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A "dangerous" lack of awareness about smoking cannabis could be putting millions of people at risk, a leading charity warned today.

Most people (88%) believe smoking cigarettes is worse than cannabis but in fact the risk of developing lung cancer is 20 times greater from a cannabis joint than a legal tobacco cigarette.

A new report from the British Lung Foundation (BLF) claims there is an alarming disconnect between the public perception of cannabis as a relatively safe drug, and the serious, even fatal impact it can have on the lungs of people who smoke it.

Dame Helena Shovelton, chief executive of the British Lung Foundation, said: "It is alarming that, while new research continues to reveal the multiple health consequences of smoking cannabis, there is still a dangerous lack of public awareness of quite how harmful this drug can be.

"Young people in particular are smoking cannabis unaware that, for instance, each cannabis cigarette they smoke increases their chances of developing lung cancer by as much as an entire packet of 20 tobacco cigarettes."

According to the BLF survey, 6.8% of 16 to 59-year-olds in England and Wales have used cannabis in the past year - approximately 2.2 million people.

This makes cannabis the most commonly used illicit drug in the UK.

Dame Helena added: "This is not a niche problem - cannabis is one of the most widely-used recreational drugs in the UK, with almost a third of the population having tried it.

"We therefore need a serious public health campaign - of the kind that has helped raise awareness of the dangers of eating fatty foods or smoking tobacco - to finally dispel the myth that smoking cannabis is somehow a safe pastime."

The BLF said its report is the most comprehensive review of research data yet compiled on the subject of cannabis use.

The survey was carried out by TNS, on behalf of the charity, among a representative sample of 1,045 people across Britain.

Respondents were asked which, out of smoking a typical cannabis cigarette (also known as a joint or spliff) and smoking a typical tobacco cigarette (either from a packet or a roll-up), increases the risk of developing lung cancer the most.

A total of 88% said tobacco cigarettes posed the greatest risk, and just 12% cannabis.

Almost a third of the those surveyed (32%) said smoking cannabis is not harmful to health, with the figure rising to almost 40% among those the under-35s.

However, smoking one cannabis cigarette a day for a year increases the risk of lung cancer by 8%, according to the BLF report.

By comparison, smoking 20 tobacco cigarettes a day for a year increases the risk of lung cancer by 7%.

Smoking a cannabis cigarette therefore increases the smoker's risk of lung cancer by as much as 20 tobacco cigarettes.

The average puff on a cannabis cigarette is two-thirds larger and is held for four times longer than the average puff on a tobacco cigarette.

As a result, someone smoking a cannabis cigarette inhales four times as much tar as from a tobacco cigarette, and five times as much carbon monoxide.

The strength of herbal cannabis in the UK, as measured by the percentage content of tetrahydracannabinol (THC) - the principal psychoactive ingredient of cannabis - almost doubled (from 5.8% to 10.4%) between 1995 and 2007.

The increasing strength of cannabis means many studies conducted with people who used cannabis in previous decades may not be accurate.

The BLF report has called for a public health education programme to raise awareness of the impact smoking cannabis has on the lungs and wider health.

And more research is also needed, according to the charity, to find out more about the health impacts of using the drug.

Further information is available at www.lunguk.org.