A group of Bucknell University student and faculty researchers has published a study that found some e-cigarettes produce carbon monoxide well above levels considered safe.

The study focused on e-cigarettes that allow users to adjust the level of power in the coil that heats and vaporizes nicotine-laced liquid in the process known as “vaping.” Some e-cigarettes allow users to adjust the power for reasons including increasing the strength and volume of the inhaled vapor.

The Bucknell researchers said they measured the carbon monoxide level produced at various power settings.

At a maximum setting of 200-watts, the e-cigarette produced carbon monoxide concentration of more than 180 parts per million. According to the researchers, the federal Occupational and Health Administration limits carbon monoxide exposure to an eight-hour average of 50 parts per million.

“We’re the first to show the relationship with carbon monoxide as a function of the power of the e-cigarette,” said Dabrina Dutcher, a professor of chemical engineering and chemistry. “The study proves that we really don’t know what’s coming out of e-cigarettes, but we now know there are potentially harmful chemical reactions.”

The researchers concluded, “vulnerable populations should be advised to either abstain from vaping or limit vaping to lower powers in order to minimize CO exposures.”

The study was published online by the journal Tobacco Control.

E-cigarettes have come under major criticism for reasons including surveys showing 28 percent of high school students vaped within the previous month. They are being blamed for reversing a downward trend of nicotine addiction among young people.

Still, vaping is believed to produce far fewer dangerous chemicals than regular cigarettes, with some leading medical experts crediting them with being a safer alternative to regular cigarettes.

Some also argue that, with the ability to adjust nicotine concentration, e-cigarettes can enable smokers to gradually wean themselves off nicotine and break their addiction.

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