Airborne radiation from a meltdown at Japanese nuclear plants poses no immediate risk to the continental United States, say University of Maryland public health and atmospheric scientists.
Drawing on research from the Chernobyl meltdown in 1986, they add that for most Japanese, the long term risk may lie in ingestion of milk, water or food, as well as direct exposure to contaminated soil.
"Radiation from Chernobyl was barely measurable in the mainland United States," says University of Maryland atmospheric scientist Russell Dickerson. "I struggled to detect any in North Dakota, at the time. How much transport we can expect from ongoing events will depend on many factors, some of which are not knowable right now. But, distance is great and so far, releases have not been of the scale seen in Chernobyl. So, there is no present danger."
The most significant release of radiation at Chernobyl involved two by-products of uranium fission - Iodine 131 and Cesium 137.
"After Chernobyl, small amounts of nuclear particles and gases were detected in other European countries," Dickerson adds. "Only about one percent of the release from Chernobyl was deposited onto the United Kingdom. The stuff tends to stay close to where it was released."
The half-life of Iodine 131 is eight days, and for Cesium 137, 30 years, though both are removed from the atmosphere fairly quickly, he adds. The real danger lies not while the particles are in the air, but once rain carries it to the soil and watershed.
"Even in most of the Ukraine and in larger areas of Europe after Chernobyl, the major routes of exposure were not directly from the air, but rather through food, especially milk, produced from contaminated areas, and from fallout deposited on the ground," says Donald Milton, director of the Maryland Institute for Applied Environmental Health and a professor at the University of Maryland School of Public Health.
"It remains highly unlikely that we will see problems with significant crop, dairy, or ground contamination in the United States as a result of the events in Japan," adds Amir Sapkota, a professor at the Maryland Institute for Applied Environmental Health and the Department of Epidemiology and Biostatistics.
'SHELTERING IN PLACE' RISKS
The Japanese government has advised people outside the immediate area of the contaminated nuclear plants to "shelter in place."
"The basic idea behind sheltering in place is that radiation is carried on particles coming from the fires and steam releases," explain Milton and Sapkota. "There is a fairly large body of data showing to what extent the indoor exposure to particles is less than outdoors. Particles in the size range that could be transported long distances penetrate houses to widely varying extents.
"If the windows are closed and there is no mechanical ventilation with outside air intake, if the windows and the rest of the house are new and very tight, the exposure could be significantly reduced. Older leaky houses would be less protective," they add. "Because much ground contamination comes from fallout in rain, the act of simply staying indoors would still provide significant protection from ground contamination, even if it only cut airborne exposure by half."
For workers inside the plant trying to contain the damage and for those working from the air above the plants, the danger is far more immediate. These exposures have already caused at least one case of acute radiation sickness, according to media reports.
"In cases with rapid onset of symptoms, the prognosis is grim at best," Milton says. "We all owe a debt of gratitude to the courageous workers who are struggling to control the plants."