Relaxing in your bath tub, surrounded by scented candles seems like a great way to relax. But new research suggests that burning certain kinds of candles may generate indoor air pollutants.

Researchers at South Carolina State University have studied petroleum-based and vegetable-source candles to determine their emissions. They let candles from different manufacturers burn for up to six hours in a small box (8 inches x 8 inches x 26 inches), and then collected and analyzed the substances released into the air.

They found that paraffin-based candles, which are the most popular kind, emitted toxic chemicals like toluene and benzene. Soybean candles did not.

The researchers said that lighting a paraffin candle every once in a while is unlikely to pose a health threat. Although, frequently lighting several candles in an unventilated space could lead to problems. Also it may aggravate asthma, cause allergy-like symptoms, or irritate the respiratory tract.

However whether some candles are safer than others is still debatable, according to one expert. “I think there’s some controversy out there as to which candles are better than others,” says George Thurston, Ph.D., an associate professor of environmental medicine at the New York University School of Medicine.

Thurston urges people to be prudent about lighting any kind of candle — whether it’s paraffin, beeswax, or soybean — in an enclosed space. “Just lighting a match to start a candle creates sulfur pollution in the air,” he adds. “It’s one of the big sources of sulfur in the indoor environment, so using a lighter would probably be cleaner.”

While you aren’t putting your life on the line by lighting a candle, he says you should exercise at least some caution. “You ought to try to minimize your exposure to paraffin wax candles,” says Thurston. “And when you light any kind of candle, you might turn on the exhaust fan; even if it ruins the mood, it vents out the fumes. People just need to use common sense.”