WASHINGTON - ONE third of people who breath in high levels of secondhand smoke have damage to their lungs similar to that seen in smokers, doctors reported on Monday.

They used a special kind of magnetic resonance imaging, or MRI, scan to look at the lungs of non-smokers who had high exposure to other people's cigarette smoke and found evidence of the kind of damage that causes emphysema.

'We interpreted those changes as early signs of lung damage, representing very mild forms of emphysema,' said Chengbo Wang, a magnetic resonance physicist at The Children's Hospital of Philadelphia, who led the study.

'Almost one third of nonsmokers who had been exposed to secondhand cigarette smoke for a long time developed these structural changes,' Mr Wang added in a statement.

'To our knowledge, this is the first imaging study to find lung damage in non-smokers heavily exposed to secondhand smoke. We hope our work strengthens the efforts of legislators and policymakers to limit public exposure to secondhand smoke.'

Mr Wang, who presented his team's findings to a meeting of the Radiological Society of North American in Chicago, said 35 per cent of US children live in homes where someone smokes regularly.

The team studied 60 adults between ages 41 and 79, 45 of whom had never smoked. The non-smokers were considered to have high exposure if they had lived with a smoker for at least 10 years, often during childhood.

'It's long been hypothesized that prolonged exposure to secondhand smoke may cause physical damage to the lungs, but previous methods of analysing lung changes were not sensitive enough to detect it,' said Mr Wang.

His team used a technique called long-time-scale, global helium-3 diffusion magnetic resonance imaging.

'With this technique, we are able to assess lung structure on a microscopic level,' Mr Wang said.

They found that 57 per cent of the smokers and 33 per cent of the nonsmokers with high exposure to secondhand smoke had signs of early lung damage as measured by the scan.

In February, US researchers reported that up to 20 per cent of women who develop lung cancer have never smoked. -- REUTERS