Apparently the fumes were so bad the crew had to wear their masks some of the flight.
"The captain also had to repeatedly ask his passengers to stop smoking marijuana because they were creating so much smoke that it actually began to seep into the pressurized cockpit. The co-pilots were forced to wear oxygen masks in order to prevent themselves from inhaling the smoke and getting high, which would have severely impaired their ability to safely maneuver the aircraft. If the pilots accidentally inhaled the smoke and failed drug tests after the flight, they could have risked losing their licenses."
Justin Bieber And Crew Reportedly Smoke Lots Of Pot, Harass Attendant On Private Flight