A massive cloud of rodioactive debris from China's nuclear test bomb blast last Sunday passed over the Washington area yesterday, raising radiation levels on the ground slightly but posing no hazards, federal and local officials said.
The roughly circular cloud, stretching from South Carolina to Canada, was expected to clear the East Coast by early today as prevailing westerly winds pushed it over the Altantic Ocean.
There is "no measured exposure to people at ground level, and there is no cause for concern and no protective actions are warranted," the Environmental Protection Agency said in a formal statement late yesterday.
The 49 fallout monitoring stations throughout the nation reporting so far the EPA have recorded generally below normal radioactivity levels of less than 6 picocuries, the basic unit for measuring radiation, said EPA spokeswoman Martha Casey. The normal level of natural or "background" radioactivity in the atmosphere, varies widely throughout the country, she said, but the average level is 10 to 15 picocuries per cubic meter of air.
Measurements must be in the "hundreds of picocuries" before they are considered hazardous, she said.
Kah-Hock Lee, radiation specialist for the D.C. Environmental Services Department, said a fallout monitoring station he maintains for EPA on the roof of D.C. General Hospital recorded a cumulative total of 1.9 picocuries of radioactivity per cubic meter of air for the 24-hour period ending at 9 a.m. yesterday.
The normal background level here, he said, is about one picocure.
He said he expects today's cumulative total to be about the same as yesterday's.
Last Sunday's nuclear explosion, the 21st in the last 13 years by the People's Republic of China, set off a large radioactive cloud that drifted steadily westward over the Pacific Ocean and reached the West Coast of the United States earlier in the week.
EPA said the bottom of the cloud is 30,000 to 40,000 feet above the ground and would pose a possible hazard only if heavy rains formed inside or above the cloud and "washed" redioactive particles to the ground. National Weather Service meteorologists said the few rain showers along the East Coast yesterday formed below the 30,000-foot level and thus did not affect the radioactive cloud.
A principal danger in fallout is a radioactive element called iodine 131, which can be transmitted to human beings in cow's milk. Large doses of iodine 131 can cause thyroid caner.
Two nuclear devices detonated by the Chinese last September and November also caused slight increases in radioactivity levels here, but posed no hazard, Lee said yesterday.
Most Chinese nuclear detonations in recent years have been above ground, causing a direct release of redioactive debris into the air. Most detonations by the United States and the Soviet Union have been underground, liming such releases.