The World Health Organization says Europeans should try to stay indoors if ash from Iceland's volcano starts raining down from the sky.

WHO spokesman David Epstein says the agency doesn't know the exact health risks from the ash cloud.

But he says the microscopic ash is potentially dangerous for people if it starts to "settle" on the earth because inhaled particles can reach the lungs and cause respiratory problems. There is a heightened danger for those suffering from asthma and respiratory diseases.

Epstein said the cloud mostly remained high in the atmosphere on Friday but the U.N. health agency was monitoring the situation closely. Some ash blanketed the ground in parts of rural southern Iceland.