WHO spokesman Daniel Epstein said the microscopic ash is potentially dangerous for people when it starts to reach the Earth because inhaled particles can enter the lungs and cause respiratory problems.

"We're very concerned about it," Epstein said. "These particles when inhaled can reach the peripheral regions of ... the lungs and can cause problems - especially for people with asthma or respiratory problems."

Other experts, however, weren't convinced the volcanic ash would have a major effect on peoples' health. Ken Donaldson, a professor of respiratory toxicology at the University of Edinburgh, said volcanic ash was much less dangerous than cigarette smoke or pollution.

Volcanic ash is made of fine particles of fragmented volcanic rock. It is light gray to black and can be as fine as talcum powder. During a volcanic eruption, the ash can be breathed deep into the lungs and cause irritation even in healthy people. But once it falls from a greater distance - like from the cloud currently hovering above Europe - its health effects are often minimal, experts say.

The Icelandic volcano that erupted Wednesday has sent an enormous cloud of microscopic basalt ash particles across northern Europe, grounding aircraft across continent. It is drifting above 20,000 feet (6,000 meters), high and invisible from the ground.

"Not all particles are created equal," said Donaldson, who has studied the impact of volcanic ash in people. "In the great scheme of things, volcanic ash is not all that harmful."

Donaldson said most Europeans' exposure to volcanic ash would be negligible and that only those in the near vicinity of the Icelandic volcano would likely be at risk.

"Once the volcanic particles are in the stratosphere, they're getting massively diluted because there's a lot of air and other particles blowing around," he said.