All e-cigarette flavors are not created equal. The taste that tickles your fancy might be hazardous to your health, according to new research from Pennsylvania State University.

Chemicals that make up the different flavors in e-liquid appear to have an effect on the production of free radicals — toxins that are often associated with cancers and other diseases, said John Richie, researcher and professor of public health and pharmacology at Penn State's College of Medicine.

The findings are another piece of evidence that vaping may not be the benign smoking substitute some hoped.

"When these products first came on the market, many people were saying they were harmless, and it was just water vapor," said Richie. "But now we know that e-cigarettes do produce free radicals, and the amount is affected by the flavorants added."

Previous studies found that diacetyl, a flavoring chemical used in e-cigarettes, has been associated with a severe lung disease known as Popcorn Lung because it afflicted workers in a microwave popcorn packaging plant. Other research has suggested that formaldehyde, a potential carcinogen, can be created when e-liquid is heated.

"We find that the heating process can produce free radicals from e-liquids," Richie said. "We know that just plain old e-liquid – propylene glycol and glycerine — can produce free radicals."

Richie and his colleagues' new research has found that the components used in various flavorants of e-liquid appear to either increase or decrease levels of free radicals.

Those flavorants are generally approved for consumption by the U.S. Food and Drug Administration, but they aren't evaluated for their safety when heated, noted the researchers.

"E-cigarettes have a coil for heating the liquid that gets quite hot and may aid the production of free radicals," Richie said. "It's important to look at the effect of flavor on these free radical levels because e-cigarettes come in hundreds of flavors, many of which are marketed toward kids, like bubblegum."

The researchers measured the free radicals produced by 50 flavors with one brand of e-cigarette. About 43 percent of the flavors were associated with significantly higher free radical levels while a few corresponded to lower levels. They then examined the individual chemicals found in the flavors.

They discovered six chemicals that that were associated with significantly increased free radical production, including linalool, dipentene and citral, which give products citrus or floral notes. One flavorant, ethyl vanillin –  for vanilla notes – decreased free radical production by 42 percent.

So just vape vanilla? Not so fast.

The flavors that were the worst in terms of free radicals included one called vanilla custard, as well as cotton candy, butterscotch, rainbow candy, bubblegum, and a cinnamon variety, Richie said.

Of course, it's not merely a matter of offending flavors. There are many manufacturers of e-cigarettes, and they can have very different formulas for the same flavor.

The researchers hope their findings may be helpful to the FDA in regulating these products.

In addition, Richie said their study results that suggest some flavor ingredients may reduce free radicals raises the possibility that some chemicals may be able to reduce or eliminate that threat in vaping products. But he said more research is needed.

"We really need studies to determine the potential toxicity of these products, and the finding that free radicals are being produced and can be affected by the flavor content raises concern," Richie said. "It gives us the opportunity to look more deeply into the potential health consequences that are associated with these flavors."