Ventilation question

A friend of mine would like to know how exactly a basement grow room is ventilated. He plans on putting an exhaust fan in the upper corner and exhausting it through the sewer but he isn't sure what to do about intake. How does he get fresh air into the room when he doesn't have windows in the basement?

first becarefull of venting in sewer line if fan stops working could cause methane gases to enter house dangerus for humans or pets, second just take the air from the basement it will wor fine

Agree with bseeds 100% on using your sewer vent. Those run 18" above roof peak for a reason and the sewer gas is that reason. Odorless (believe that or not) and lethal. Now go here to calculate your air needs.

http://botanical.com/hydro/air/calculating_fan_requirements.html