Yes... Any second hand smoke or smoke in it's own is harmful to a child... NOT just marijuana smoke but any type if smoke at all can cause breathing difficulties, Asthma, pneumonia, respiratory problems and so on... Frequent exposure will most likely cause at least one of these issues... Tobacco smoke is very bad, (even second hand mixed with marijuana) not only can it cause one of the above problems, but can also cause cancer, Heart disease, stroke and so on...
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Not very much if you discount the tobacco that may be mixed with it. The only 'serious' claim that just being around pot users can affect your own system was made by an athlete who had failed his drug test and who claimed it was because he had attended parties where pot had been smoked.
Unless you were to be surrounded by people blowing marijuana fumes in your face in an unventilated room, you should be fine.
Seeing as first hand marijuana smoke has no negative side effects, it's second hand smoke is not at all dangerous.