Passengers who came down with a mystery illness on a Delta Airlines flight were found to have elevated levels of carbon monoxide in their bodies.

ABC News reported that several passengers became ill on the Delta flight from Atlanta to Denver on Sunday.

They were found to have elevated levels of carbon monoxide in their bodies, Tulsa Fire Department Stan May said.

Once passengers went outside and breathed fresh air, their levels returned to normal, he said.

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"The flight crew of Delta flight 1817 from Atlanta to Denver elected to divert to Tulsa when a few customers reported feeling ill," Delta said in a statement. "The safety and security of our customers is our top priority."

It was initially reported that 100 people were assessed by emergency services, with nine having the same symptoms - but no cause was established.

The number of patients complaining of symptoms was later raised to 12 by the Tulsa Fire Department.

May said one person was taken to hospital, but it was not related to the carbon monoxide.

Passengers were re-screened to be put on another Denver-bound flight.

- nzherald.co.nz