Fact: Moderate smoking of marijuana appears to pose minimal danger to the lungs. Like tobacco smoke, marijuana smoke contains a number of irritants and carcinogens. But marijuana users typically smoke much less often than tobacco smokers, and over time, inhale much less smoke. As a result, the risk of serious lung damage should be lower in marijuana smokers. There have been no reports of lung cancer related solely to marijuana, and in a large study presented to the American Thoracic Society in 2006, even heavy users of smoked marijuana were found not to have any increased risk of lung cancer. Unlike heavy tobacco smokers, heavy marijuana smokers exhibit no obstruction of the lung’s small airway. That indicates that people will not develop emphysema from smoking marijuana.