From my college days, I do not remember anyone smoking one joint a week. No one was drinking one beer or smoking one cigarette either. What a bunch of crap. This is a study from the same people who want outdoor smoking bans because second hand smoke will give you instant cancer.
According to the Substance Abuse & Mental Health Services Administration, 4.2% of Americans smoke more than once a year but less than once a month - so it's not hard to believe that some smoke only once a week.