From newson6.com:
Tulsa firefighters said the Delta airline passengers who became ill on a flight that was diverted to Tulsa were suffering from elevated carbon monoxide levels in the blood.
....
The district chief that responded to the incident said they initially had reports that 12 to 15 passengers complained of nausea, headache and dizziness, TFD told News On 6. EMSA and Hazmat crews responded and pulled everyone out of the plane.EMSA treated nine people, according to spokesperson Kelli Bruer. The fire department said the patients recovered with fresh air and the administration of oxygen.
Another passenger was transported to the hospital on an unrelated complaint of hypertension, a TFD spokesperson said.
The MD 90 plane was evaluated by Tulsa Hazmat crews who did not find elevated CO levels. The plane was released to Delta maintenance, according to the Assistant Chief's Office. TFD noted that people in the front of the plane did not experience symptoms of illness.
And btw from Avherald:
Accident: Germanwings A319 between Stuttgart and Hamburg on May 11th 2016, two cabin crew poisoned by carbon monoxide
Emergency services, paramedics, arrived and took measurements for carbon monoxide, one of the flight attendants showed a value of 9 (normal values 0-3) prompting the paramedics to immediately call an emergency doctor, who confirmed carbon monoxide poisoning, a second ambulance was called, one paramedics team per flight attendant, with the emergency doctor coordinating efforts to stabilize the patients, carbon monoxide levels in the blood still rising until reaching a value of 13.