Carbon Filter: Vent or Scrub?

I bought a HTG inline fan and I'am doing a DIY carbon filter. I have 6 plants but want want to expand to 16 plants eventually.

It's in a closet in a room. Closest isn't sealed but has door shutters.

Would it be more effective to vent the air from my carbon filter to the attic or have the carbon filter just keep scrubbing the air in the room and exhausting it in the room?

Unless the room is very cool I would think venting the air to the attic would get the heat out of the room. My vote would be for the attic.