ECs are marketed as safe products delivering pure nicotine and releasing harmless water vapor that vanishes in seconds [
3],[
4], but is this true? The research field is characterized by severe methodological problems, severe conflicts of interest, relatively few and often small studies, inconsistencies and contradictions in results, and a lack of long-term follow-up. Therefore, no firm conclusions can be drawn on the safety of ECs [
5],[
6], and much is left to subjective interpretations. Most probably ECs are less harmful than conventional cigarettes, but they can hardly be called safe. An experimental study found that cells exposed to high-nicotine vapor showed a similar pattern of gene expression to those exposed to tobacco smoke [
7]. Very short-term experimental exposure to EC vapor showed effects that are reminiscent of the obstructive effects seen with smoking [
8]-[
10], even though the impact on lung function was smaller than with smoking. An experimental animal study found that EC fluid can exacerbate allergy-induced asthma symptoms [
11]. Furthermore, a study found that the vapor induced release of cytokines and pro-inflammatory mediators [
12] and many studies have found cytotoxicity [
13],[
14] and harmful substances in fluid and vapor (e.g., fine or ultrafine particles [
15]-[
17], harmful metals [
13],[
18], carcinogenic tobacco-specific nitrosamines [
19]-[
23], carbonyls [
19],[
21],[
24],[
25], volatile organic compounds [
19],[
26], polycyclic aromatic hydrocarbons [
18],[
22]) or in urine [
18]. It is true that most studies found low or very low concentrations, but values below the threshold-limit do not necessarily protect against a negative health effect of 200 to 300 daily inhalations [
27] over decades.
The EC is a radically different product than the CC and, therefore, it seems wrong to base an assessment of the safety of EC on comparisons with CCs only. Of special concern are compounds not found in CCs: the glycols (propylene glycol and sometimes glycerin) are major ingredients of ECs [
28]-[
32] used to create the visible fume. A report commissioned by vapers and vendors of ECs concluded that
“estimated levels of exposure to propylene glycol and glycerin are close enough to threshold-limit values to warrant concern” and that
“the threshold-limit value is based on uncertainty rather than knowledge” [
33]. Several studies on glycols have raised health concerns [
34]-[
37]. Other concerns are the flavors, metals, and silicone [
13],[
14],[
38]. Finally, nicotine itself is probably not harmless [
39], and it is highly addictive. Studies show that non-smokers passively exposed to ECs absorb nicotine [
18],[
40],[
41].