For years, passengers and crew have reported mysterious illnesses after breathing strange fumes on some airplane flights. NBCLA has uncovered growing evidence that explains where those fumes are coming from and why they could be making some travelers sick.
Just how safe is the air you breathe on commercial airplanes?
An untold number of air travelers have complained of experiencing tremors, crippling headaches and other neurological problems after breathing strong fumes while in flight.
"The flying public has a right to know what they're breathing," said Aviation attorney Alisa Brodkowitz.
Brodkowitz wants to know what happened to two of her clients, twin sisters Victoria and Valerie Vaughn, after they left Los Angeles International Airport last January, on a Southwest Boeing 737 to Nashville, Tenn.
According to a petition filed in Los Anegels Superior Court, the plane filled with a strange mist, then made an emergency landing. Ever since that flight, the Vaughn sisters say they've been seriously ill.
"They experienced severe headaches, tremors, cognitive problems, memory problems," says Brodkowitz.
The mystery of what's happened to the Vaughn sisters and other travelers might be starting to unravel, nearly a decade after Congress ordered the National Academy of Sciences to study the air on commercial planes.
That study, and other studies done abroad, reveal that toxic jet engine oil fumes sometimes "unintentionally enter the cabin" of a plane.
Dr. Clement Furlong at the University of Washington, who's studying the problem of toxic air on planes, believes a chemical found only in jet engine oil -- called TCP -- has poisoned passengers and crew.
"These are pilots and crew from all over the world, they describe similar symptoms," Furlong said.
Former flight attendant Karen Burns says she experienced symptoms, like tremors and memory loss, for three years after breathing fumes on one particular flight.
Angie Estes, 77, says she still has daily tremors that began the day after she flew home to Seattle from visiting family in Phoenix.
"It comes on so fast," says Estes.
These alleged illnesses have even been documented in a film made by former commercial pilot Tristan Loraine, who believes he was poisoned by fumes on one of his flights.
"My doctors decided that the medical effects and symptoms which I had were such that my health had been compromised to the point that it was no longer safe for me to fly," Loraine said.
So how do toxic fumes end up in the air you breathe on a plane?
Numerous studies now blame what's called a "bleed air system," which is used on almost all commercial jets made by Boeing and Airbus.
In a bleed air system, the plane's engines suck in fresh air from the outside and circulate it through the plane.
But the studies show, if there are oil leaks in the engine, which are not uncommon, the system can actually pump toxic fumes through the cabin and that can cause nervous system disorders.
Both Boeing and Airbus tell us that air on its planes can occasionally become contaminated. Boeing claims those fumes can only cause short-term health problems for travelers, not long-term illnesses.
Airbus says it's not aware of any significant problems from the contaminated air.
But some travelers who say they've fallen ill, like Valerie and Victoria Vaughn, want more information.
That's why their lawyers took Southwest Airlines to court this past week. Southwest says there were no toxic fumes on the Vaughns' flight from LAX, but the airline agreed in court to turn over evidence from the grounded plane to the Vaughns' lawyers and allow them to test materials from the 737 to see if any chemicals might have been in the cabin air.
And Congress is now calling for a long-term solution to the problem of contaminated bleed air.
The U.S. House just passed a bill calling for the FAA to study requiring sensors and filters on planes that would detect and remove toxic fumes inside an airplane. The technology already exists, but would be costly to install.
"Airline manufacturers should equip their planes with filters that would filter out chemicals so people don't breathe them," Brodkowitz said.
What to Do If You Are Exposed to Toxic Fumes on an Airplane
Do you have a story for us to investigate? E-mail Joel at: Joel.Grover@nbcuni.com
Click here to check out more NBC Los Angeles investigative reports.