Everywhere you turn these days it seems there's a new study warning about the health risks of yet another product, food or pastime.

Now it's the turn of scented candles, air fresheners and some cleaning products, because although they might smell great, new research suggests a chemical in them can react with the air to produce something far less pleasant, and possibly even deadly.

Bad smell

A small study by Professor Alastair Lewis of the National Centre for Atmospheric Science at the University of York, and a team from the BBC show Trust Me, I'm A Doctor, has found that a chemical used in some scented candles, air fresheners and cleaning products can lead to the production of a different, nasty, carcinogenic chemical in the home.

Professor Lewis measured levels of volatile organic chemicals in six modern houses, and found that one chemical detected in high quantities was limonene, commonly used to give scented products like candles and cleaning products a citrus smell.

Health hazard?

While limonene itself isn't a health hazard, it may cause problems when it's floating around inside our homes. When it's exposed to ozone in the air, every two molecules of limonene could produce one molecule of the chemical formaldehyde.

[Related story: 8 of the world’s smelliest plants]


The Health Protection Agency says formaldehyde is a possible human carcinogen, as well as being toxic and corrosive. It can also cause sore throats, coughs, scratchy eyes and nosebleeds.

The York study found that the levels of formaldehyde in the houses studied correlated with the levels of limonene, and Professor Lewis admits the high concentration of limonene found was a “big surprise”, and says the subsequent formation of formaldehyde was “significant”.

Healthy ventilation

Instead of snuffing out your scented candles and forgetting the air freshener and cleaning products, try opening a few windows.

“At issue here is the high concentrations that appear to be building up in modern homes,” says Professor Lewis, who points out that today's homes release so little energy and air that the high concentrations of formaldehyde linger longer and can cause long-term harm.

“The advice of course is to think about ventilation in homes, and for the manufacturers of these products to take account of the secondary products that might start to form in poorly-ventilated homes.”

Plant power

But if you don't want to open windows because it's so cold outside, there is another, less chilly option. Buy some plants.

As well as looking nice, houseplants can absorb pollution, including formaldehyde. In the study, although limonene levels remained high, formaldehyde levels came down when plants were brought into the houses.

Although the BBC suggests the best plants to absorb formaldehyde are English ivy (Hedera helix), geraniums, lavender and many ferns, other plants are also thought to gobble up this chemical nasty, including spider plants, certain lilies including the peace lily, philodendron, rubber plants and aloe.

How do you keep your home smelling nice? Tell us in the Comments box below.