E-cigarette vapor linked to cancer in mice
Mice exposed to e-cigarette vapor containing nicotine had an increased risk of developing lung cancer and pre-cancerous changes in the bladder.
While these results can't predict how e-cigarette vapor might affect people, they highlight the need for more studies into the potential toxicity of e-cigarettes.
Tobacco smoke is known to be both addictive and toxic. It's addictive because it contains nicotine, a chemical that stimulates the nervous system and causes physical dependency. It's toxic because the smoke contains a host of compounds that can cause cancer and other health problems. Nicotine itself can be transformed into toxic compounds during the process of curing and burning tobacco.
In the past decade, e-cigarettes have been marketed as a less harmful alternative to smoking tobacco. But there is still much to learn about both the short- and long-term health effects of these products. In addition to nicotine, they contain other chemicals, including flavorings. Recently, concerns have been raised about the skyrocketing number of children and adolescents using e-cigarettes.
In previous work, researchers led by Dr. Moon-shong Tang from the NYU School of Medicine found that mice exposed to nicotine-containing e-cigarette vapor for 12 weeks showed DNA damage in their lungs, heart, and bladder. Their lung cells also showed a reduced capacity for DNA repair.
That same study discovered that a chemical process found in human cells could convert nicotine into potentially cancer-causing compounds. To understand how these processes may relate to cancer risk, the researchers performed a longer-term study in mice.
One group of 40 mice was exposed to e-cigarette vapor containing nicotine for 4 hours a day, 5 days a week for just over a year. A control group of 18 mice was exposed to e-cigarette vapor without nicotine. A third group of 18 only breathed filtered air.
At the end of the exposure period, the researchers looked for cancer and pre-cancerous changes. The study was funded by NIH's National Cancer Institute (NCI) and National Institute of Environmental Health Sciences (NIEHS). Results were published on October 22 in Proceedings of the National Academy of Sciences.
Nine out of the 40 mice (22.5 percent) exposed to the nicotine-containing vapor developed one or more visible lung tumors. By comparison, only one mouse in one of the control groups developed lung cancer.
Although visible tumors weren't found in the bladders of any of the mice, tissue analyses found pre-cancerous changes in the bladders of 23 (57.5 percent) mice exposed to the nicotine-containing vapor. Similar changes were found in only one of the control mice.
The researchers caution that these results can't prove how e-cigarette vapor affects the human body. The research had several limitations, including using a relatively small number of mice that are naturally prone to developing lung cancer. However, the findings do suggest caution for e-cigarette use and highlight the need for further study.
"Our study results in mice were not meant to be compared to human disease, but instead argue that e-cigarette smoke must be more thoroughly studied before it is deemed safe or marketed that way," Tang says.
-by Sharon Reynolds
For further information on this and other health topics, visit the web site of the National Institute of Health at www.nih.gov.