Updated: Thursday, 11 Mar 2010, 7:16 PM CST
Published : Thursday, 11 Mar 2010, 7:16 PM CST

Allan Chernoff (CNN)

The “fume event” problem is one that all airlines share. Indeed, Northwest flight attendants complaining of headache and tremors, say their airline also suffered a series of three “fume events” over the past several weeks on the same aircraft flying between Frankfurt and Detroit. The airline, though, won’t confirm precisely what happened.

“We are investigating each case of employee illness, but at this time we can not pinpoint a specific cause,” said Anthony Black, a spokesperson for Delta, which recently merged with Northwest.

Aircraft manufacturer Boeing concedes there is a chance of fume events, but the company says the air on board its aircraft is “safe and healthy.”