Reuters reported on December 13, 2007 on a study published in the December, 2007 issue of the Journal, Thorax, that pot smoking increases the risk of chronic bronchitis 2 times non pot smokers and asthma is increased 70%. Each joint seems to be the equivalent of 2.5 - 5 tobacco cigarettes.
Cannabis smokers had twice the risk of chronic bronchitis and a 70 percent increased risk of asthma diagnosed after age 16 years, they report in the medical journal Thorax.
Based on observed changes in lung function tests, the investigators calculate that one cannabis joint does as much harm as 2.5 to 5 tobacco cigarettes. "This provides patients with a good measure of the relative risks of cannabis and tobacco smoking," Beasley said.