Published: Apr 21, 2010 10:51 PM EDT

The electronic board at Europe's biggest airports are showing about 80 percent of flights on schedule today.

They are also pretty full. With more than 100,000 flights scrapped worldwide in the last week thanks to the ash cloud emitted by an Icelandic volcano, airlines today began filling vacant seats with travelers who have been stranded for days.

Airlines say their losses have topped $2 billion. The International Air Transport Association says economic fallout is worse than the three-day worldwide shutdown that followed the September 11 terrorist attacks.

Still, scientists at a Swiss institute say an analysis of air samples collected over Zurich last weekend show the safety concerns were warranted.

And they say the volcano could be getting more dangerous. One scientist says the composition of the magma is changing and could become more explosive. If that continues or a second nearby volcano erupts, Peter Ulmer says it could last for weeks and be similar to the eruption of Mount Vesuvius that buried Pompeii in 79 A.D.