This study defined the marijuana usage as 4 days a week. That is not uncommon now, among teenagers, who often view marijuana as harmless and even sometimes deny that it is a drug.
From another article on the same study:
What isn’t clear from this study is what quantity of weed causes damage, and what age (if any) might be safe for regular use.
Marijuana use is up among American teens, who are now more likely to smoke pot than tobacco, according to a 2011 University of Michigan study.
That study found one in every 15 high-school seniors getting high on a daily or near daily basis, the most substantive rates seen since 1981. One hypothesis for the resurgence is that teens perceive few risks associated with the drug, with many refusing to even call it a drug.
In Canada, the prevalence of pot use among Canadians aged 15 and over decreased to 9.1 per cent in 2011 from 10.7 per cent in 2010.
Still, the rates for youth aged 15 to 24 were three times higher than for their over-25 counterparts: 21.6 per cent versus 6.7 per cent.
Would that be considered a massive amount to ingest compared to what you might get from incidental expose to second hand smoke?