Environmental allergies affecting only left sinus cavities, why is this?

Anatomy: Allergies cause swelling of the tissues that line the nose and sinuses. Swelling of the nasal tissues cause the drainage openings of the sinus to constrict. Once they constrict completely, mucous from the sinus cannot drain. If one side of the nose is shifted (think deviated septum) then that side constricts before the other. Try to shrink the tissues with antihistamines and, my fave, neilmed.