Smoke travels through vent

He also suggested that I get a strong personal HEPA air filter and that I place it right under the HVAC vent

I have a bit of a legal issue on my hands. I signed a lease to rent a room in this house. I still have 10 months left on the lease but I really want to find a new room to rent. The problem is that I work a job that has regular drug tests. The owner of the house apparently does not. I smell him smoking something good every single night. I can’t have even a trace of drugs in my system or I will lose my job. At first I would just shove a towel under my door but that didn’t stop the smell. Recently, I discovered that the smell is coming from the HVAC system. I know the homeowner hangs out in the living room and watches TV in the evenings. I also know that the couch is right next to the HVAC vent. This must be how the smell is getting into my room. It must be coming in through the ductwork. Since this discovery I have started closing the vent in my room in the evenings but it doesn’t seem to help much. Besides, it is always hot here so I can’t go all night without A/C. A friend of mine suggested that I get a really strong air filter for the HVAC system. He said that there are air filters that can filter out smoke but I find it hard to believe any air filters are that strong. He also suggested that I get a strong personal HEPA air filter and that I place it right under the HVAC vent. I just hope that that will be good enough. I still am scared of the next time I get tested.
Air conditioner service