Ventilation systems get dirty and cleaning them periodically is necessary for the efficiency and longevity of the system. It is also believed to  impact IAQ, though this is somewhat controversial, since there are not currenlty any studies to prove this.

Go through the points covered in the "How can I prevent ventilation IAQ problems?" section and correct anything found.

You can prevent ventilation IAQ problems by running fans continuously from prior to occupancy through all hours of occupancy. Ensure that there are open outside air intakes and no obvious source of pollutants in front of them. Put thermostats under lock and key or, better, use programmable thermostats that allow occupants to adjust the temperature up or down a few degrees for comfort, but do not allow other changes.

Any of the following indicates a problem with the ventilation system and IAQ: There is an odor issuing from the vents; you see pieces of fiberglass caught in grilles, or find debris in areas below vents.; you can smell odors when you first walk in,  such as people's perfume, cooking odors, bathroom odors, or solvents; people start sneezing or their noses run when the A/C fan comes on; the systems are more than 10 years old and have never been cleaned (unless the filtration is unusually good); thermostats are under the control of occupants who can turn them on or off at will, and adjust the temperature to anything they like; and/or the air is stale, stuffy, or dead.

Ventilation systems can bring in pollutants from outside; they can be the source of pollutants; they can cause pollutants to flow from one location in a building to another; or they can fail to dilute or remove pollutants from a building or a portion of it. When you add these factors up, you have the single biggest cause of IAQ problems: improperly designed, installed, operated, or maintained ventilation systems.