Suggest Treatment For Dark Circle Under The Eyes

just had a visit from my sister from out-of-state. The whole family of 4 has dark circles under their eyes. Even the 3 year old, who is not blood related, but adopted. Ages 62, 47, 9 and 3. Someone thinks they are getting carbon monoxide poisoning. Is this true or is there some other reason? We did not say anything to them about their appearance.