In addition to scores of canceled flights, the World Health Organization has warned people across Europe to stay indoors or use masks when the ash begins to fall.

Volcanic ash is made up of tiny pieces of glassy sand and dust produced when explosive eruptions demolish solid rock or spray lava into the sky, where it solidifies before falling.

Inhaling these particulates can cause irritation of the eyes, nose, and throat. Finer particles can penetrate deep into the lungs and cause breathing problems, particularly among those with respiratory issues like asthma or emphysema.

But other scientists believe that ash fall from Eyjafjallajökull will be too limited and scattered to