Europeans should try to stay indoors if ash from Iceland's volcano starts settling, the World Health Organization warned Friday as small amounts fell in Iceland, Scotland and Norway.  WHO spokesman Daniel Epstein said the tiny ash is potentially dangerous for people when it starts to reach the Earth because particles   taken in by people can enter the lungs and cause breathing problems. And he also said Europeans who go outside might want to consider wearing a mask. Other experts, however, weren't convinced the volcanic ash would have a major effect on peoples' health and said WHO's warnings were " _ ." They 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.  "Not all particles are created equal," said Ken Donaldson, a professor at the University of Edinburgh, "In the great scheme of things, volcanic ash is not all that harmful." And he said most Europeans' exposure to volcanic ash would be ignored and that only those in the near districts of the Icelandic volcano would likely be at risk. Dr. Stephen Spiro, a professor of the British Lung Foundation, said the further the particles travel, the less dangerous they will be. "The cloud has already passed over northern Scotland and we haven't heard of any ill effects there," he said. Spiro said to wear masks or stay indoors to avoid volcanic ash was "over the top" and "a bit hysterical." The text is mainly about   _  .
the effect of volcanic ash the health risk of volcanic ash the disadvantages of volcanic ash the opinions on health risk of volcanic ash
Question: Europeans should try to stay indoors if ash from Iceland's volcano starts settling, the World Health Organization warned Friday as small amounts fell in Iceland, Scotland and Norway.  WHO spokesman Daniel Epstein said the tiny ash is potentially dangerous for people when it starts to reach the Earth because particles   taken in by people can enter the lungs and cause breathing problems. And he also said Europeans who go outside might want to consider wearing a mask. Other experts, however, weren't convinced the volcanic ash would have a major effect on peoples' health and said WHO's warnings were " _ ." They 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.  "Not all particles are created equal," said Ken Donaldson, a professor at the University of Edinburgh, "In the great scheme of things, volcanic ash is not all that harmful." And he said most Europeans' exposure to volcanic ash would be ignored and that only those in the near districts of the Icelandic volcano would likely be at risk. Dr. Stephen Spiro, a professor of the British Lung Foundation, said the further the particles travel, the less dangerous they will be. "The cloud has already passed over northern Scotland and we haven't heard of any ill effects there," he said. Spiro said to wear masks or stay indoors to avoid volcanic ash was "over the top" and "a bit hysterical." The text is mainly about   _  .
A. the effect of volcanic ash
B. the health risk of volcanic ash
C. the disadvantages of volcanic ash
D. the opinions on health risk of volcanic ash
Answer: