Scientists have found six substances strongly linked to bladder cancer in the urine of people who had used electronic cigarettes, and sometimes other tobacco products, a finding that may lead to new clinical recommendations for those who vape.
The review research, published in the journal European Urology Oncology, assessed the results of 22 different studies which analysed the urine of e-cigarette users, or those using tobacco products like cigarettes.
In the study, the researchers, including those from the University of North Carolina (UNC) in the US, checked for evidence of cancer-linked compounds, or biomarkers of those compounds.
"Smoking is the No.1 modifiable behavioral risk factor for bladder cancer," said Marc Bjurlin, study co-author from UNC.
"There is now evolving literature showing that people who vape may have similar carcinogens in their urine as combustible cigarette users," Bjurlin said.
While public health agencies including the US Food and Drug Administration and the Centers for Disease Control and Prevention have warned that there are health risks of vaping, their safety profile has not been "definitively characterised," the scientists noted.
"The first and foremost side effects that we're seeing from electronic cigarette use are lung and pulmonary related," Bjurlin said.
"We won't see the side effects for these other kinds of carcinogenic pathways until much later down the exposure pipeline," he added.
In the study, the researchers analysed possible exposure to substances that can cause bladder cancer in particular, since carcinogens could be processed in the body and then passed in urine.
Bjurlin and his colleagues reported 40 different compounds in vapes that can be processed in the body to produce 63 different toxic, cancer-causing chemicals, or carcinogens.
According to the researchers, six of the chemicals have a strong link to bladder cancer.
Some of the studies they analysed noted that e-cigarette users had "significantly" higher levels of several carcinogens that can be converted in the body into substances linked to bladder cancer in their urine.
"This finding shows us that people who vape will be exposed to a variety of different carcinogens," Bjurlin said.
"People who have decades of exposure to these carcinogens from vaping may be at risk for developing malignancies, especially bladder cancer," he added.
However, the researchers said they do not know the levels of all of the cancer-causing substances in the urine of users from the studies.
"The study population was quite heterogeneous, meaning that often studies looked at dual users, meaning those who used e-cigarettes and combustible cigarettes," Bjurlin said.
"That made it difficult to assess whether the carcinogen found in the urine was actually from the e-cigarette use or from the cigarette use," he added.
While there is no definitive case linking bladder cancer to vaping yet, the scientists believe it is reasonable to suspect that decades down the road after exposure to the byproducts, e-cigarette users may be at risk of developing bladder cancer.