The National Academies of Sciences, Engineering, and Medicine (NASEM)have published one of the largest reports to have ever investigated the effects of electronic cigarettes.

The report—which can be accessed in full here—reviews over 800 scientific studies and draws several conclusions on various health aspects that may be affected by the use of electronic cigarettes, also known as e-cigarettes.

The need for an in-depth analysis of e-cigarettes is evident given their widespread use. According to the Centers for Disease Control and Prevention (CDC), over 12% of the American adult population have at least tried an e-cigarette, and approximately 3.7% of Americans use e-cigarettes regularly.

E-cigarettes are even more prevalent among young people. Over 20% of adults aged between 18 and 24 have tried them, according to the CDC, although this number tends to decline with age.

Despite their widespread use, the health effects of e-cigarettes are still unclear.

"E-cigarettes cannot be simply categorized as either beneficial or harmful," says David Eaton, chair of the NASEM committee that wrote up the new report.

"In some circumstances, such as their use by non-smoking adolescents and young adults, their adverse effects clearly warrant concern. In other cases, such as when adult smokers use them to quit smoking, they offer an opportunity to reduce smoking-related illness."

David Eaton

Hopefully, the new NASEM report will help to elucidate this complex issue. Here are some of its conclusions, grouped by how strong the evidence is that supports them. The strength of the evidence ranges from "conclusive"—which is the strongest—to "insufficient" or "no evidence."

E-cigarettes, nicotine, and cancer

The report found conclusive evidence that the vast majority of e-cigarettes contain a wide variety of substances that are potentially toxic.

However, the report also found "conclusive evidence that completely substituting e-cigarettes for combustible tobacco cigarettes reduces users' exposure to numerous toxicants and carcinogens present in combustible tobacco cigarettes."

In terms of nicotine intake, the report uncovers "substantial evidence" that nicotine exposure from e-cigarettes "among experienced adult e-cigarette users can be comparable to that from combustible tobacco cigarettes."

There is also substantial evidence that "heart rate increases after nicotine intake from e-cigarettes," and that "some chemicals present in e-cigarette aerosols (eg, formaldehyde, acrolein) are capable of causing DNA damage and mutagenesis."

This is important because we can plausibly infer from this that long-term use of e-cigarettes could raise the risk of cancer, write the authors of the report. In fact, the report refers to animal studies that used intermediate biomarkers of cancer and whose results supported this hypothesis.

However, the authors also emphasize that these studies only amount to a level of "limited evidence," and that "[w]hether or not the levels of exposure are high enough to contribute to human carcinogenesis remains to be determined."

Additionally, "[t]here is no available evidence whether or not e-cigarette use is associated with intermediate cancer endpoints in humans," the report says.

Respiratory and developmental effects

"There is no available evidence whether or not e-cigarettes cause respiratory diseases in humans," the report concludes.

But the in-depth analysis of existing studies also found moderate evidence that e-cigarettes may lead to increased coughing and wheezing in teenagers, as well as an increase in asthma exacerbations.

The effects on adult users may differ slightly, however. The report found "limited evidence for improvement in lung function and respiratory symptoms among adult smokers with asthma who switch to e-cigarettes completely or in part."

Limited evidence was also found "for reduction of chronic obstructive pulmonary disease (COPD) exacerbations among adult smokers with COPD who switch to e-cigarettes completely or in part."

In terms of pregnancy and developmental effects, the report found "no available evidence whether or not e-cigarettes affect pregnancy outcomes," and "insufficient evidence whether or not maternal e-cigarette use affects fetal development."

The report mentions that more research is needed to fully understand if and under what circumstances e-cigarettes can help adults to quit smoking, and it states that the use of e-cigarettes should be discouraged among teens by restricting access and through educational and public awareness campaigns.