New York: Air-fresheners may be very pleasing to your nose but catastrophic to your lungs, says a study conducted in New York.

A potentially harmful smog can form inside homes through reactions between air-fresheners and ozone, researchers at the US Environmental Protection Agency (EPA) say.

The reactions generate formaldehyde, classed as a probable carcinogen, and related compounds that many experts believe are responsible for respiratory problems, 'Nature' magazine quoted them as saying.

They studied the reactions between ozone gas and fragrance molecules such as pinene and limonene, which are emitted by air-fresheners that plug into electrical outlets.

Ozone, produced when vehicle emissions react with sunlight, is a common urban pollutant, and environmental bodies have set limits on outdoor levels of it, 'Nature' said.

"If you open a window on a high-ozone day, you could trigger these reactions," says Mark Mason, an environmental scientist at the EPA's National Risk Management Research Laboratory, who led the study.

Some people actually use ozone generators in their homes to remove unwanted odours and 'clean' their air, which could create indoor ozone levels that are much higher than those in the study.

"If you are concerned about indoor air, you should not introduce any extra chemical sources to your home, and that includes volatile organic compounds and ozone," the magazine quoted Frank Princiotta, director of the EPA's Air Pollution Prevention and Control division, as saying.