Studies have demonstrated that THC may act as a temporary (1-2 hours) bronchodilator, especially when ingested. The
New England Journal of Medicine, published a 1973
study that stated, “Marihuana smoke, unlike cigarette smoke, causes broncho-dilation rather than broncho-constriction [narrowing of the air passages] and, unlike opiates, does not cause central respiratory depression [i.e. a decrease in breathing].”
Another long-term
study (20 years) done from 1985 to 2006, examined 5,000 subjects from three American cities. The researchers questioned the participants about their cannabis and cigarette use, and tested lung capacity as well as rate of air flow out of their lungs. To their surprise, the researchers found that at low to moderate levels of cannabis smoking, lung volume and air flow rates both increased with each “joint year” (365 joints), up until seven joint-years (or 2,555 joints).
http://www.medicaljane.com/2013/02/02/asthma-and-cannabis-marijuana-shown-to-manage-asthma-symptoms/