B737-800
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Flight Crew Last 90 Days 488; Flight Crew Type 2500
Physiological - Other
Aircraft Equipment Problem Less Severe; Flight Deck / Cabin / Aircraft Event Illness / Injury; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
Air Traffic Control Provided Assistance; Flight Crew Diverted; Flight Crew Landed As Precaution; Flight Crew Overcame Equipment Problem; General Flight Cancelled / Delayed; General Maintenance Action
Aircraft
Upon arrival at the aircraft to begin my flying day; I noticed an unpleasant odor in the cabin. I asked the flight attendants if they noticed it and without hesitation they said that they did and that the smell was in the entire cabin. I called Maintenance and made a logbook entry describing the odor. Maintenance reviewed the logbook entry and completed a task card. The header of the task card said that if the crew reported a smell such as the one I had described; that it could indicate a leaking oil seal which could contaminate bleed air. Maintenance completed this task card. It took at least an hour. Maintenance said they could find no issue and returned the aircraft to service. We boarded and departed for ZZZ. Approximately 30-45 minutes after takeoff; a Flight Attendant called and said that four passengers in 'the exit rows' were commenting about the odor and asking about it. A short time later; that same Flight Attendant called again and reported that the same passengers were complaining of some discomfort with the smell. She went on to say that she was experiencing a headache. The Flight Attendant said that the smells were intermittent and didn't seem overly concerned.After the first call; I began sending detailed ACARS messages to Dispatch explaining the events. We discussed diversion options; but decided it was reasonable to continue to ZZZ. A few more minutes passed and the same Flight Attendant called again. She said that she was feeling dizzy and nauseous. She went on to say that the B Flight Attendant was feeling the same way. She went on to say that two or three other passengers had started to report discomfort and the first four passengers were now reporting similar symptoms but no dizziness. Meanwhile the First Officer and I were both experiencing very minor eye discomfort. I contacted Dispatch via ACARS stating that diversion was the next step. Dispatch suggested ZZZ1. We were very near ZZZ3 at FL 360 and we agreed with that plan. We notified ATC of a destination change saying that we had an unpleasant odor in the cabin. We proceeded directly to ZZZ1 and landed without incident. Paramedics met our flight. The EMT's evaluated several passengers as well as all of the flight attendants. To the best of my knowledge; no person received any treatment except for the evaluation. It has been suggested that I should have [advised ATC]. While I don't think we had an emergency and I don't think [advising ATC] was appropriate; on the chance that folks wiser than I am think [advising ATC] was called for; I am submitting this report.
B737-800 Captain reported unpleasant odor during preflight. Aircraft was evaluated and returned to service by Maintenance. Odor returned during cruise and flight was diverted for medical evaluation.