The quality of the air in your grow room is one of the factors that determine the unique microclimate that you wish to create for your cannabis plants to grow.
This include proper ventillation, circulation, humidity and a number of more elusive variables, such as the air ionization.
Perfect humidity varies in different stages of the plant's life.
Though preferably high at the beginning, it should not be too great in the later stages of flowering, as you wouldn't like to see your future marijuana being covered in mould.
One basic piece of equipment that you will sooner or later need is a decent fan.
Those also vary, as some are meant to take the air in/out of the box, while others are merely there to ensure the circulation inside - what you will need depends on the size of the room and the type of lighting you use.
With multiple HPS lamps, that emit lots of heat, you might need somewhat stronger ventillation then with CFL or LED systems.
You will find that ventilators come both plain, as well as with attached sensors and knobs.
More advanced fans can regulate their speed automatically, depending on the temperature, humidity or even negative pressure that can sometimes happen in closed grow boxes.
In order to reduce the smell outside of the growing area, you might need carbon filters that work in tandem with fans.
Those are rather bulky cylinders filled with activated carbon that captures the strong aroma of your cannabis plants.