Dao suffered a broken nose, a concussion and lost two front teeth in the violent removal, one of his attorneys, Tom Demetrio, said at a press conference earlier on Thursday.
Dao's daughter, Crystal Dao Pepper, told the news conference that the family was "horrified, shocked and sickened" by what happened to her father.
He promised to review the airline's policies to make sure something like that never happens again, and said United will no longer use police to remove bumped passengers.
After what occurred, Dao "has no interest in ever seeing an airplane" and will likely be driven to Kentucky, Demetrio said. Neither Pepper, nor Dao's lawyers, have received an apology from United.
The lawsuit will be filed in Cook County Circuit Court in IL, the lawyer said, indicating it would target both the airline and the city of Chicago, whose Department of Aviation was involved in removing Dao from the plane.
Other attorneys said state courts are typically more favourable to plaintiffs. "But what this City Council can do is assure the public that this will never happen again in Chicago's O'Hare and Midway airports". WATCH: MUNOZ INTERVIEW WITH ABC NEWSIn an initial response to the incident, Munoz seemed to blame Dao, describing him as "disruptive and belligerent".
Chicago Alderman Michael Zalewski, head of the committee, called the incident a nightmare that should have been avoidable.
Callan said he had reviewed United's contract of carriage, the fine print that passengers agree to when they buy tickets.
This update has been corrected to show that the United vice president is John Slater, not Jeff Slater. United has no set policy for physically forcing passengers to deboard, he added.
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"Prince George (who is 3) will be a page boy, and Princess Charlotte (2 on May 2) will be a bridesmaid", the statement added. But pitching her look perfectly for her sister's wedding is set to be one of her most fascinating sartorial challenges yet.
"To be quite frank, Chicago employees should not be doing the dirty work for the friendly skies airline", said Alderman Ed Burke, who played video of Dao being removed. They said the city of Chicago was also responsible for what happened to their client.
The aviation city chief says the standard protocol is to detain an individual until Chicago police arrive, not to go on the plane when there is a simply a "customer issue".
The confusion about the security force starts with the fact that airport security in Chicago is handled differently than it is in other big cities such as Los Angeles, New York and Minneapolis. The stock closed down 1.2 per cent yesterday. I share all of those sentiments, and one above all: "my deepest apologies for what happened", Munoz said in a statement.
The news of the scorpion sting could not have come at a more coincidental time than this week, days after a United passenger was forcibly dragged from his flight to Chicago. Three officers were suspended pending an investigation and the incident triggered a global outrage for United and CEO Oscar Munoz, who rejected calls for his resignation.
Dr. David Dao was forcibly removed on Sunday from a plane that was full of paying passengers, setting off a firestorm United is still reeling from. "I've concluded that based upon hundreds of tales of woe, of mistreatment, by United", Demetrio said.
Video of the Kentucky physician being pulled from his seat after he refused to leave the full plane has been viewed by people around the world. Three aviation officers have been place on leave due to the incident.
In the future, law enforcement officers will not be involved in removing a "booked, paid, seated passenger", Munoz said.
The United flight incident shined a spotlight on Chicago's aviation police.