Chronic -
1) of long duration
2) Subject to a habit or disease for a lengthy period
Syn. continuing, lingering, persistent, prolonged, protracted
The term chronic is sometimes used as in definition #2 - "A history of CO inhalation and an awareness of the typical distributions of lesions are important for recognition of the effects of CO poisoning, especially when patients are in the chronic stage." (Uchino et al., 1994, Neuroradiology, 36, 399-401)
Note: In this condition, ie. chronic CO poisoning, we are concerned with how long the insult (exposure) lasts, not how long the resulting effects last.