A key ingredient in the aroma from citrus fruits such as oranges and lemons appears to protect rats from the symptoms of asthma, new research shows.

Study author Dr. Ehud Keinan explained that the citrus ingredient is called limonene, and it likely protects against asthma by “burning” inhaled ozone, which can increase inflammation in the lungs.

Other scents - such as those emitted from pine trees, geraniums and roses - contain similar ingredients to limonene, Keinan said, which may help explain why asthma is much more common in urban areas that lack vegetation.

“In rural populations, people are very much exposed to these compounds,” he said.

The researcher, who is based at the Technion-Israel Institute of Technology in Israel, told Reuters Health that squeezing an orange peel releases liquid that contains a high concentration of limonene. He said he has heard stories of people who say they experienced relief from asthma and other lung diseases after spending time around limonene.

He added that he and his colleagues, who report their current findings in the journal Bioorganic & Medicinal Chemistry, are currently investigating how limonene and similar substances may help alleviate asthma in humans.

A growing body of research suggests that ozone, which is a key component of air pollution, can encourage changes in the body that result in persistent inflammation in the airways.

Limonene helps rid the body of ozone because it reacts with ozone, muting its toxic effects, Keinan explained.

To investigate whether limonene could protect lungs from asthma, Keinan and his team induced the symptoms of asthma in rats, them let them smell limonene or eucalyptol, the key ingredient in the odor of eucalyptus, which does not react with ozone.

The researchers checked the rats for asthma symptoms repeatedly over a period of 20 hours to five days. They found that only rats exposed to limonene “didn’t show any symptoms of the disease,” Keinan said.

These results suggest that inhaling limonene may protect people from developing asthma, or alleviate symptoms in those already diagnosed, he noted.

SOURCE: Bioorganic & Medicinal Chemistry, December 8, 2004.