You have raised lots of issues here. Firstly, it is important to note that the dangers linked to second-hand inhalation of tobacco (passive smoking), such as the development of some cancers, along with heart and lung conditions are usually due to exposure over a long period of time. Even though it can be pretty unpleasant if you're a non-smoker, you're not going to develop these problems simply hanging out with tobacco smokers from time to time. It's really long-term exposure that is the problem. So having lived in a home where both parents (and other family members) smoked cigarettes regularly could be an issue.
We have no evidence of similar problems for cannabis smoke exposure because no research has really been conducted in the area. Given cannabis smoke contains more carcinogens (i.e., compounds that are known to be associated with an increased risk of developing cancers) than tobacco smoke, it is reasonable to suggest that being around cannabis smokers frequently over a period of time could cause some health problems. This is particularly true for babies and children, at a time where the brain is undergoing rapid development.
You sound as though you are more concerned with the impact of passive cannabis exposure on your mental health. It is possible to experience mild intoxication from inhaling the cannabis smoke of others if you are in a very confined space that is not well-ventilated. This is more likely when the newer, stronger strains are being smoked as they contain higher levels of THC (the chemical that produces the high) in side-stream and exhaled smoke. THC has been found in non-smokers' oral fluid and blood up to 3 hours after being exposed to cannabis smoke and reported the same kinds of intoxication effects as smokers depending on their level of exposure (i.e., whether or not the room was ventilated).
Even so, could passive inhalation of cannabis lead to developing a serious mental illness? There are no reported cases of anyone attributing their exposure to second-hand cannabis smoke as the cause of their schizophrenia or other mental health issue. While the risk is extremely small, however, if you have any concerns about your mental health or know you have a family history of the condition, it is best to avoid any exposure to cannabis.
First published: April 2018