PRINCETON, N.J. — When airports across Europe reopened after the closure caused by the eruption of Iceland's Eyjafjallajokull volcano, it was not because the amount of ash in the atmosphere had dropped, but because the risk that the ash posed to airplane safety had been reassessed. Was it new scientific information that led to the lifting of the flight ban, or was it a reflection of the hardship, both personal and economic, that the ban was causing?

Over six days, about 95,000 flights were canceled, at a cost to airlines of more than $1 billion. An estimated 5 million people were stranded or delayed. The British economy lost £1.5 billion, and others were similarly affected. Flower growers in Kenya, who depend on air transport to take their short-lived product to Europe, suddenly had no income. Sixteen cancer patients in critical need of bone marrow for transplants were put at risk because the matching marrow could not be flown in from the United States or Canada.

In the past, jets flying into ash spewed from volcanoes in the U.S., Indonesia, the Philippines and Mexico have temporarily lost engine power, and in one case, dropped thousands of feet, although all managed to land safely. But there was no evidence that the more widely dispersed ash blowing over Europe from Iceland would have caused similar problems. The decision to ground flights was based on the view that any level of ash in the atmosphere posed some risk to aircraft, and that no matter how slight that risk might be, the government's job was, as British Prime Minister Gordon Brown put it, "to make sure that safety was paramount."