The Associated Press reports that emergency and fire crews rushed to Charlotte-Douglas International after a dozen passengers and crewmembers on a flight from St. Thomas, the U.S. Virgin Islands, complained of sickness.

A local television station reported that seven US Airways crewmembers were taken from Charlotte-Douglas International Airport Jan. 16 for treatment after they complained of headaches and nausea. Five passengers from that flight were treated on the scene.

Shortly after the plane took off from St. Thomas, crewmembers noticed a slight odor on board and alerted authorities in Charlotte, the AP reported.

Investigators were working to determine what caused the odor, and the plane was taken out of service.

Passengers and flight attendants have filed multiple lawsuits against Southwest Airlines, Boeing and Airbus in federal court, alleging that jetliners they were aboard leaked toxic air that sickened them.

The latest lawsuit seeking damages was filed in a federal district court in Birmingham, AL in late October by twin sisters who allege they were seriously injured during a suspected toxic air fume event on a Southwest Airlines flight from Los Angeles and now suffer from disabling neurological and respiratory symptoms.

Victoria Vaughn Holsted of Franklin, TN, and Valerie Vaughn of Birmingham, AL allege in their complaint that as a result of a toxic air exposure onboard a Boeing 737-300 jet aircraft operated by Southwest Airlines on January 27, 2009, they have suffered severe injuries, including motor skill deficiencies, loss of balance, vision impairment and uncontrollable tremors.

The sisters' complaint alleges that Southwest Airlines negligently operated and maintained the jet aircraft, causing their injuries.

"We are filing this lawsuit in part because we want to help our clients learn what chemicals they were exposed to on that flight so their doctors can provide them with proper medical treatment," stated Seattle WA-based Alisa Brodkowitz, one of the attorneys for Holsted and Vaughn.

Brodkowitz is also representing former flight attendants suing Boeing and Airbus for their roles in other contaminated cabin air incidents.

Two former flight attendants are suing Airbus, alleging that toxic "bleed air" from aircraft engines caused serious permanent injuries.

According to the lawsuit filed in U.S. District Court in Broward County, FL, Lucy Mayorga and Adriana Moravcik suffered inhalation injuries on Oct. 8, 2005 aboard a U.S. Airways Airbus A319-112 aircraft. Upon takeoff, they noticed a strange chemical smell, followed by their eyes watering, their throats tightening and debilitating symptoms, including headache, upper respiratory irritation and difficulty breathing.

They were later diagnosed with breathing injuries, and their symptoms continued to worsen until they had to give up their jobs. The lawsuit goes on to allege that defective design allows toxic chemicals from engine oil and hydraulic lubrication products to contaminate air drawn in to the cabin through the engines.

The problem of toxic 'bleed air' on airplanes has been known to the airline industry since the 1950s, says Brodkowitz. "The aircraft manufacturers have turned a blind eye to this problem and failed to equip their planes with sensors or filters to keep toxic chemicals out of the cabin. The only things filtering this stuff out of the cabin are the lungs of passengers and crew members."

Brodkowitz also represents former flight attendant Terry Williams who has sued Boeing, alleging that bleed air fumes sickened her aboard an American Airlines MD-82 in 2007, eventually causing symptoms so intense that she could no longer work.

Initially, airline passengers breathed in air supplied directly from the atmosphere using compressors. But this was an expensive proposition, so in the 1960s a system was developed to draw the "bleed" air from the engines.

Air is drawn out of the compression section of the engine and cooled. It then enters the cabin, where it mixes with re-circulated air that has passed through filters designed to remove bacteria and viruses. These air filters do not remove any fumes or vapors from the engine. So if engine oil or hydraulic fuel leaks, because of poorly designed or faulty seals, or even over-filled tanks, toxic chemicals can contaminate the air supply.

The bleed air can be laced with a chemical, tricresylphosphate (TCP), an organophosphate, or other toxic mixtures of chemicals that have been linked to serious respiratory problems, memory loss, neurological illnesses and even brain damage.

According to the Association of Flight Attendants (AFA): "the air you breathe in the aircraft cabin is supplied from the engines or auxiliary power unit (APU) and sometimes it gets contaminated with engine oils or hydraulic fluids that get heated to very high temperatures, often appearing as a smelly haze or smoke. That haze/smoke that enters the cabin air is a toxic soup and can contain carbon monoxide gas as well as chemicals that can damage your nervous system called tricresylphosphates. Exposure to TCPs can initially cause stomach ache and muscle weakness, followed by delayed memory loss, tremors, confusion, and many other symptoms," AFA added.

University of Washington Professor Clem Furlong is in the final stages of developing a blood test that will confirm whether 92 people who suspect they've been poisoned by toxic fumes in airplanes actually were. Brodkowitz said the Vaughn sisters were among the nearly 100 people who took Furlong's new blood test for TCP poisoning.