 of the studies) |
Very low
Initial rating: high RoB: −1.5, inconsistency: −1, indirectness: −0.5, imprecision: −1, publication bias: −1 |
|
| ECU vs TS | All | Cross- sectional | 5 | 676 | Mixed findings: Statistically significant better biomarkers in ECU (3/5 of the studies) |
Very low
Initial rating: low RoB: −1.5, inconsistency: −1, indirectness: −0.5, imprecision: −1, publication bias: −1 |
|
| Oral health | |||||||
| Periodontal | ECU vs NS | Exclusive | Cohort | 1 | 59 | No statistically significant differences in clinical parameters |
Very low
Initial rating: high RoB: −1.5, inconsistency: −1indirectness: −0.5, imprecision: −1publication bias: −1 |
| All | Cohort | 2 | 10,020 | Mixed findings: Statistically significant worse periodontal self-reported parameters (1 study), and no statistically significant difference in clinical parameters (1 study) |
Very low
Initial rating: high RoB: −1.5, inconsistency: −1, indirectness: −0.5, imprecision: 0, publication bias: 0 |
||
| ECU vs NS | Exclusive | Cross-sectional | 3 | 198 | Mixed findings: Two studies showed no statistically significant difference in clinical parameters and showed mixed finding between worse and better parameters self reported outcomes |
Very low
Initial rating: low RoB: −1.5, inconsistency: −1, indirectness: −0.5imprecision: −1publication bias: −1 |
|
| All | Cross-sectional | 10 | 221,991 | Mixed findings:No statistically significant difference in clinical parameters (6/10 studies), significant worse clinical parameters in ECU (3/10 studies), worse oral hygiene and self-reported complains (2/10) and better oral hygiene (2/10 studies) |
Very low
Initial rating: low RoB: −1.5, inconsistency: −1, indirectness: −0.5imprecision: 0publication bias: −1 |
||
| ECU vs TS | All/ Exclusive | Cohort | 1 | 58 | No statistically significant difference in clinical parameters |
Very low
Initial rating: high RoB: −1.5, inconsistency: −1, indirectness: −0.5, imprecision: 0, publication bias: −1 |
|
| Exclusive | Cross-sectional | 2 | 208 | Mixed findings: Statistically significant better self-reported parameters in two studies, one of which also showed no statistically significant difference in clinical parameters |
Very low
Initial rating: low RoB: −1.5, inconsistency: −1, indirectness: −0.5, imprecision: −1, publication bias: −1 |
||
| All | Cross-sectional | 5 | 312 | No statistically significant difference in clinical parameters in all studies (n = 4); better parameters (missing teeth) in two studies |
Very low
Initial rating: low RoB: −1.5, inconsistency: −1, indirectness: −0.5, imprecision: −1, publication bias: −1 |
||
| Peri-implant | ECU vs NS | Exclusive | Cross-sectional | 1 | 80 | No statistically significant difference in clinical parameters |
Very low
Initial rating: low RoB: −1.5, inconsistency: −1, indirectness: −0.5, imprecision: −1, publication bias: – |
| All | Cross- sectional | 6 | 442 | Mixed findings: 30% of the periodontal parameters around the implants showing no significant differences, 30% showing statistically significant better outcomes and 40% showing statistically significant worse outcomes. |
Very low
Initial rating: low RoB: −1.5, inconsistency: −1, indirectness: −0.5, imprecision: −1, publication bias: −1 |
||
| ECU TS | Exclusive | Cross- sectional | 1 | 80 | Mixed findings: No statistically significant difference in BOP and PI, but statistically significantly better outcomes in PD and bone loss (RBL) in ECU |
Very low
Initial rating: low RoB: −1.5, inconsistency: −1, indirectness: −0.5, imprecision: −1, publication bias: −1 |
|
| All | Cross- sectional | 6 | 193 | Mixed findings: 50% of the periodontal parameters around the implants showing No significant differences (50%), statistically significant better outcomes (33%) and statistically significant worse outcomes (17%) |
Very low
Initial rating: low RoB: −1.5, inconsistency: −1, indirectness: −0.5, imprecision: −1, publication bias: −1 |
||
| Respiratory health | |||||||
| Lung function | ECU vs NS | Exclusive | Cohort | 1 | 21 | No statistically significant difference |
Very low
Initial rating: low RoB: −1.5, inconsistency: −1, indirectness: −0.5, imprecision: −1, publication bias: −1 |
| All | Cohort/ RCT | 3 | 247 | No statistically significant difference |
Very low
Initial rating: high RoB: −1.5, inconsistency: 0, indirectness: −0.5, imprecision: −1, publication bias: −1 |
||
| Exclusive | Cross-sectional | 1 | 60 | Statisticallysignifucantly worse results in lung function |
Very low
Initial rating: low RoB: −1.5, inconsistency: −1, indirectness: −0.5, imprecision: −1, publication bias: −1 |
||
| All | Cross sectional | 6 | 228 | No statistically significant difference (4/6 of the studies) |
Very low
Initial rating: low RoB: −1.5, inconsistency: −1, indirectness: −0.5, imprecision: −1, publication bias: −1 |
||
| ECU vs TS | All | Cohort/ RCT | 6 | 599 | No statistically significant difference in 4/5 of the studies (80%), the other two showed better outcomes |
Very low
Initial rating: high RoB: −1.5, inconsistency: 0, indirectness: −0.5, imprecision: −1, publication bias: −1 |
|
| All | Cross-sectional | 4 | 162 | No statistically significant difference |
Very low
Initial rating: low RoB: −1.5, inconsistency: 0, indirectness: −0.5, imprecision: −1, publication bias: −1 |
||
| ECU vs DU | All | Cohort/ RCT | 1 | 55 | No statistically significant difference |
Very low
Initial rating: high RoB: −1.5, inconsistency: −1, indirectness: −0.5, imprecision: −1, publication bias: −1 |
|
| Respiratory symptoms | ECU vs NS | Exclusive | Cohort | 1 | 21 | No statistically significant difference |
Very low
Initial rating: high RoB: −1.5, inconsistency: −1, indirectness: −0.5, imprecision: −1, publication bias: −1 |
| ECUvs NS | All | Cohort | 1 | 21 | No statistically significant difference |
Very low
Initial rating: high RoB: −1.5, inconsistency: −1, indirectness: −0.5, imprecision: −1, publication bias: −1 |
|
| All | Cross-sectional | 4 | 640 | No statistically significant difference in 75% (3/4) of the studies |
Very low
Initial rating: low RoB: −1.5, inconsistency: 0, indirectness: −0.5, imprecision: −1, publication bias: −1 |
||
| ECU vs TS | All | Cohort/ RCT | 4 | 367 | Statisticallysignifucant better respiratory symptoms in 75% (3/4) of the studies |
Very low
Initial rating: high RoB: −1.5, inconsistency: 0, indirectness: −0.5, imprecision: −1, publication bias: −1 |
|
| All | Cross-sectional | 3 | 486 | Mixed findings: no statistical signifucant difference in 2/3 of the studies), the 3rdshowed better respiratory symptoms |
Very low
Initial rating: low RoB: −1.5, inconsistency: −1, indirectness: −0.5, imprecision: −1, publication bias: −1 |
||
1Groups are e-cigarette users (ECUs), non-smokers (NSs), traditional smokers (TSs), and dual users (DU).
2Direction of association is determined by the direction of 75% or more of studies, otherwise studies were considered as having mixed findings.