Wind
The presence of the wind, whether as a gust of searing desert air or the unnatural chill that suddenly envelops the room is used to symbolize the presence of the demonic. This is appropriate given the invisible but tangible presence of the wind as a parallel to the invisible but likewise tangible presence of the demon tormenting Regan. The use of the wind as a symbol for the demon Pazuzu is also a nod to Babylonian mythology as the demon was once worshipped as an evil deity of the wind.
No Positive Male Figures
The lack of positive male figures in the film is used as a symbolic parallelism to the apparent lack of presence of the Judeo-Christian God in the lives of Regan and Chris, thus opening them up to supernatural assault.
Religious Iconography
Religious iconography is scattered throughout the film largely because of the characters, the setting and the themes tackled. More importantly the religious iconography is also used to highlight the apparent strength and the sheer evil of the demon proving that to it truly nothing is sacred. It mocks the church by desecrating image of the Virgin Mary, in a church no less. It compels Regan to masturbate with a crucifix and vomits on the priests as they wield rosaries and crucifixes.
Darkness
As the film nears the climax the scenery and lighting scheme grows increasingly darker, as if to emphasize and match the growing evil influence the demon has over Regan.
Captain Howdy/Pazuzu
In addition to using the wind as a manifestation of the demon it begins to manifest itself later on in the film now more directly as either a horrifying pale face with angry eyes or as the silhouette of the four-winged, beast-headed effigy of Pazuzu. These images become increasingly visible and always in short, horrifying flickers but consistent enough to frighten. The increased frequency of the manifestation of these images are used to emphasize its’ supposed dominance over Regan and her family.
