As surprising as it is, people with asthma or breathing disorders can actually benefit from marijuana. It allows the lungs to loosen and helps the hairs in the lungs not be agitated anymore after a few coughs, and the body becomes relaxed as well as the brain which can help control the triggered attack of an asthma attack.
Marijuana doesn't do anything for someone's lungs. Anything other than air or medicine in the lungs can cause all kinds of maladies. Marijuana is good at relieving pain when nothing else works, hence the medical marijuana debate in several states. Smoking anything into someone's lungs that isn't a corticosteroid can be dangerous on any level.