Cannabis contains most of the chemicals of a tobacco cigarette, but the damage induced to the lungs can be higher.

A 2002 report of the British Lung Foundation showed that 3-4 cannabis cigarettes daily induced the same damage to the lungs as over 20 tobacco cigarettes. But researchers find it difficult to separate the effects of cannabis smoking from those of tobacco smoking in patients, as most of them smoke both.

"I've got a collection of young people who have lungs that look like they're 65-year-olds." said Dr Onn Min Kon, a consultant physician of respiratory medicine at St Mary's Hospital in London, referring to some of his cannabis smoking patients.

"If I don't stop smoking I won't be around much longer - there is no cure for emphysema, the holes in my lungs are getting bigger. There should be adverts showing people like me. I'd never even heard of emphysema when I was diagnosed, and I don't think people know there is a risk of getting it." said a 37 years old patient of Kon, who smoked cannabis for 20 years. She started with two joints daily and eventually reached 10 a day, while an average tobacco smoker smokes 13-14 cigarettes daily.

Emphysema has no cure and usually this progressive condition normally encountered with older people who have smoked tobacco throughout their lives is deadly. These cases prompted Kon to plan a study making a comparison between the lungs of patients who smoke cannabis with those who smoke only tobacco. He will employ lung function tests and CT scan images to assess the lung damage.

"Even people with apparently normal lung function could have problems, such as holes in their lungs, which would show up on these scans. These people may have no symptoms but could still have respiratory diseases," he said.

"Research carried out by the British Lung Foundation found that smoking cannabis alone can cause severe lung damage. It is vital that people are fully aware of the dangers so they can make an educated choice and know the risks fully." said a spokesperson of the British Lung Foundation.