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Breathing in second-hand cigarette smoke has been shown to increase the risk of cancer in humans.

Now, researchers have shown that passive smoking affects plants too, which can take up nicotine from contaminated soil and plumes of smoke.

The finding may explain why some spices, herbal teas and medicinal plants have high concentrations of nicotine in them, despite none being allowed in insecticides.

Researchers have shown that passive smoking affects plants too, which can take up nicotine from contaminated soil and plumes of smoke (illustrated with a stock image)

Nicotine was frequently used as an insecticide until it was banned by the European Union in 2009 because of its toxicity.

But a surprisingly large number of food crops and plant-derived products still contain very high levels of nicotine.

Dirk Selmar and his colleagues at the Technical University of Braunschweig, Germany, set out to uncover whether there are reasons for this beyond the use of illegal insecticides.

They used peppermint plants, which contain minimal traces of nicotine, in a series of mulching and fumigation experiments.

He said: 'Tremendously elevated nicotine levels were detected after fumigation with cigarette smoke.'

The researchers showed for the that that peppermint plants (a stock image is shown) can take up high concentrations of nicotine from contaminated soils, as well as taking in the drug from smoke

They also showed for the first time that that peppermint plants can take up high concentrations of nicotine from contaminated soils.

The team analysed plants in soil mulched with cigarette tobacco for more than nine days to find that the resulting nicotine concentrations in them were several times higher than the maximum residue level set by European authorities.

The study, published in Springer's journal Agronomy for Sustainable Development, suggests that the reported high levels of this substance may indeed originate from tobacco.

The researchers found a drastic decrease in nicotine concentration in plants as time progressed.

This is likely because nicotine was taken up by the roots of the peppermint plants and processed in their leaves.

'Our results suggest that the widespread occurrence of nicotine in medicinal, spice and food plants may, at least in part, be due to other nicotine sources apart from the illegal use of insecticides,' Dr Selmar said.

As well as being interesting to the food industry, the findings prove that substances such as alkaloids, can be transferred from one plant, after its death, to another.

Such 'horizontal transfer of natural products' sheds light on the hitherto unexplained success behind farming practices such as crop rotation and the co-cultivation of certain vegetables.