The cloud, caused in the aftermath of Japan's earthquake and tsunami, passed over the US and the Caribbean on Tuesday and is headed for Europe. It is expected to hit western Europe within two days.
Officials are doing their best to calm the public.
"There are no possible health consequences for France," NSA president André-Claude Lacoste said, explaining radioactive elements were at "extremely low levels, 1,000 to 10,000 times less than those of Chernobyl".
"The cloud has crossed great distances, so there is a strong chance it will be highly diluted when it gets here," Yannick Rousselet of Greenpeace told Paris’s Metro freesheet. But he added, “The problem is that we don’t know exactly what it is made up of.”
Highly toxic elements could still be harmful even after the cloud has travelled all that distance, he says.
“It is estimated that one millionth of a gramme of plutonium can give someone bronchopulmonary cancer.”
And Criirard, an organisation founded after Ukraine’s Chernobyl nuclear accident, complained Tuesday about the lack of information.
“The passing of contaminated air masses over France should not give rise to too much concern,” a statement by the group said. “But, given the crucial lack of information, some of Criirad’s statements have to be conditional.”