Date: 202001
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class D ZZZ
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Human factors: Troubleshooting
Anomaly: Flight Deck / Cabin / Aircraft Event Illness / Injury; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
Detector: Person Flight Attendant; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Returned To Departure Airport; Flight Crew Overcame Equipment Problem; Flight Crew Landed in Emergency Condition
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: I was the PF (Pilot Flying); the First Officer was the PM (Pilot Monitoring). During climbout we noticed fumes that would not dissipate. We contacted the flight attendants and asked them if they smelled anything. They said there was an exhaust smell throughout the entire cabin and that they and a passenger were experiencing adverse symptoms. We transferred control and I ran the Smoke; Fire or Fumes in Passenger Cabin or Flight Deck checklist. Through the checklist we decided to divert back to our origination airport. I found the Air Quality Event checklist and referenced that. We advised ATC and requested ARFF to meet the aircraft. We had an uneventful overweight landing. We had a large margin in our landing data. All passengers were offered medical assistance. To my knowledge all declined. Our three flight attendants asked to be medically evaluated due to varying degrees of dizziness; burning eyes; burning throats; light headedness; etc. Two flight attendants were given oxygen. All flight attendants declined transport to the hospital. We debriefed over the event. The Smoke; Fire or Fumes in Passenger Cabin or Flight Deck checklist was ran. In the moment there was confusion about a part of the checklist regarding turning on all lights in the cabin. It was daylight and since we did not know the origin of the smell we elected not to have all lights turned on. The reasoning was not clear at the time why the checklist was directing that action. We were within minutes of landing so we elected to take the safest course of action to continue with the checklist. We believed the smell to be dissipating after completing the checklist items. During the debrief the flight attendants thought the smell was intensifying with the aircraft below 10;000. Inflight; the flight attendants had said the smell was consistent.The cause was unknown; even after a ground-run of the aircraft. Recommendations would be for a small note to be added for the reasoning of turning on the cabin lights. I can only speculate about the reasoning at this point. Another strong recommendation would be for checklists like the 'Air Quality Event' and security 'Threat Levels' to be in more accessible 'books' than the FOM. They share a lot of similarities. They are run and used like checklists; they are both difficult to find and both have time critical information in them. Events like smoke and fumes have a lot of moving parts to juggle and it would be safer if information found in the checklists were more intuitive to find instead of being part of the treasure hunt that are our manuals.




Synopsis: Air carrier Captain reported a fumes event resulting in a return to the departure airport.