There are a few misconceptions here that need to be addressed. First, marijuana is not smoked like cigarettes are. Second hand smoke from cigarettes is (in the general sense) thick and prolonged. Marijuana is not smoked (again in the general sense), one joint after another.
Cigarettes and their smoke has many carcinogens. The delivery system for cigarettes is very efficient, even a short inhale and quick exhalation is enough to deliver a significant dosage of nicotine. Marijuana, on the other hand needs several seconds to exchange with the alveoli.
There are hazards in almost all environments. Especially regarding children. I notice no concern for insecticide sprays, fresh paint, and common chemicals in almost all households.
Most of the research on marijuana is quite promising. Along with its positive effects on cancer treatment side effects and glaucoma, new research is finding very interesting benefits from canabinoids. Canabinoids are showing ways to stop the activity of the gene responsible for metastasis in breast and other types of cancers. It appears to relieve convulsion, inflammation, anxiety, and nausea. It also has antipsychotic benefits and appears to offer a new (potential) treatment for schizophrenia.
To put all my cards on the table, there is also some "substance induced psychosis.