Lighting a scented candle at the end of a long day helps many of us unwind.

But that flickering flame might not seem quite so relaxing once you know it is spreading pollution around your home.

Candles release carbon and metal particles which could raise the risk of heart and lung problems, a study has warned.

The scented candle market is worth £90million in Britain and they have become a favourite gift, used to freshen living rooms or add a gentle glow to bathtimes.

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But the US study found burning candles of any type in the home increases particles of pollution by 30 per cent - and opening doors and windows does very little to help.

Frying and burning food have the same effect. So lighting a scented candle after burning the dinner might only make matters worse, the Daily Mail reports.

Lead author Dr Neil Klepeis, from San Diego State University, said he wants to help families reduce exposure to pollutants, especially if there are children in the house.

Environmental scientist Professor Barbara Maher, from the University of Lancaster, was not surprised at the US team's findings.

"My research team have directly measured the particles released by candles and found they contain carbon compounds and metals, the latter most probably from the industrial production process,' she said.

"This has made me change my own behaviour - I used to like to light a scented candle at home but now think it is a bad idea. We don't know exactly what harm these particles cause but they force the body to launch an inflammatory response to clear them. Indoor pollution is likely to be linked to the same health problems, from lung cancer to strokes, that we see from outdoor pollution."

The US researchers scanned 300 family homes for pollution including fine particles between 0.5 and 2.5 micrometres in size - a range that includes dust, fungal spores and vehicle emissions.

Smoking at home was found to be the major source of airborne pollution. But burning candles and frying food with oil raised levels by almost a third.

Air pollution expert Professor Ian Colbeck, from the University of Essex. said he believed the risk of health problems was the same for scented and unscented candles.

The US study, published in the journal PLOS ONE, concluded that indoor pollution was even worse for big families living in small houses.