The thought of lighting a scented candle can be very appealing, especially during winter or on a romantic evening.
But new research suggests that scented candles can be harmful, giving off potentially dangerous amounts of the toxic substance formaldehyde.
A study by Alastair Lewis, of the National Centre for Atmospheric Science, at the University of York, in the UK, found that an ingredient commonly used to give candles their scent changes into formaldehyde on contact with the air.
The ingredient is limonene, which is used to give a citrus tone to a candle’s aroma.
In its unaltered state, limonene is considered so safe that it is used to flavour food, and to give cleaning products and air fresheners a lemony scent. But limonene reacts with naturally occurring ozone when released into the air, causing one in every two limonene molecules to combine with ozone to form formaldehyde.
It has long been known that limonene, which occurs naturally in plants, can degrade into formaldehyde, but almost every test into its harmful effects was carried out decades ago. Lewis is worried because the concentrations of limonene he found in scented candles are up to 100 times higher than previously thought.
He said the ventilation of modern homes was such that high concentrations of formaldehyde linger longer and so can cause long-term harm.
Formaldehyde is listed as possibly a carcinogen, as well as being toxic and corrosive.
But Lewis has found that household plants such as English ivy, geraniums, and many ferns and lavender can reduce the amount of formaldehyde in the air.