The Ebola crisis and heightened concerns about the risk of spreading disease during air travel have focused concern on what airlines do to keep planes clean.

It's a murky area without clear regulatory standards. The Federal Aviation Administration says it doesn't regulate or inspect cleaning and referred a reporter to the Occupational Safety and Health Administration, which says it has nothing to do with aircraft cleanliness. OSHA suggested contacting the FAA. The FAA then suggested the Food and Drug Administration. The FDA says it inspects food and water safety on commercial aircraft only.

Airlines say they set their own standards without regulators and give voluminous instructions to contractors. They use chemicals approved by aircraft manufacturers and conduct their own quality-control inspections.

With more than two billion people flying every year, "commercial air transport is potentially an efficient means for spreading communicable disease widely by surface contact and proximity to infected people," the World Health Organization cautions in its Guide to Hygiene and Sanitation in Aviation. Much of the risk comes from being in close contact with an infected person. But contaminated surfaces on airplanes also can spread disease.

Some airlines are reluctant to discuss how much cleansing airliners get. Typically, planes get a once-over straightening-up between flights and usually a more thorough cleaning overnight or between long international flights. Periodically planes get scrubbed from nose to tail when they undergo major maintenance work.

Delta Air Lines and United Airlines say their aircraft that fly into and out of hot spot zones such as western Africa, where several countries are under the threat of the Ebola virus, get a thorough cleaning with disinfecting solution per guidelines issued by the WHO.

Because of the Ebola threat, United increased its cleaning regimen on aircraft flying between Houston and Lagos, Nigeria, on the WHO's recommendation. The Centers for Disease Control and Prevention also issued guidelines for protecting crew members and reporting ill passengers. Airlines say they are complying.

Medical studies have shown that air travelers face higher rates of infection: One study pegged the increased risk of catching a cold at 20 percent. Much of the danger comes from the people within two rows around you.

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