Candles create an atmosphere that may be romantic but is also laced with cancer chemicals, researchers have warned.
Burning paraffin wax candles is an unrecognised source of indoor pollution, say scientists.
Levels of chemicals released by candles can build up in unventilated closed rooms, the American Chemical Society's annual meeting in Washington DC was told.
They include harmful substances such as toluene and benzene, which are known to cause cancer.
Candle emissions could also irritate the lungs and trigger asthma attacks, said the researchers.
Some cases of indoor allergy may actually be a response to inhaling candle chemicals, it was claimed.
People who frequently used candles, for instance to help them relax in the bath or provide the right ambience for dinner, are most at risk.
Dr Amid Hamidi, from South Carolina State University in Orangeburg, who co-led the study, said: “An occasional paraffin candle and its emissions will not likely affect you. But lighting many paraffin candles every day for years or lighting them frequently in an unventilated bathroom around a tub, for example, may cause problems.”
The scientists suggested switching to candles made from beeswax or soy, which were believed to be safer. To investigate candle emissions, the researchers burned a range of candles in the laboratory and collected the mixture of substances they gave off.
These were then analysed in a machine which separated out the individual elements and identified their atoms by weight. Paraffin-based candles produced “clear sharp peaks” for many chemicals.
Burning candles does not produce high enough temperatures to combust the heavy molecules contained in paraffin wax completely, said the scientists. This led to the formation and emission of hazardous molecules such as toluene and benzene.
Dr Joanna Owens, of Cancer Research UK, said: “There is no direct evidence that everyday use of candles can affect our risk of developing cancer. In terms of cancer, a far more significant type of indoor air pollution is second-hand cigarette smoke.”
Dr Noemi Eiser, of the British Lung Foundation, said: “We would like to reassure people that occasional use of paraffin candles should not pose any risk to their lung health.”