An A321 diverted to a nearby airport when a passenger and one or two flight attendants became nauseous and light headed. After landing the source of their discomfort was determined to be the odor of prescription grade eucalyptus oil used by two passengers as a respiratory aid.

Narrative

Approximately 1-1/2 hours into the flight the Lead Flight Attendant informed us that they had a passenger and possibly a Flight Attendant becoming ill with nausea and light headedness. He indicated he would get more information and get back to me ASAP. About three minutes later a second Flight Attendant called the cockpit and advised she was feeling nauseous and felt like passing out. These were two separate calls. I spoke with the second Flight Attendant about diverting and she indicated she felt that was best. I then made then made the decision to divert; since the Flight Attendant was unable to do her duties onboard. I informed the passengers of the diversion for medical reasons.We declared a medical emergency with Center and started our descent. I then received the call back from 'A' Flight Attendant asking if we were really diverting to ZZZ. I confirmed and realized the second Flight Attendant had not communicated with the others. We landed uneventfully and were met by gate agents; security; police; and paramedics. The paramedics came on board and retrieved the Flight Attendant and passenger to be monitored in the terminal. After a period of recuperation; both declined hospital care.Once the passengers deplaned I spoke with the remaining flight attendants. The flight attendants indicated a smell of ammonia. At this time we did not know if it was cabin related or aircraft related. As time went on the ill passenger indicated the passengers sitting next to her put some kind of ointment under their noses. After that; she began feeling ill and so did the Flight Attendant serving off the cart in that area. We determined that the source of the smell was a prescription grade eucalyptus oil and not aircraft related. TSA and the Flight Attendant both confirmed the eucalyptus oil as being the source that was smelled.The Flight Attendant was relieved of duty and we awaited her replacement and continued on to our destination.