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A Misleading Map of Radiation Spread

What looks like an ominous cloud actually poses little serious health risks for the US.
March 17, 2011

The New York Times website has published an alarming interactive map that shows a massive plume of radiation moving across the Pacific and reaching Southern California tomorrow. The map was created by the Comprehensive Test Ban Treaty Organization, an arm of the United Nations. The text around the map explains—in bold type—that the map does not indicate levels of radiation, and that the plume would be so dilute by the time it reaches the U.S. that it would have “extremely minor health consequences.” But unfortunately, the picture is going to be far more powerful than the words. For a dispassionate look at the health impacts of radiation, see our story today.

Here’s what’s wrong with the map. First, is uses a logarithmic scale: parts of the plume indicate radiation levels that are several orders of magnitude lower than others. The yellow parts of the map show levels that are one thousand times higher than the purple ones. But just looking at the map, you wouldn’t notice this at first. The levels all seem pretty close to each other, in part because of the gradual differences in color between them. Second, the map uses “arbitrary units.” We have no idea whether even the highest concentration parts are dangerous, let alone the dilute parts. But given data from Japan, even the highest concentrations shown are at levels that are not dangerous to human health. The levels that reach the U.S. would be far lower. They might be just barely detectable above natural background radiation.

Yesterday in Japan, at the gate of the severely damaged Fukushima plant, radiation levels were at a few millisieverts per hour. The levels rapidly drop to a few microsieverts moving away from the plant. Judging by the scale of the map, the yellow sections—the most intense—would likely correspond to microsieverts. Radiation sickness doesn’t set in until a person has received a dose of about 1 sievert—a million times more than a microsievert. But, of course, this is all a guess, because the map doesn’t provide the data. All it gives is relative concentrations, and those are misleading.

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