As a health professional, I started telling smokers to switch to smokeless tobacco in 1994, based on decades of definitive epidemiological evidence for the relative safety of smokeless tobacco. But I was slow to endorse e-cigarettes as a reduced-risk option for smokers.
There is no parallel body of evidence for e-cigarettes. But then, there is no scientific evidence that would link vapor inhalation to cancer, heart attacks or strokes. That is significant, but as a pathologist, I must consider whether long-term vapor consumption can cause respiratory problems. There is little human experience with intense, long-term inhalation of propylene glycol, vegetable glycerin and other agents, including flavorings.
Konstantinos Farsalinos and colleagues in 2014 reported a laboratory analysis  of 159 e-cigarette liquids from 36 manufacturers/retailers in six European countries and the United States. They found that almost 70 percent of the samples contained varying amounts of diacetyl (DA) and/or a similar flavor compound, acetylpropionyl (AP). Although these substances naturally occur in fermented products like cheese and beer, they are also added to foods to provide a butter-like flavor. They are generally recognized as safe by the Food and Drug Administration when added in small quantities to foods.
DA and AP are known to cause bronchiolitis obliterans, a serious, sometimes-fatal lung disease in exposed workers (see here , here  and here ), most notably in plants producing buttered popcorn .
Farsalinos estimated the amount of DA and AP that vapers would inhale using the tested liquids. He concluded:
The median daily exposure levels were slightly lower than the strict [National Institute on Occupational Safety and Hazards]-defined safety limits for occupational exposure and 100 and 10 times lower compared with smoking respectively; however, 47.3 percent of DA and 41.5 percent of AP-containing samples exposed consumers to levels higher than the safety limits.
Farsalinos’ study should have prompted e-liquid suppliers to abandon those agents. They have not.
Raquel Rutledge, a reporter for the Milwaukee Journal Sentinel, recently worked with a Marquette University chemist to test liquids sold in local vape shops. She “bought five e-liquids dubbed top-sellers by sales clerks… and had them tested for [DA and AP]… All five contained both chemicals.” According to her story , some had high levels.
Staff at the vape shops selling the liquids were unaware of the presence of these toxins.
It is unacceptable for any vape shop to sell liquids with flavoring ingredients that are proven respiratory toxicants. Vapers should only use liquids that are certified to be free of these agents.
- “reported a laboratory analysis”: http://www.ncbi.nlm.nih.gov/pubmed/25180080
- “here”: http://www.ncbi.nlm.nih.gov/pubmed/21800743
- “here”: http://www.ncbi.nlm.nih.gov/pubmed/17541015
- “here”: http://www.ncbi.nlm.nih.gov/pubmed/17464280
- “buttered popcorn”: http://www.ncbi.nlm.nih.gov/pubmed/12151470
- “her story”: http://www.jsonline.com/watchdog/watchdogreports/lab-tests-reveal-popular-e-cigarette-liquids-contain-harmful-chemicals-b99583582z1-334833961.html